rr166 an appraisal of existing seismic hazard estimates for the uk

HSE Health & Safety

Executive

An appraisal of existing seismic hazardestimates for the UK Continental Shelf

Prepared by The Mallard Partnership for the Health and Safety Executive 2003

RESEARCH REPORT 166

HSE Health & Safety

Executive

An appraisal of existing seismic hazardestimates for the UK Continental Shelf

D.J. Mallard B.Sc.(Eng) M.Sc. C.Eng. M.I.C.E. FGSB.O. Skipp B.Sc. Ph.D. C.Eng. F.I.C.E. C.Geol FGS

& W.P. Aspinall B.Sc. Ph.D. C.Geol FGS The Mallard Partnership

26 Gloucester Street Winchcombe

GLS4 5LX

The first seismic hazard estimates for the UKCS were made for HSE in 1986, with later assessments being made in 1993 and 2002.Since the work on the most recent of these studies was completed, a draft ISO standard and a NORSOK standard have been published. Also, a medium-sized earthquake has occurred in the central North Sea, in an area previously considered to have low seismic activity. In this report, the most recent estimates (given in report OTO 2002/005) are synthesized with those from the earlier studies, using a specially-designed rating scheme, to generate hybrid contour maps of the peak ground acceleration (pga) hazard. The reliability of these hybrid results is assessed by comparison with provisional, newly-calculated, de-minimis hazard figures based on catalogue completeness considerations. Until such time as a uniformly-derived hazard map is developed it is recommended that the 10-4 p.a.probability of exceedance pga hazard figure at any point in the offshore UKCS east of longitude 10°W should be taken to be highest of the figures given by: the new map of allowable minimal hazard levels, the new synthesized hazard map, and the estimate given by report OTO 2002/005. As there is no current information on hazard exposure at the lower frequencies that are of concern when designing offshore installations, recommendations are made as to how such estimates could be calculated. The characterization of ground motion is discussed, noting the significance of the differences between hard and soft sites and current thinking on vertical motion. Attention is drawn to policy and technical issues that remain unresolved and suggestions made as to how those issues could be addressed.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

HSE BOOKS

© Crown copyright 2003

First published 2003

ISBN 0 7176 2778 0

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Applications for reproduction should be made in writing to: Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to [email protected]

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CONTENTS Executive summar y v

1 INTRODUCTION 1

1.1 Background and objectives 1 1.2 Scope 3 1.3 Approach adopted for conducting this appraisal 4 1.4 Structure of this report 6

2 GENERIC ISSUES RAISED BY THE REPORTS CONSIDERED IN THIS APPRAISAL 8

3 SOME FUNDAMENTAL ASPECTS OF THE PRACTICE OF SEISMIC HAZARDASSESSMENT 11

3.1 General matters 11 3.2 Modelling seismicity 13 3.3 The value of hazard maps at low probabilities 17 3.4 The role of geological information 20 3.5 The effects on hazard levels of varying model parameter values 21 3.6 Concluding remarks 22

4 SYNTHESIS OF PGA HAZARD MAPS 23

4.1 The existing hazard maps 23 4.2 Making a synthesis of the existing hazard maps 27 4.3 Discussion 33

5 CHARACTERISING EARTHQUAKE GROUND MOTION FOR THE UKCS 42

5.1 Introduction 42 5.2 The ground motion characterisations proposed in the existing reports 44 5.3 The adequacy of the ground motion characterisations proposed in the existing

reports 45 5.4 Time histories 52

6 VERTICAL MOTION 54

7 TOWARDS ESTIMATING THE 1HZ HAZARD ACROSS THE UKCS 56

8 CONCLUDING REMARKS AND RECOMMENDATIONS 58

8.1 The ground motion hazard in terms of pga 58

9 REFERENCES 66

9.1 The reports forming the input to this appraisal 66 9.2 References quoted in main text 66 9.3 References quoted in appendices 68 9.4 A brief selected bibliography 68

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10 Appendix 1: Notes on the individual Reports 71 10.1 Report 1 – Principia Mechanica Ltd. (1986) 10.2 Report 2 – BGS & Ove Arup and Partners (1997) 10.3 Report 3 – NORSAR & NGI (1998) 10.4 Report 4 – EQE International Ltd. (2002)

71 72 74 77

11 Appendix 2: Attenuation Relationships 80 11.1 Report 1 – Principia Mechanica Ltd. (1986) 80 11.2 Report 2 – BGS and Ove Arup & Partners (1997) 80 11.3 Report 3 – NORSAR & NGI (1988) 81 11.4 Report 4 – EQE International Ltd. (2002) 83

Figures

Following page 83:

Figure 1 Report 1: replicated 10-2 p.a. pga contoursFigure 2 Report 1: replicated 10-4 p.a. pga contoursFigure 3a Report 2: replicated 10-2 p.a. pga contours (east of 10ºW)Figure 3b Report 2: replicated 10-2 p.a. pga contours (west of 1ºW)Figure 4a Report 2: replicated 10-4 p.a. pga contours (east of 10ºW)Figure 4b Report 2: replicated 10-4 p.a. pga contours (west of 1ºW)Figure 5 Report 4: replicated 10-2 p.a. pga contours (east of 10ºW)Figure 6 Report 4: replicated 10-4 p.a. pga contours (east of 10ºW)Figure 7 UK Continental Shelf Designations [2001] showing areas covered by existing reportsFigure 8 Hybrid 10-2 p.a. pga hazard contour map (east of 10ºW)Figure 9 Hybrid 10-4 p.a. pga hazard contour map (east of 10ºW)Figure 10 Hybrid 10-4 p.a. pga hazard contour map compared with onland site-specific

hazard estimates Figure 11 Macroseismic earthquake magnitude completeness thresholds in and around

the British Isles Figure 12 Minimal 10-4 p.a. pga hazard contour map determined from catalogue

completeness considerations Figure 13 Difference contours for hazards indicated by Figure 9 and Figure 12 Figure 14 Combination 10-4 p.a. pga hazard contour map (the hazard levels plotted are

the higher of those indicated at each point by Figure 9, or by Figure 12) Figure 15 Comparison of PML [1981] spectral shapes for hard and soft sites Figure 16 Comparison of various spectral shapes for bedrock/hard sites Figure 17 Comparison of various spectral shapes for soft sites Figure 18 Difference contours for hazards indicated by Figure 14 and Figure 6 (i.e.

Figure 14 minus Figure 6) Figure 19 Recommended 10-4 p.a. pga hazard contour map (the hazard levels plotted

are the higher of those indicated at each point by Figure 14 or by Figure 6)

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Executive summary

The first seismic hazard estimates for the offshore UK Continental Shelf (UKCS) were made for HSE in 1986, with later subsequent studies being published in 1993 and 2002. Since the work on the most recent of these studies was completed, a number of developments have taken place. Two design standards for offshore structures have been published (a draft ISO standard and a NORSOK standard), and a medium-sized (4.2ML) earthquake has occurred in the central North Sea, in an area previously thought to be characterised by low seismic activity.

With this background, the Offshore Safety Division of the Health and Safety Executive (HSE OSD) commissioned The Mallard Partnership to carry out an evaluation of several existing hazard studies (i.e. not only the most recent report, but also some of the earlier work) in the light of recent methodological advances. HSE OSD also wished to be informed about such future work as might be necessary to address any issues revealed by this appraisal that cannot be resolved using available information. As part of the exercise, comparisons were to be made with the requirements of various design standards, including the new draft ISO standard.

The existing reports made available by HSE OSD for this appraisal are:

Principia Mechanica Ltd. (1986) 'North Sea seismicity'. UK Dept. of Energy Offshore Technology Report No. OTH 86 219, HMSO, London.

BGS & Ove Arup and Partners (1997) 'UK continental shelf seismic hazard'. Health and Safety Executive Offshore Technology Report OTH 93 416, HMSO, London.

NORSAR & NGI (1998) 'Seismic zonation for Norway'. Report for the Norwegian Council for Building Standardization.

EQE International Ltd. (2002) 'Seismic hazard œ UK continental shelf. Health and Safety Executive Offshore Technology Report 2002/005.

The seismic hazard results provided by each of these studies, which are presented solely in the form of peak ground acceleration contour maps for certain annual probabilities of exceedance, and for various differing parts of the UKCS area, are taken and re-plotted to a uniform format for direct comparison and appraisal.

The strengths and weaknesses of the individual reports are discussed in depth, and a specially-devised scheme is used to weight their various results impartially, according to their respective merits. The results are then combined to generate a set of hybrid seismic hazard maps, representing an appropriate amalgamation of the information that is provided by the four separate studies. For seismic hazard at the 10-4 p.a. probability of exceedance, the hybrid contour map of offshore peak horizontal ground acceleration (pga) is compared with corresponding pga hazard levels determined in site-specific studies at a number of coastal sites around Britain.

The reliability of the 10-4 p.a. probability of exceedance pga hybrid contour map is assessed by comparison with a provisional map of ”minimal‘ pga hazard levels, determined independently, and specifically for this appraisal, from generic catalogue completeness

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considerations. Difference contours between this minimal hazard map and the levels of hazard indicated by the hybrid map are plotted to illustrate variations across the expanse of offshore waters around the British Isles. The two maps are then combined to form a newly derived 10-4 p.a. pga hazard contour map for areas of the offshore UKCS east of longitude 10˘W (the hazard levels plotted being the higher of those indicated at each point on either map).

The spatial differences between the hazard levels indicated by this combination map and by the results in the most recent of the existing reports (EQE International Ltd., 2002) are then explored. Whilst these differences are modest, and explicable, there are indications of systematic divergences in different parts of the UKCS area. A final composite map is constructed, therefore, showing the pga contours that are obtained by selecting the higher of these alternative hazard estimates at each point on the map.

Thus, this final composite map provides a transparently-derived estimate of seismic hazard levels which takes appropriate account of the different opinions that have been expressed in the existing hazard studies and includes precautionary adjustments in areas with no known earthquakes.

It is recognised that the other limitations in the existing reports are likely to have had some effects on the reliability of this reconciliation of their results, which cannot be resolved within the scope of the present appraisal. Nevertheless, for the purpose of providing guidance on seismic hazard levels in UK offshore areas, it is judged that the final composite pga hazard map derived here will suffice until a more authoritative study, providing a robust, coherent and uniform assessment across the whole UKCS area, can be undertaken.

A significant deficiency in the information currently available to HSE OSD arises from the fact that the existing reports provide little or no information on low frequency ground motion hazard exposure and its variability. As it is precisely these lower frequencies that are of most concern in the design of offshore installations, recommendations are made as to how simple provisional estimates of the 10-4 p.a. probability of exceedance ground motion at 1Hz could be made using different amplification ratios, depending on whether the site concerned is hard or soft. For the present, any such estimates could only be regarded as provisional as several technical elements of the problem need more detailed consideration before improved regulatory guidance for decision-making can be fully developed.

Consideration has also been given to the spectral shapes that should be used to characterise the earthquake ground motion across the offshore UKCS, stressing the need in defining such characterisations to distinguish between hazard spectra and design spectra. In comparison with the other available alternatives, the spectral shape recommended in the NORSAR & NGI (1998) and EQE International Ltd. (2002) reports is considered a not unreasonable characterisation of the ground motion hazard for hard sites in areas close to the British Isles. This shape is likely to be rather less satisfactory, however, for more distant localities in the far west of the UKCS. Whether or not this same spectral shape should be adopted as a design spectrum for hard sites is a matter of policy: to-date, most authorities have preferred to employ a standard piecewise-linear spectrum for this purpose. The proposals made in these same two reports concerning the treatment of soft sites is one that utilises an approach which relates only to a ground motion characterisation intended as a design input (these reports, therefore, do not touch on the ground motion hazard characterisation for such sites). However, there are many issues that would have to be resolved before it could be recommended that this proposal could be accepted as a standard design spectrum. One major factor influencing policy decisions on spectral shapes is the existence of the recent draft

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document produced by ISO which deals precisely with this topic: on a purely technical level, for much of the UKCS the shape recommended in the current draft would appear to be somewhat over-cautious at frequencies below about 0.4Hz.

In the related matter of selecting corresponding time histories appropriate for dynamic analysis, the existing reports are found to offer only limited assistance: several technical questions would need to be developed and resolved before an adequate generic approach could be formulated for representing conditions across the whole area of the UKCS.

With regard to vertical ground motions, only very limited information is provided in the four existing reports: a cautious position, for the time being, would be to accept the recommendations made in the NORSAR & NGI (1998) report. In fact, this is a topic which, generally, has received insufficient attention in recent times and, whilst it seems possible that the Norwegian proposal could be over-conservative, further work would be necessary to resolve whether or not this is the case.

As a final general suggestion to HSE OSD, having completed the appraisal, it is concluded that a site-specific analysis of its exposure to earthquake ground motion hazard should probably be undertaken in the case of any particularly sensitive location or special structure.

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1 INTRODUCTION

1.1 BACKGROUND AND OBJECTIVES

Over the last twenty years or so a large number of reports have been produced providing probabilistic estimates of the hazard due to earthquake ground motion across and around Britain. Even allowing for the fact that the concern here is just with the offshore areas of the UK Continental Shelf, there have still been several such studies, emanating from sources in the UK and elsewhere, and they have produced differing results. Because of the nature of probabilistic seismic hazard assessments (see Section 2 below), the existence of several sets of results makes complex the provision of definitive guidance either for the satisfactory design of new safety-critical facilities or for the carrying out of proper safety reviews of existing facilities.

At the request of the Offshore Safety Division of HSE [HSE OSD], an appraisal is here presented of four of the most significant and recent hazard assessments which deal with the offshore areas. The objective is to provide recommendations as to the position that should currently be adopted for passing on, with confidence, to users: in short, HSE OSD wish to know how best use can now be made of the results of the four studies in the light of current and developing engineering practice.

Clearly, in making such recommendations, there are many issues that have to be considered: for example, the following questions were posited in the original proposal for this project:

In terms of hazard levels

· Which report is best founded - how do the reports compare, one with another -which results to use?

· How robust are the general hazard levels particularly against the occurrence of new earthquakes?

· How do the hazard results compare with other seismic hazard maps, existing or in the pipeline (e.g. CEN, ISO)?

· How robust are the low hazard levels assigned to areas of sparse earthquake information? In such areas, are there any geological factors not fully accounted for?

· What values of hazard should be used for sites in the locality of the 1931 North Sea earthquake?

· How do the hazard contours compare with existing site-specific hazard results on land?

· What is the nature of any conservatism in the hazard modelling?

· What are the features of hazard modelling and contouring software that need to be understood in appraising the relative merits of the different reports?

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In terms of seismic actions:

· What hazard exceedance levels are appropriate for offshore structures taking account of existing and possible future regulatory stipulations or guidelines and target reliabilities?

· How suitable are response spectra (piecewise-linear or URS), especially in the frequencies most relevant to offshore facilities, with regard to: (i) the design of new facilities, and (ii) safety reviews of existing facilities?

· What should be the role of artificial and real time histories and how should they be chosen?

In addressing those questions on this list which remain relevant, along with the many others which arise, one factor which clearly has to be taken into account is the fact that there now exists a draft International Standard on the topic, namely ISO/CD 19901-2 (Petroleum and natural gas industries - Specific requirements for offshore structures - Part 2: Seismic design procedures and criteria). Whilst this document is not yet a formally-accepted International Standard, because of its potential significance it is mentioned wherever appropriate in the discussion which follows, using the shorthand nomenclature —ISO (2001)“. Also, it may be noted that another, potentially relevant, standard has recently been published for Norway, namely NORSOK Standard N-003 (Actions and actions effects, Rev. 1, dated February 1999).

Finally, it is understood that one factor motivating the need for the present appraisal was the occurrence of a significant earthquake in the central North Sea which post-dated all the hazard studies available to HSE OSD. This event, on 7 May 2001, had a local magnitude of 4.2ML and was located in an area where, previously, there had been little recorded seismicity. By its very occurrence, this earthquake provides a focus for some of the general issues that need to be considered in this appraisal. For example:

· What is the effect on calculated hazard levels of such a ”new‘ event, particularly in an area where there have not been many recorded earthquakes?

· Given this evidence of an event in such a remote location, what is the likelihood that similar events might have occurred in the past without being recorded?

Essentially, both of these issues test the security of existing hazard estimates against an earthquake that might occur tomorrow. Involved in the first issue is the decision as to whether the event can be treated straightforwardly as a zonal earthquake, or whether it should be taken to be a manifestation of fault-specific seismicity. The second issue highlights the importance of taking proper account of recording thresholds when making an interpretation of activity rates.

When the available information is scrutinized in detail, it is found that the 2001/05/07 event raises yet another profoundly important issue, namely the potential implications for safety in the offshore UKCS of induced seismicity œ that is, ”provoked‘ earthquakes, which are explicitly caused by human activity, or ”triggered‘ earthquakes, which are brought forward in time by human intervention. However, as is noted in Section 1.2 below, this is a topic that is outwith the scope of the present appraisal.

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1.2 SCOPE

As has already been explained, this appraisal is concerned with making the most appropriate use of existing estimates of hazard exposure, it having been agreed by all parties that the computation of yet another set of estimates would not be appropriate, or possible, within the scope of the present project. (This said, it has been found necessary to carry out some new independent calculations of minimal hazard levels.)

It follows, therefore, that the scope of this appraisal is unavoidably constrained by the scopes of the existing hazard assessments and the fact that those scopes vary quite markedly introduces further complexities (and constraints). The assessments that have been made available to this study by HSE OSD are identified in the following list, along with the numbering system that has here been adopted as the simplest means of referring to them:

Report 1: Principia Mechanica Ltd. (1986) 'North Sea seismicity'. UK Dept. of Energy Offshore Technology Report No. OTH 86 219, HMSO, London.

Report 2: BGS & Ove Arup and Partners (1997) 'UK continental shelf seismic hazard'. Health and Safety Executive Offshore Technology Report OTH 93 416, HMSO, London.

Report 3: NORSAR & NGI (1998) 'Seismic zonation for Norway'. Report for the Norwegian Council for Building Standardization.

Report 4: EQE International Ltd. (2002) 'Seismic hazard œ UK continental shelf. Health and Safety Executive Offshore Technology Report 2002/005.

In terms of the variations in areal coverage, although the present appraisal is required to address the whole of the offshore UK Continental Shelf (UKCS), only one of the reports on which it is based (Report 2) set out to cover all of that area. (In fact, the Designated Area of the UKCS has locally been marginally extended by statutory additions since Report 2 was written.) Report 1 considers only the UK sector of the North Sea, Report 3 covers only Norway and the surrounding seas, and the results presented in Report 4 only extend westwards out to 11ºW.

Inevitably, this situation results in the emphasis in the discussion that follows being on the UK sector of the North Sea, where comparisons can be made between at least three of the existing reports. Although it is understood that the area that is of most interest to HSE OSD extends much further to the west (out to about 14ºW), there are, unfortunately, severe constraints on what can be said about the extremities of that area because of the lesser quantity of information provided by the existing studies. This said, some of the comments that are made below in the context of the North Sea relate to methodological matters and, as such, have generic applicability.

For the area even further to the west, i.e. out towards the mid-Atlantic spreading ridge, special technical considerations apply (see Section 3 below) and this circumstance, coupled with the very limited amount of available information (which is provided by just one of the existing reports), means that only brief comment is possible within the scope of the present document.

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As agreed with HSE OSD, this appraisal concentrates on the single topic of earthquake ground motion hazard, i.e. the probabilistic exceedance levels of various intensities of ground motion, under free-field conditions, at the sea bed. Therefore, like the reports on which it is based, there is no discussion here of the separate but related hazard due to ground rupture (or surface faulting, as it is sometimes called). In similar vein, no special consideration is given here to the potentially significant issue of induced seismicity, whether triggered or provoked, which can complicate hazard assessments and which, as has already been noted, may well have been manifest in the North Sea by the recent magnitude 4.2ML (5.0MT) Ekofisk event of 2001/05/07 (Braunmiller et al. 2001).

Because the concern here is with hazard, no attempt is made to comment in any detail on the site response analyses that appear in some of the existing studies. (Where appropriate, such analyses can be used to assist in defining design basis earthquake ground motions: they do not, however, form part of a hazard assessment, see Section 5 below.)

In connection with the earthquake ground motion hazard estimates provided by the four reports, it is important to note that limitations are introduced by the fact that, in all cases, these estimates are presented only in graphical contoured form: nowhere are the actual calculated numerical values reported.

Finally, it is necessary to define the range of probabilistic ground motion parameters which are of most interest to HSE OSD and which, therefore, need to be discussed in this appraisal:

o in terms of probabilities of exceedance, it was confirmed at the meeting at Rose Court on 31 August 2002 that HSE OSD are primarily concerned with hazard levels in the range 10-2 to 10-4 p.a., and

o in terms of the frequencies of interest (over and above the usual concern with peak ground acceleration), at the same meeting, it was agreed that, although different frequencies would, inevitably, be of concern for different structures and different modes of vibrational behaviour, it would be sensible to take 1Hz as representing simplistically the ground motion of most engineering concern for the majority of offshore structures.

(In this last context, it may be noted that 1Hz is also used as a 'default' frequency in the draft ISO document referred to in Section 1.1 above.)

1.3 APPROACH ADOPTED FOR CONDUCTING THIS APPRAISAL

1.3.1 Personnel

This appraisal has been carried out by:

B O Skipp (Independent consultant);W P Aspinall (Aspinall and Associates), and D J Mallard (The Mallard Partnership),

all of whom are members of the Seismic Hazard Working Party (SHWP).

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The SHWP is a multi-disciplinary group of specialists set up in 1982 by the CEGB for the specific purpose of, first, developing and, then, applying seismic hazard assessment methodologies which are in keeping with state-of-the-art scientific advances and recognised international practices but which are tailored to the particular setting of the UK.

1.3.2 Methodology

It is important to emphasise the fact that it was never the intention that the present study should conduct and present a Peer Review of each of the four reports that have been supplied by HSE OSD. However, in order to come to the required outcome, it is inevitable that some judgements have had to be made as to the relative strengths and weaknesses of those reports. As is commonly the case, these judgements have been guided by the use of various 'tests' which, in this instance, focus on the significant stages involved in the process of hazard assessment, namely:

· the database that has been used

· the methodologies employed for treating all the components of that database

· the seismotectonic synthesis of the treated database

· the methodologies employed in constructing the seismic source model(s)

· the parameterisation(s) of the seismic sources

· the selection of appropriate strong motion attenuation relation(s)

· the program(s) and algorithm(s) used to compute the hazard exposure in terms of horizontal peak ground acceleration (pga)

· where appropriate, the methodology employed to obtain an expression of the hazard exposure in terms of the ordinates of uniform risk spectra (URS)

Clearly, in many cases, these matters can be effectively and simply judged by comparison with established practices (as set down in guidelines and the scientific literature, etc.). However, for reasons which relate to the ubiquitous importance to seismic hazard assessments of making expert judgements in the face of the many uncertainties that are involved, see Section 3 below, there are three additional considerations which have a bearing on any ratings which can properly be accorded to the likely reliability of such assessments. These are:

· the make-up, experience, and size of the assessment team;

· the methodology employed for making decisions, and

· the adequacy of the reporting of all the information and activities involved, including the decision-making process.

It is important that the size of the assessment team and the use, within that team, of expert judgement is in accord with good practice. Of necessity, expert judgement plays a significant

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part in these studies and, nowadays, both the making of such decisions and their reporting should respect established practices for the use of expert judgement, as exemplified, for example, in the principles laid down by Cooke (1988).

It is similarly essential that a user, or a reviewer (or any other reader), can easily follow: (a) what has been done, and (b) the flow of information, so as to be able to replicate all, or any part, of the process (in order, for example, to check calculations, etc.).

1.3.3 Procedures

The decision-making procedures developed by the SHWP over the last twenty years (many of which are described in Mallard et al., 1991) have been employed, wherever possible, in carrying out this project so as to arrive at a consensual view on important matters of judgement. Thus, whenever appropriate, the major decisions have been based on the open elicitation of expert judgements from the three specialists involved. (In larger SHWP projects, with bigger teams of specialists, the gatherings where such decisions are made are treated as Plenary Meetings, being defined as those meetings at which the expert judgements of all participants are formally elicited.)

For the record, four such meetings have been held during the course of the present study, as follows:

1) 12 July 2002 at the Winchcombe office of The Mallard Partnership;

2) 31 July 2002 at the Institution of Civil Engineers, Westminster;

3) 26 September 2002 at the Winchcombe office of The Mallard Partnership

4) 21 October 2002 at the Winchcombe office of The Mallard Partnership

1.4 STRUCTURE OF THIS REPORT

This report begins, in Section 2, with a brief summary, in the form of a list, of the generic shortcomings of the four reports on which this appraisal is based. Then Section 3 provides a short general introduction to some of the fundamentals involved in the practice of seismic hazard assessment. It is considered that these two sections need to be included because, between them, they explain why some of the actions taken in conducting this appraisal have been necessary and why some of the decisions and recommendations that follow have been made.

For the whole of the UKCS, Section 4 synthesises to the widest and most defensible extent possible (given the nature of the source material) the horizontal pga hazard levels derived in the four reports. The discussion presented in this section touches on the relevant detailed concerns that have been expressed by HSE OSD, including the special circumstances that obtain in the immediate vicinity of known large earthquakes, such as the 1931 North Sea event, and in areas where there is a lack of seismological data.

Section 5 discusses the merits or otherwise (as seen purely from the perspective of the strong motion seismology involved) of the various spectral shapes which appear currently to be in

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play for characterising the horizontal component of earthquake ground motion across most of the offshore UKCS, making the necessary distinction between hazard spectra and design spectra. Following on from this, Section 6 deals briefly with what can currently be said concerning vertical motion.

Section 7 suggests how pga hazard estimates, including those shown on all the maps presented in Section 4, could be used to quantify the 1Hz horizontal ground motion hazard. Still using just the existing information, this simple procedure would provide some measure, at least, of the likely range of variation in a hazard parameter which is rather important more meaningful for this application.

Finally, Section 8 summarises the findings of this appraisal, making recommendations where appropriate but also sounding warnings as to the qualifications that have to be placed on such outputs as have been presented. The need, in any future studies, for a proper treatment of the uncertainties involved in seismic hazard assessment is stressed. Attention is drawn to a number of outstanding policy and technical issues and suggestions are made as to how those issues might best be resolved. In this context, given the rather scant attention that seems, so far, to have been paid to: a) the most significant evidence provided by the available geological database (e.g. evidence of neotectonic fault movement), and b) the particular hazard-modelling issues that are raised by known major earthquakes, Section 8 discusses whether and where site-specific hazard assessments are desirable or necessary. Finally, mention is made of just some of the techniques which would allow the management by HSE OSD of seismic hazard issues to remain in step with current thinking.

References quoted in this report are listed in Section 9.

This report includes two appendices, as follows:

- For completeness, Appendix 1 lists those comments, pertaining to each of the individual reports, which have been noted, at one time or another, during the course of this appraisal and which seem to be worth recording. It should be noted, however, that these comments are selective (and, probably, partial) and certainly not exhaustive and that Appendix 1 does not correspond in any way to a formal Peer Review: the lists which appear there are no more than the accumulated jottings of individual members of the appraisal team.

- Because of the acute importance of strong motion attenuation in estimating seismic ground motion hazard, Appendix 2 records (again, in note form) selected salient details concerning the various attenuation relationships that have been adopted in the four reports for computing their hazard results.

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2 GENERIC ISSUES RAISED BY THE REPORTS CONSIDERED IN THIS APPRAISAL

The intention in this section is to summarise, in list form, those issues raised by one, or more, of the reports on which this study is based which affect the carrying out of the appraisal and, in some cases, the conclusions that can be reached. The methodological importance of a number of these issues is discussed in Section 3 where, for some of them, examples are provided of their potential significance for hazard estimates.

It may be noted that sets of selected comments relating to the individual reports are given in Appendix 1 although, as has already been said in Section 1, neither the material presented in that Appendix, nor, indeed, the list given here, should be construed as constituting a formal peer review of the four reports. In making the following comments, it is recognised that the four reports reflect evolving scientific and methodological advances and that they can only sensibly be judged in comparison with the accepted practices at the time they were written.

The generic issues which inevitably have had an influence on the present appraisal are now listed:

- none of the reports is entirely satisfactory in that, in one way or another, they all raise technical concerns;

- none of the reports is entirely satisfactory in that, in one way or another, they all fail to describe in sufficient detail what has actually been done (where insufficient detail is given of the process that has been followed, i.e. where there is a lack of transparency, this makes difficult the drawing of comparisons between the reports);

- the different reports use different attenuation relations: this means, in effect, that their precise definitions of ground motion hazard vary;

- all of the reports introduce unnecessary uncertainties, a propensity which is far from ideal, given all the inevitable uncertainties that are encountered in making seismic hazard assessments. Such situations arise, for example, where conversions are made between magnitude scales in Reports 2, 3 and 4 in order to use otherwise incompatible attenuation relations

- the different reports present hazard estimates at different annual exceedance probabilities, as shown on Table 2.1.

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Table 2.1 Summary of hazard levels assessed in the existing reports

pga hazard: Prob = 1 x 10-4

p.a.

Prob = 1 x 10-3

p.a.

Prob = 2.1 x 10-3

p.a.

Prob = 5 x 10-3

p.a.

Prob = 1 x 10-2

p.a.

Prob = 2 x 10-2

p.a.

Report 1 [PML 1986] Yes No No No Yes Yes

Report 2 [BGS/Arup 1997] Yes Yes No No Yes No

Report 3 [NORSAR/NGI Yes Yes Yes No Yes No 1998]

Report 4 [EQE 2002] Yes Yes Yes Yes Yes No

- in all of the reports, the sea bed is treated as though it corresponds to an onland free-field condition: this is not a criticism of what has been done, as there is no sensible alternative, but this circumstance could potentially have some effect on the hazard levels that are actually experienced, and none of the reports mentions this;

- in all of the reports, the actual presentation of the results is far from reader-friendly: in the worst cases, it is not easy even to discern where changes in hazard levels occur;

- none of the reports actually show the punctual results which emerge from their hazard calculations: this is a very significant failing because, depending on the technique used, the production of smoothed contours can have a marked effect on the apparent results through reducing the amplitude of both peaks and troughs (effectively, all local maximal and minimal values of hazard are hidden);

- none of the reports has mapped the hazard in terms of the ground motion at the periods that are likely to be of greatest engineering concern for offshore structures although, admittedly, this might have made for a lot more work as, unlike the pga, the motion at such lower frequencies can be markedly affected by local soil conditions;

- none of the reports seems adequately to have dealt with the problem of the influence on estimated hazard levels of circumstances where the possibility of recording earthquakes is very limited, e.g. through a lack of instrumental coverage, etc.;

- none of the reports includes adequate discussion of the robustness of its results. The methodologies used in Reports 1 and 2 preclude any discussion of the confidence levels associated with their results. This is not the case with Reports 3 and 4 but, whereas Report 3 includes some information on the variability of its results with confidence level, there is no such discussion in Report 4, where even the confidence level associated with the results presented is not actually stated.

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- none of the reports provides sufficient discussion of the uncertainties involved. It would have been interesting and instructive if, for example, Reports 3 and 4 given some indication of the differences in hazard levels predicted individually by the two alternative sets of zonal seismic sources they have allowed for;

- none of the reports explores the stability of its results, even to the extent of considering the effects of the occurrence of a single new earthquake;

- in the reports which calculate hazard levels for those areas of the UK Continental Shelf where there is likely to be some influence from the mid-Atlantic spreading ridge, insufficient attention appears to have been paid to the markedly different style of seismogenesis which prevails there;

- in all of the reports, the treatment and incorporation of geological, geophysical and tectonic information is, generally, rather poor: properly used, such data can often be informative and, under certain circumstances, can be crucial;

- only one of the reports (Report 3) deals with vertical motion;

- none of the reports deals with the hazard associated with induced seismicity.

For lists of the more detailed shortcomings of the individual reports that have been noticed in the course of making this appraisal, reference should be made to Appendix 1.

For the record, it may be noted that a synopsis of some of the material contained in Reports 3 and 4 has been published separately by Bungum et al. (2000). As that paper does not formally come within the scope of this appraisal, it has not been examined critically, or in detail, in pursuit of additional information not appearing in the reports themselves. On the face of it, however, there is no substantive additional material in the paper that bears on the present discussion.

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3 SOME FUNDAMENTAL ASPECTS OF THE PRACTICE OF SEISMIC HAZARD ASSESSMENT

Like Section 2, the material presented in this section is included because it explains some of the difficulties and constraints involved in making the present appraisal. Whereas Section 2 identifies the particular issues that arise because of information that is, or is not, contained in the four reports, the intention here is to put such issues into context by discussing briefly the relevant fundamental principles that need to be employed in the proper carrying out of seismic hazard assessments.

N.B. this discussion deals only with those aspects of the methodology that are of immediate concern to the present appraisal: there are many other important principles and aspects (including, for example, the distinction between ”natural‘ and induced earthquakes) which are not mentioned.

3.1 GENERAL MATTERS

All earthquake hazard assessments are made for a particular purpose with a particular state of knowledge and this status must be always be recognised. In the course of each assessment it is necessary continually to review the balance that has been struck between the uncertainties which inevitably exist and the degree of conservatism that has been allowed for. For any given type of project, it is important that the judgements made in this regard are consistent (e.g. from one region to another, or from one site to another) and that those judgements are consistently secure against the future arrival of additional data (e.g. the occurrence of a new earthquake).

The first point to be made in this brief discussion concerns the need to distinguish between probability and frequency of occurrence. Whilst the recurrence of events too rare for statistics to be accumulated cannot be enumerated in terms of annual frequencies, using Bayesian methods, it is perfectly reasonable and proper to evaluate their annual probabilities of occurrence.

In many seismic hazard assessments, the ground motion of interest is referred to having a —frequency“ of exceedance or a —return period“ of so many years. This is unfortunate as such terms should, preferably, be reserved for situations where the results really are objective frequencies based on observation: in the present context they give a wrong and misleading impression.

As Allen (1995) says:

—Whether we like to admit it or not, earthquake hazard assessment is essentially a predictive effort, with all the frailties and uncertainties of any predictive science.”

Thus, the results of an earthquake ground motion hazard assessment are no more, and no less, than subjective probabilities. That is to say, they are degrees of belief based on a given state of knowledge (and, as such, philosophically and technically identical to the odds offered on a

11

horse race by a bookmaker). The all-important distinction is that probabilities may change with our state of knowledge whereas frequencies do not.

Even where there are no project-related constraints, the inevitable gaps or shortcomings in the database, where there is little or no scientific evidence, ensure that the simplistic concept of making an objective seismic hazard assessment, i.e. one with no subjectivity, is likely to be impossible.

Where a decision has to be made on the basis of uncertain evidence, the only practical recourse is to make use of expert judgement. As has been discussed by Mallard et al. (1991), in making those expert judgements, the specialists involved have to work with:

· the local database;

· the deficiencies in the local database, and

· an awareness of trends, patterns and relations established with much larger databases from much bigger areas or from other analogous parts of the world.

As well as being governed by the local seismotectonic environment and the quality of the available database, however, their judgements are also likely to be conditioned by the objectives they are attempting to meet. These objectives relate to the technical and cultural circumstances which surround the particular project they are dealing with, including:

· the appropriate annual exceedance probability for each seismic hazard

· the lifetimes of the facilities involved

· the cost and programme constraints on the project

· regulatory requirements

· public concern

There is nowadays widespread recognition of the importance of expert judgements in the practice of seismic hazard assessment (see, for example, IAEA [1991]) and a good deal of guidance is now available concerning their proper elicitation and reporting. (In the context of the present appraisal, for example, where expert judgements have been made but not properly reported, this makes comparison with other assessments increasingly fraught.)

Of the five central principles laid down by Cooke (1988) for the sanction of expert judgement in scientific or technical decision-making, namely: reproducibility; accountability; empirical content; neutrality, and fairness, the hardest to achieve is neutrality because, between them and to varying extents for different projects, the constraints and conditions listed above conspire to introduce some degree of conservatism into the judgements.

The difficulty raised for the present appraisal by the ubiquitous presence of expert judgement throughout the process of seismic hazard assessment is that the four reports each represent a wholesale accumulation of such judgements. (Even where a reviewer is faced with a single assessment, and the judgements made in that assessment have been properly elicited and reported, he may agree with some of those judgements but not with others.)

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Also, in the normal course of events, it can often be presumed that a later study is likely to be more reliable than an earlier one because of advances in databases, methods and understanding, and because it can take advantage of the correction of any errors that have been exposed in the meantime. Indeed, this is understood to be the position taken, thus far, by HSE OSD in relation to the four reports considered here.

However, in the case of seismic hazard assessments, the value of such advances might be perceived, in the mind of a third party, to have been undermined by the exercise of what he considers to be misguided, inappropriate, selective, faulty, or otherwise questionable, judgement(s).

One approach that can be used to shed light on the overall effects of the judgements that have been made is to consider how probabilistic hazard results, which appear from experience to be low, for example, compare with those that would be given by simple deterministic methods. While Mallard (1993) argues for always having a probabilistic perspective on hazard levels, it is equally important that probabilistic results should not appear nonsensically high or low given the local seismological circumstances.

3.2 MODELLING SEISMICITY

So far as is known, all the existing probabilistic hazard studies of the British region (or parts of the British region), and, certainly, all of the four reports considered here, have made the presumption that seismogenesis can be treated as a Poisson process, with all that that entails, namely:

- that there is independence between the events (so, for example, all aftershocks and foreshocks should be removed from the catalogue that is used);

- that there is stationarity in the process (i.e. there is time invariance of the mean occurrence rate and of the other moments of its distribution, e.g. variance), and

- that the possibility of simultaneous events is negligible.

Beyond this, most studies then presume that the numerical seismicity for each source can be represented by the simple Gutenberg - Richter equation, i.e. Log N = a - bM. This said, one of the reports considered here (Report 2) adopts a generalisation of this expression, due to Lomnitz-Adler and Lomnitz (1979).

The significance to the present appraisal of such presumptions is that there are some seismotectonic settings where these may well not be the most appropriate models to use and that one such setting could exist along the mid-Atlantic spreading ridge with its attendant structural features. Because none of the existing reports seems to have made any significant adjustment to its methodology to reflect this special source of seismicity, there is very little that can be said here about that westernmost part of the area of concern.

For many years, one of the problems with seismic hazard assessments has been that perceived hazard levels have continually been ratcheted upwards by the arrival of additional data or findings, not only locally (e.g. another earthquake or the discovery of a fault), but also as a result of new evidence from elsewhere. To counteract such problems, it is important

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nowadays to derive hazard estimates which, as far as possible, are secure against the arrival of any further relevant information. With this objective, it essential to bear in mind the fact that what is not known is just as important as what is known. This is a consideration which is particularly relevant and important to the present appraisal in the context of assessing appropriate activity rates for offshore areas where there are few, if any, known earthquakes.

As is noted in Section 3.5 below, the activity rate assigned to a seismic source directly affects local hazard levels and is, therefore, of primary significance. Clearly, the absence from an area of any known earthquakes means one of two things: either there have not been any earthquakes, or there has been no system sensitive enough to record their occurrence.

This quandary can be partially resolved by estimating the maximum size of local earthquake that could have escaped being reported in different periods. This, in turn, can indicate levels of magnitude above which the record should be consistent and complete. Such an approach to the problem of missing earthquakes requires that cognisance be taken of historical and instrumental factors similar to those considered with respect to those earthquakes, if any, that were reported.

In any hazard assessment, therefore, it is essential to determine locally valid thresholds for each historically or instrumentally coherent period, above which the retained earthquake record provides a ”complete‘ dataset. Whilst these 'magnitude completeness thresholds' are apparently simple combinations of magnitudes and dates, their importance should not be underestimated. Without reliable thresholds, it is not possible to quantify properly seismicity (both activity rates AND b-values depend on them). Also, the construction of zonal seismic source models needs to be founded on the locations of only those earthquakes that belong to complete datasets.

The principle which has to be borne in mind in assigning magnitude completeness thresholds - whether this is being done across a whole map for the purposes of constructing a zonation, or to a single zone for the purposes of calculating activity rates - is that those thresholds must reflect conditions in that part of the area of interest where the circumstances are least propitious for recording earthquakes. Reflecting this geographical dependence, it follows that, sometimes, different sets of completeness thresholds have to be used for different purposes within a single hazard assessment.

In practice, the setting of completeness thresholds is a complex issue. Although some practitioners favour an analytical approach for assessing catalogue completeness, such methods are undoubtedly questionable and the use of independent historiographic or instrumental reasoning is to be preferred, see, for example, Vere-Jones (1987).

When reliable magnitude completeness thresholds have been defined and the portion of an earthquake catalogue that is thereby identified as being complete has been separated out, it is possible to make unequivocal statements about what earthquakes have occurred. However, the complete earthquake dataset still represents only a sample of the manifestations of a geological process, operating on a geological time-scale, and, in order to make robust inferences as to the long-term average rate of occurrence of earthquakes, it is necessary to recognise the fact that the completeness thresholds also define what earthquakes have not occurred.

Reflecting the uncertainties involved, a good deal of work has been done on the most appropriate ways of modelling activity rates for attempting to judge the 10-4 p.a. probability of

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exceedance hazard over, say, the next 50 years faced with an earthquake record which may only be complete for 200 or 300 years at best.

If it is presumed that earthquake generation in a moderate seismicity environment is a Poisson process that can be described by a Gutenberg-Richter model (which, as is noted above, is the usual presumption), there are a whole range of possible long-term average activity rates which could have produced even that part of the known historical and instrumental earthquake record which is considered to be complete.

It is important to understand that the validity of the Poisson model assumption is not necessarily statistically undermined by variations about the average return period which are of order six or seven times up or down (at about the 5% significance level). That is to say, it would not be inconsistent with Poisson behaviour if an event has an average return period of 150 years yet does not occur in a single period of 1000 years, or if an event which has happened twice in 1500 years has an average return period of 10000 years.

Thus, the confidence limits which can be placed on apparent mean annual activity rates vary markedly with the number of events that have occurred. As an illustration of the variability in confidence levels:

Consider four situations each suggesting a mean annual activity rate of 0.05 for magnitude 4 events:

· For a zone with 50 magnitude 4 events observed in 1000 years, the 95% confidence limits for the average annual activity rate at magnitude 4 are about 0.035 and 0.063




The implication of this is that, for any given seismic hazard assessment, judgement, conditioned by the application (i.e. user requirements), has to be exercised to decide how much reliance is placed on the historical record. It may be perfectly reasonable to use the historical record directly when addressing high probabilities and short facility lifetimes (although, even in such circumstances, there may be problems). At the other extreme, for very low probabilities and very long facility lifetimes (e.g. for nuclear waste repositories), the historical earthquake record obviously becomes less and less important, and more and more reliance has to be placed on other, usually geological, information.

In the middle of this range are facilities where the requirement is for ground motions having probabilities of exceedance around 10-4 p.a. for facility lifetimes of about 50 years. For such projects, the objective in deriving activity rates is not simply to obtain the closest possible fit to the seismicity which has occurred in the past. When dealing with the uncertainties which

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attend such probabilities, it cannot be presumed that the parameters of the time-invariant Poisson process are fully revealed by the available earthquake catalogue: as has already been said, even that part of the historical record which is complete needs to be regarded as being only a sample of earthquake occurrence. Not to allow for the possibility that the historical record is insufficient to reveal all the possible fluctuations in earthquake occurrence is to ignore a fundamental aspect of the physics of seismogenicity that could well be significant at this level of probability of occurrence.

Where there are no earthquakes in the catalogue with magnitudes above the local completeness thresholds for long periods of history, this means, by definition, that none have occurred. Such negative evidence, combined with the numbers of known events whose magnitudes exceed the local completeness thresholds (which, thus, form the 'complete' dataset), can be used to calculate an activity rate distribution for a logic-tree formulation of the hazard model.

Based on what is known to have happened and what is known not to have happened, it is possible to calculate a range of possible long-term average activity rates which are all equally likely to have produced that part of the known historical record which is complete. These activity rates are derived, using the adopted values for the b-value and the maximum magnitude, on the premise that earthquake occurrence is a Poisson process (so that the expression which yields the relative likelihood of different values of activity rate has the form of the standard Gamma distribution probability density). The 'span' covered by the full range of possible individual activity rates is a function of both the length of the complete record and the number of events in the complete dataset. When the complete dataset contains a lot of events, the span is not so wide as it is when there are very few events or none at all. Thus, where relatively little is known about earthquake occurrence, as in areas far offshore, the span can cover as much as an eighty-fold variation in activity rate. Under such circumstances, any simplistic estimate of an overall average activity rate is likely to be extremely imprecise.

The contrast between a hazard result obtained using such a treatment and one which just uses the known earthquake record will obviously be much more marked where there is a wide span of potential activity rates because known events are rare as a consequence of limited perceptibility. The effect on hazard results is most clearly illustrated by considering the case of an area which has had so few historical earthquakes that there are effectively no events from which to calculate an activity rate, as in the following hypothetical case which could relate directly to areas of the North Sea:

- take a site in the middle of a large area (a radius of 100 km is assumed here for enumerating hazard levels) of presumed uniform seismicity;

- assume, for simplicity, a single focal depth of 10km, a b-value of 1.28, a maximum magnitude of 6.5Ms and that the Principia Mechanica Ltd. (1982) pga attenuation relation is valid;

- for these circumstances, Table 3.1 shows the calculated expected 10-4 p.a. probability of exceedance pga for a variety of scenarios where only thresholds or numbers of earthquakes change:

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Table 3-1 Generic examples of the dependence of pga hazard on earthquake catalogue completeness thresholds

Scenario Catalogue

completeness Thresholds

No. of earthquakes

above thresholds

Expected 10-4 p.a. pga

hazard


hazard


hazard

1 5Ms 1900-2000 0 22.7%g 11.1%g 3.9%g

2 5Ms 1900-1980 4.5Ms 1980-2000 0 19.7%g 9.0%g 3.0%g

3 5Ms 1900-1980 4.5Ms 1980-2000 1 23.9%g 11.9%g 4.3%g

4 5Ms 1900-1970; 4Ms 1970-2000 0 12.9%g 4.9%g 1.3%g

5 5Ms 1900-1970; 4Ms 1970-2000 1 16.3%g 6.9%g 2.0%g

6*

5Ms 1900-1970; 4Ms 1970-1990; 3.5Ms 1990-2000 *split b-value: b=1.28 for M$4; b= 0.8 for M<4

0 11.3%g 4.0%g 0.9%g

(N.B. comparison between Scenarios 2 and 3 or 4 and 5 on this table effectively provides some indication of the influence on hazard levels of the occurrence of a new earthquake)

Were the threshold magnitudes to be higher than those used in this example, or the completeness periods shorter, the calculated hazard levels in any particular case would be higher.

It may well be the case that similar considerations underpin the recommendation contained in IAEA (1991) that a minimum hazard of 0.1g should be allowed for anywhere in the world.

3.3 THE VALUE OF HAZARD MAPS AT LOW PROBABILITIES

Comparisons with site-specific studies shows that typical hazard maps for regions of modest seismicity, like the UK, can only give an approximate indication of the relative severity of ground motion at hazard levels of the order of 10-4 annual probability of exceedance.

This is because, in hazard mapping, the incorporation of uncertainties is only possible at an extremely coarse scale, incapable of representing the subtleties that can have a local influence on the hazard at such probability levels. Normally, the results from any site-specific study would be expected to be higher than those from a typical hazard map:

(a) sometimes, because of the need, from detailed local investigations, to include potentially active fault sources, and

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(b) more commonly, because unequivocal (and, therefore, universal) source zone boundaries are very rarely encountered in moderate seismicity regions with the result that the source boundaries in site-specific models are often artefacts chosen, usually with an element of conservatism, with respect to the particular position of the site in question.

Given their potential significance to the outcome of the present appraisal, these two circumstances are now briefly discussed.

3.3.1 Effect on hazard levels of potentially active faults

In regions of moderate seismicity, the identification of an active fault (or a fault that might be active) need not necessarily sharply increase the hazard at a nearby site. Even so, such a circumstance can have an appreciable effect on the hazard, depending on the distance between the site and the fault, and the effect may vary at different frequencies. Obviously, one of the factors which governs hazard sensitivity in this regard is the level of ambient hazard in the absence of the fault source.

Table 3.2 shows an example, taken from a real site-specific study, which illustrates the potential effect on calculated hazard levels of including a fault source. This example explores the effects on the preferred results, shown in bold italics, achieved by varying just the active-status of a nearby (2km away) fault and the likelihood that a significant local earthquake (4.4MS) was on that fault.

Table 3-2 An example of the influence of a nearby fault source and its active-status on pga hazard

Model

Active-status of fault

Earthquake on fault ?

Expected 10-4 p.a.

pga (%g)

No fault 0 0 20.3

With fault 0.8 0.5 26.2

With fault 0.8 0.8 0.8

0.75 0.25 0.1

28.8 23.4 21.5

With fault

1 0.5 0.2 0.1

0.5 0.5 0.5 0.5

27.5 24.1 21.8 21.1

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3.3.2 Importance of zonal boundaries

In all seismic hazard models, the boundaries between zones of uniform seismicity should ideally represent some change in the characteristics of seismicity, such as activity rate, depth distribution, focal mechanism, etc. However, these boundaries should not be perceived as being necessarily directed towards seismotectonic posterity. (In the case of site-specific studies, experience shows that they may need to varied from one site to another, even where those sites are fairly close together.) While there may be instances where hard and fast seismotectonic boundaries, such as changes in geological structure, earthquake focal mechanisms, stress state, etc., can be established and used in any hazard model, this will not often be the case. More commonly, boundaries are introduced to mark differences in the statistical character of earthquake occurrence.

This last consideration is of major importance since a zone is characterised primarily by the earthquakes it has produced and any proposed boundaries should be checked against the distribution of that part of the total earthquake population which, across the whole area, is 'complete', as discussed in Section 3.2 above. This requirement gives rise to the recognition that some boundaries may have to be drawn as constructs which do not mark any definite observed change in seismogenesis but, rather, correspond to changes in the extent of knowledge, e.g. an offshore boundary may be invoked to mark the extent of some instrumental recording threshold. Where appropriate, therefore, thresholds of completeness can be used to judge how far offshore boundaries ought to be.

With such principles, it is possible (albeit, with greater ease, in the case of site-specific seismic source models) to develop a zoning methodology which can be employed on a reasonably systematic basis and which does not depend on loose, unproven, or speculative geological controls. It is important that the drawing of zone boundaries in a hazard model is not based on arbitrary decisions.

In conventional practice, the inclusion of an earthquake in one zone means its exclusion from all other zones and what often seems to be forgotten, particularly in hazard mapping exercises, is the need for consistency throughout the process of defining zone boundaries. It is just as important that there is uniform seismicity with consistent seismological characteristics across zones where there are not many earthquakes as it is for zones where there are. Ideally, one should have a uniform degree of confidence that each and every zone in the whole model is a discrete seismotectonic entity. While such requirements can increasingly be relaxed with distance in the case of site-specific studies (leading to the acceptable notion of simplified 'background' zones which have little effect on calculated hazard levels), this is not the case with hazard maps where every zone has to be regarded as being as important as every other zone.

Two last points are worthy of mention in this very brief discussion of the overall significance of zone boundaries:

- whilst local pga hazard levels are most affected by the locations of the nearest zone boundaries, when source models are used to predict ground motion at lower frequencies, the significance of more distant boundaries may be heightened, and

- in many parts of the world, there will always be concern that even properly tested zonations based on the patterns revealed by moderate magnitude earthquakes may well give a false impression as to the likely whereabouts of the larger events

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which are so much rarer in the historical record, i.e. zonations, when we understand them better, may need effectively to be magnitude-dependent.

Amplifying a point which has already been made in Section 3.3 above, uncertainty concerning zone boundaries can give rise to special problems when the resulting zonation is going to be used for constructing hazard maps. Experience of site-specific studies shows that often the uncertainties arising from the available database, coupled with variations in expert opinion, are such that it is necessary to allow for several appropriately weighted alternative zonations, rather than a single set of zone boundaries. (The uncertainties which such alternative zonations usually need to represent relate to the precise location of a boundary, or to doubts as to whether there should be a boundary at all, or to both.). Whilst this situation can easily be handled by the logic-tree formulation of a site-specific hazard model, such complications are beyond practical resolution at the scale of the grid spacings that are typically used to compute regional hazard maps using conventional models. With all the possible combinations of zonation that may need to be taken into account, it is hard even to imagine a calculational route for computing the 10-4 p.a. probability of exceedance hazard at a sufficiently large number of locations to plot its variability as a continuous surface.

Nevertheless, even in the nuclear industry, probabilistic national hazard maps have been employed in Germany, Switzerland and the former Soviet Union. In the latter case, while the nature of the map was somewhat different, the seismic source zones in which the most significant earthquakes were presumed always to occur were rather small and based almost entirely on historical evidence with the result that the map had to be changed drastically in Armenia, as a result of the 1988 Spitak earthquake, and in the Crimea. Demonstrable frailty like this is always the potential fate of regional hazard maps which is why IAEA (1991), for example, calls for site-specific hazard assessments.

Away from the nuclear industry, some of the problems associated with boundary locations in hazard mapping are recognized in certain calculational programs (e.g. SEISRISK III) which are designed specifically for that application in that they allow simplistically for a predetermined fixed degree of positional uncertainty in those locations. A somewhat more sophisticated approach to the issue, however, is exemplified in two of the reports considered here (Reports 3 and 4) where a limited number of alternative wholesale zonations is allowed for.

3.4 THE ROLE OF GEOLOGICAL INFORMATION

It is self-evident that an understanding of the geology of the region of interest, and of the past and current crustal processes affecting that region, is a fundamental requirement for the construction of robust seismic hazard models. However, all too often, the way the geological database is interrogated for such purposes results in little more than over-simplistic and cavalier arguments in support of particular zonations or fault sources. In such circumstances, the link between the geological story and the seismic hazard model can be, at best, tenuous and far from transparent (except, of course, in highly active areas with well-mapped active faults).

For seismic hazard assessments, the main purpose in examining geological information is to see how the far longer geological record informs the modelling of seismogenesis, which, otherwise, has to be based only on the relatively very short historical and instrumental records. The primacy of geological information is amply demonstrated by the fact that, in

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many regions of the world, including the UK, the most reliable criteria of all for demonstrating whether a particular fault is, or is not, active depend on geological evidence.

Properly interrogated, geological information (along with several other types of non-seismological data) can assist in many aspects of hazard model construction, not just the obvious ones mentioned above, i.e. in defining the boundaries of area zone sources of seismicity and defining fault-specific sources of seismicity. For example, it may provide insights by recording evidence of the occurrence of pre-historic earthquakes, thereby affecting perceptions of activity rates or maximum magnitude values.

At the highest level, an understanding of the crust, its history of assemblage, past and current stress regimes and geodynamic processes, may provide information on the likely stress tensor at seismogenic depths, which, in turn, can suggest constraints on the potential for seismic action. By comparison with global data, such understanding may also, again, provide a pointer to the size of the likely maximum earthquake.

Finally, at a more local level, such information may provide hypotheses against which the pattern of actual known earthquake occurrence can be tested (to point up the role of basin boundary faults and sediment loading, for instance) and indicate the rate of deformation that may have obtained throughout the duration of the current tectonic regime.

Thus, there are many potentially significant geological factors that can help inform a thorough and comprehensive seismic hazard assessment.

3.5 THE EFFECTS ON HAZARD LEVELS OF VARYING MODEL PARAMETER VALUES

As well as the geometry and make-up of the seismic source model, estimates of hazard exposure are obviously affected by the parameters that are assigned to the various sources and by the strong ground motion attenuation relation(s) that are employed in the hazard modelling. It may be helpful, in concluding this discussion of some of the fundamentals involved in the practice of making such estimates, to list in summary form the effects that variations in the assigned parameters and selected attenuation relations can be expected to have on the results down to 10-4 p.a. probability of exceedance in typical British hazard assessments:

source parameters:

· increasing activity rate increases hazard

· increasing b-value reduces hazard

· increasing minimum magnitude reduces hazard

· increasing maximum magnitude produces very minor increase in pga hazard but somewhat greater increase in low frequency motion

· increasing focal depth reduces hazard

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attenuation relations:

· increasing attenuation reduces hazard

· increasing attenuation scatter increases hazard

3.6 CONCLUDING REMARKS

As has already been said, the material presented in this section has been included here because it is all, in one way or another, direct relevant to the issues that have had to be faced in making the current appraisal. In particular, the illustrative numerical examples that have been presented will be referred to in later sections of this report.

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4 SYNTHESIS OF PGA HAZARD MAPS

4.1 THE EXISTING HAZARD MAPS

4.1.1 Introduction

As has been noted, all four of the reports considered in this appraisal present their pga hazard estimates only in the form of mapped contour plots. Also, while Reports 3 and 4 adopt similar formats for plotting their estimates, the results from Reports 1 and 2 are each presented on alternative, and different, styles of mapping. Thus, between them, the four reports use three presentational formats for depicting their results.

Clearly, the first requirement in the process of making a synthesis of these results is to bring them all to a common basis. This task is somewhat diminished by the fact that, although Report 3 provides essential background to Report 4, in terms of actual hazard estimates, it deals exclusively with Norway and its adjacent sea areas and, thus, provides no input to this synthesis which is only concerned with hazard levels across the offshore areas of the UK Continental Shelf (UKCS).

Bringing the results from the three remaining reports to a common basis involves overcoming the following complications:

- the different areal coverages: whereas Report 2 provides pga hazard maps extending across almost the whole of the UKCS, Reports 4 and 1, respectively, cover increasingly limited parts of that area;

- the different mapping styles: Report 4 employs the Mercator projection whereas Reports 1 and 2 use variants of a Conic projection;

- the different mapping scales;

- the different annual exceedance probability levels at which the contour maps of pga hazard are drawn;

- the different pga hazard contouring intervals;

- the different degrees of graphical resolution at which the pga hazard is plotted, and

- the different units that are used for expressing the pga hazard (%g, decimal g, or m/s/s).

Given all these complications, it is not a straightforward matter to re-present the results of the three reports on a common basis: for the record, the procedure that has been adopted is now described in the rest of Section 4.1. Then, in Section 4.2, a description is given of the method by which a synthesis has been made of those results in the face of the complexities involved. Finally, Section 4.3 discusses this synthesis and its reliability.

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4.1.2 Rendering the existing hazard maps to a common format

The point has just been made that, whilst each of the three reports derives hazard exposure results at several annual probabilities of exceedance, they vary in selecting which ones to use. (The actual make-up of each suite of results is shown in Table 2.1.)

Fortunately, all three reports present results for the two annual exceedance probability levels, 1x10-2 p.a. and 1x10-4 p.a., which, as is noted in Section 1.2 above, mark the limits of the range of interest for the present appraisal. Therefore, the process of bringing the results to a common basis has here been pursued at these two probability levels, the consequence of this decision being that it is necessary to re-plot six maps in total (two from each of the three reports).

The first stage in this 'normalisation' procedure is the conversion of the reported hazard results, which are presented only in the form of maps, into digital data. By this means, they can then all be plotted in a suitable selected common format, and can then all be subjected to the various treatments necessary to meet the needs of this appraisal.

For this project, the digitising was done by initially superimposing scanned images of the individual report plots (in TIFF or JPEG format, depending on whether the original was a B&W line drawing or a colour plot, respectively) onto a bare base-map of the region containing the UKCS. The base-map was generated for this purpose using a computer graphics package with mapping capabilities, CoPlot 6.1. This overlaying process involved rotating, scaling and aligning the scanned images to get as exact a fit as possible to the corresponding latitude and longitude grids of the base map, before digital re-sampling could be undertaken.

In addition to being able to switch between one map projection and another within the same plot, the CoPlot program also allows the co-ordinates of individual points on a map display to be retrieved by selecting them with the mouse pointer. Thus, the geographic locations of points on the overlaid contour plots could be manually retrieved, generating a file of corresponding co-ordinates for selections from that particular plot. Although an exact calibration of the precision of this step was not attempted (because the quality and resolution of the original plots did not merit the additional processing), it is estimated that the match was everywhere better than about 0.03 degrees of latitude or longitude. Appropriate use of the zoom capability facilitated positioning the sample points to such a precision as was justified by the detail in the scanned plots and by the width of reproduced contour lines.

By sequentially clicking along each of the contour lines for the hazard levels represented on the plot, a string of samples was obtained which were, of necessity, irregularly spaced, and could only be taken at places where the hazard level was defined (strictly, the samples correspond to points on contours which mark a change from one range of values to the next range of values). The density of the manual sampling was modulated to capture as faithfully as possible changes in the curvature of the contours, with more samples being taken at places where deviation in direction was most rapid.

In the context of the subsequent numerical processing of the results obtained by this digitisation, it should be noted that complications arise in treating the terminations of contours where they disappear on land or beyond the boundaries of the area of a hazard map.

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(Potentially, such discontinuities can generate difficulties in maintaining good behaviour in any trend- or surface-fitting algorithm.) For the present exercise, it was deemed expedient to provide a few dummy values at such localities, taking advantage of other sources of information concerning hazard trends in and around the British Isles. In this way, it was ensured that the fitting did not result in nonsensical hazard levels for these peripheral locations.

Another factor that had to be taken into account, if proper comparisons were to be made between the different sets of results, was their conversion to a common unit of measurement. As noted above, in defining the hazard, some reports use different units for pga, and all employ different contour intervals for displaying their results. For the present appraisal, so that comparisons can conveniently be made with other studies, pga is described in terms of decimals of g.

This decision requires that some of the reported contours (for instance, those defining steps expressed in terms of m/s/s) have to be redrawn to mark the locations they would have occupied had the original display shown hazard intervals expressed in terms of decimals of g. In order that these revised contours could be correctly located relative to the digitised samples derived from the original contour plots, some positional interpolations were necessary.

Then, because the point samples used in digitising each set of original results were, inevitably, irregularly spaced, further interpolations were necessary so as to have values on a uniform spatial grid, both for re-mapping (as a necessary check to confirm fidelity to the originals), and for ease of comparison and manipulation.

To meet both these demands, a powerful algorithm, due to Inoue (1986), for the least-squares smooth fitting of irregularly-spaced data using two-dimensional cubic B-splines was chosen. Inoue‘s scheme, which is best explained by drawing a simple physical analogy (see below), determines a set of optimum smoothing functions for this type of surface-fitting problem. Optimal parameterisations of these functions are determined by jointly minimising the l2

norm of data residuals (i.e. in the least squares sense), and the first and second spatial derivatives of the data: these three are, respectively, measures of the total misfit, fluctuation, and roughness of the functions concerned. In this method, the requisite functions are represented by cubic B-spline expansions with equi-spaced knots: other formulations are possible, but the cubic spline is a well-established treatment and, as far as is known, no compelling arguments exist for preferring any alternative functional form. Adding the extra condition of minimizing the derivative norms stabilizes the linear equation system for solving for the spline function expansion coefficients in the basic misfit problem.

This approach can be regarded in three ways:

- from a mathematical viewpoint, as has just been noted, it provides stabilization of the spline-fitting problem in two dimensions and in the presence of noise in the data: this is a unique and important capability that can be used to advantage in the present appraisal (where it was always likely that it would be necessary to assign different weights to the different sets of results);

- from a stochastic perspective, it is equivalent to solving for the maximum-likelihood estimate among admissible functions, making a priori the assumption that the first and second derivatives are zero everywhere due to random errors, and

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- in terms of a physical analogy, it can be viewed as a finite-element approximation to the flexure of a plate with internal tension, acted on by external forces (the external ”forces‘ on the surface or ”plate‘ represent the pulls exerted by the data points acting through ”spring‘ connections whose stiffnesses are proportional to the uncertainties associated with each value).

In any application, the Inuoe algorithm requires appropriate values to be chosen for two, dimensionless, modelling parameters which jointly control the ”roughness‘ of the fitted function (i.e. how the response of the surface to perturbations is damped at short wavelengths), and the ”tension‘ within the functional surface (i.e. the elastic ”tightness‘ that determines the curvilinear behaviour of the function between data points). In the context of the physical analogy described above, these parameters equate to constraints which determine the degree of balance between the potential energy in the springs and the strain energy in the plate. Thus, by carefully selecting values for these parameters, the variational smoothing in the spline-fitting process can be controlled.

In practice, an empirical balance can be struck between minimising the l2 norm misfit to the data samples on the one hand (the total misfit reduces asymptotically to an absolute minimum by ever-increasing tension in the spline), and the retention of continuous, realistic curvilinear behaviour between and through data points (in both dimensions simultaneously) on the other. In the present case, this balance is achieved by a process of progressive adjustment - that is, by varying both parameter values while requiring that the resulting fitted functions replicate exactly the locational characteristics of the contours of the given dataset and, at the same time, that spurious features or overshoots were not introduced. In particular, it was judged important that the fitting procedure did not introduce any contours higher or lower than those depicted in the original plots.

The values that were assigned to these parameters were then used unchanged for the processing of all six datasets involved in the synthesis. Whilst, to some extent, these values were arbitrary selections, the fact that all the datasets involved similar numbers of sample points, with generally similar spatial distributions, encouraged the view that holding the fitting parameters constant represented the most obvious way of achieving a uniform and coherent treatment.

The other parameter value that had to be selected for this spline-fitting exercise was the number of knots, on a uniform grid spacing, at which the function coefficients should be estimated. Whilst the spatial density of these knots determines the extent to which local variations are faithfully represented (at all scales), beyond a certain point, the use of additional knots provides no information gain, and incurs unnecessary computational overheads.

For the present purpose, the areas in which contoured useful information is provided by the three reports are completely contained within a rectangular area of 14 degrees of longitude (i.e. from 10ºW to 4ºE) by 16 degrees of latitude (i.e. from 48ºN to 64ºN): N.B. the issue of the pga hazard across the UKCS to the west of 10ºW has to be addressed separately, see below. Four knots per degree was found to be more than adequate for replicating accurately all the features in the original contour plots, with the tension and roughness parameters held fixed as described above. Indeed, it was found that one knot per degree would have provided results that were visually virtually indistinguishable from those generated with 4 knots per degree.

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For the record, in this exercise, the Inoue (1986) two-dimensional cubic B-spline fitting algorithm was implemented in FORTRAN code by one of the authors (WPA), compiled with a MS PowerStation v1.0a FL32 FORTRAN compiler, and run on a PC under MS-DOS. This program produced, as output, a file of the spline expansion coefficients for each knot at the 3,584 individual locations on the selected two-dimensional geographic grid of 56 by 64 points. A second FORTRAN program was then used to evaluate the corresponding smoothed function values at equi-spaced intervals with a density of 20 points per degree of latitude and longitude, thereby producing a file of (89,600) pga point estimates on a rectangular 14º x 16º geographic grid.

In this way, the results of the three individual reports were transformed into a uniform digital data format, allowing them to be re-plotted on a common map projection (the Conic projection being preferred) and to appropriate common scales.

4.1.3 Re-presentation of the existing hazard maps

For the whole area of the UKCS (which lies between the latitude extremes of 48.17ºN and 63.89ºN and the longitude extremes of 23.96ºW and 3.4ºE), the pga hazard estimates given in Reports 1, 2 and 4 at 1x10-2 p.a. and 1x10-4 p.a. probability of exceedance are now presented, in terms of decimals of g (using common scales and the Conic projection), on six maps, as follows:

Figure 1 shows the 10-2 p.a. pga hazard map from Report 1 Figure 2 shows the 10-4 p.a. pga hazard map from Report 1 Figures 3a and 3b show the 10-2 p.a. pga hazard map from Report 2 Figures 4a and 4b show the 10-4 p.a. pga hazard map from Report 2 Figure 5 shows the 10-2 p.a. pga hazard map from Report 3 Figure 6 shows the 10-4 p.a. pga hazard map from Report 3

In the interests of improved clarity and detail, and because only one of the reports (Report 2) provides any results which extend significantly further west than 10ºW, this longitude has been adopted as the western boundary for all but two of the figures listed above, i.e. with the exceptions of Figures 3b and 4b, all of the standard re-plotted pga hazard maps cover the area which lies between 48ºN and 64ºN and 10ºW and 4ºE.

The other two re-plotted maps (Figures 3b and 4b), both of which emanate from Report 2, show, at a reduced scale, the area further to the west, i.e. these figures show the area which lies between 48ºN and 64ºN and 0ºEW and 24ºW. This said, apart from very small localised areas, 10-2 p.a. pga hazard map presented in Report 2 indicates that the hazard at this probability level is everywhere less than 0.05g. This conclusion, therefore, provides the only information that can be shown on Figure 3b (and on Figure 3a).

4.2 MAKING A SYNTHESIS OF THE EXISTING HAZARD MAPS

4.2.1 Introduction

Before discussing the approach that has here been adopted for synthesising the re-plotted hazard maps from the three reports, two points need to be made:

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(i) thus far in this exercise, the boundaries of the hazard maps presented in the original reports have been retained. However, even the outermost of some of these boundaries do not correspond to the currently defined limits of the UKCS as supplied by HSE OSD to the present appraisal. Figure 7 shows all of the boundaries involved and, thus, indicates some of the complexities involved in synthesising the existing hazard maps. As Figure 7 shows, different parts of the UKCS can be covered by one, two or all three of the existing hazard maps (and there are some small peripheral areas that, strictly, are not covered at all), and

(ii) the re-plotted hazard maps presented in Section 4.1.3 above are all predicated on the assumption that each of the three reports is dealing with precisely the same parameter when they present their pga hazard results. Whilst, within the scope of the present appraisal, no other assumption is possible, this is, in fact, a simplification of the true situation. For the future, it should be noted that there are actually subtle differences in this regard between the three reports, as a consequence of the fact that the different reports use different attenuation relations. (All three use 'imported' attenuation relations or conversions based on various datasets of strong motion recordings that have been selected and treated by third parties.) To assist in comprehending the complexities involved in this issue, for each report, Appendix 2 lists some of the salient facts concerning the different attenuation relations that have been used and the treatments that have been applied to them.

In determining how the pga hazard results from the three reports should be synthesised, the decision has here been made to concentrate on the 10-4 p.a. probability of exceedance figures as these are the ones where the differences between the methodologies used and the assumptions made are most apparent.

4.2.2 The options available

Faced with three sets of results for the 10-4 p.a. pga which cover different but overlapping areas of the UKCS, a choice has to be made as to how those results should best be synthesised. So far as is known, there is no accepted practice for making such a choice, nor is any guidance provided by any of the usual authorities or institutions. In fact, there are many options that are available and many philosophies that could be employed, including:

- to accept exclusively, and in their entirety, the results from the study which covers the largest part of the area of interest (i.e. Report 2);

- to rely solely on the most recent of the studies (Report 4) across its smaller area of coverage and revert to Report 2 for the outer areas, remembering that the earliest report (Report 1) covers the smallest area of all, or

- to switch between the reports, locally adopting the most pessimistic (or, depending on one‘s stance, the most optimistic) of the three sets of results.

In one way or another, each of these options uses the existing results without modification. Alternatively, as the first option is undoubtedly attractive because of its many practical advantages, it might be considered that the best thing would be simply to modify its results in some fashion by applying an appropriate multiplier across the whole map. (In making such choices, it is always sensible to avoid, if possible, options which lead to practical difficulties in drawing contours across the boundaries between different source maps.)

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One of the points to be considered in this debate is the extent of the differences between the three sets of results. Indeed, before selecting any particular approach, it might be argued, almost as a fundamental principle, that checks should first be made as to the meaningfulness of the differences between the various results. Even accepting the pragmatism that is apparently embodied in such an approach, it has here been decided to eschew any attempt at exploring and analysing the magnitudes of the differences between the three sets of results. There are two reasons for coming to this decision:

- on grounds of practicality: given the scope of the present appraisal, it is not possible to embark on time-consuming analyses which are unlikely to affect subsequent decisions: simple visual comparison of Figures 2, 4a and 6 shows: (a) that there can be quite large differences between the hazard exposure at particular points as estimated in the different reports, and (b) that a proper statistical analysis of these differences would be a considerable undertaking, and

- on the basis of the principles involved in hazard assessments: it is important (and especially so in the case of hazard mapping studies) that judgements as to the reliability of such studies are made, not just on the results, but also on the whole process by which those results were derived.

Given this decision, it now becomes necessary to decide how the 10-4 p.a. probability of exceedance pga hazard results should most appropriately be merged into a robust, yet not over-conservative, synthesis. In selecting or developing a methodology, it has to be remembered that none of the reports is regarded as being entirely satisfactory and that, whilst the westernmost part of the area of interest is covered by only one report, the North Sea area is covered by all three.

With the results re-formatted onto a common grid, in theory, it would be possible to come to a different decision at each grid point by treating the results (from one, two, or three reports, as appropriate) as alternatives that are weighted in accordance with some perceived likelihood that they are the correct value for that location.

Such a major undertaking is clearly outside the scope of the present appraisal. Nevertheless, it has been decided that the preferred route is still to mimic the logic-tree approach, but in a much simpler way by taking each set of results in toto and treating it as an alternative 'value' which is assigned a single overall 'weight'.

By this means, all the generic factors that are considered to work in favour of, or against, the reliability of each set of results can be taken into account. What cannot be taken into account are local variations in reliability within each assessment arising from particular factors (such as the local zonation, the local integrity of the earthquake catalogue and/or its magnitude completeness thresholds, the local b-value(s), the local focal depth distribution(s), etc.). Thus, each study has to be appraised in an inclusive, total, sense, and rated accordingly.

4.2.3 Rating the existing hazard maps

In deciding on the single, overall, weight which should be assigned to each of the studies, a staged procedure has been used, as follows:

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· identify all the generic decisions which have an effect on the 10-4 p.a. probability of exceedance hazard results (N.B. not just the pga hazard)

· order those decisions in terms of hazard sensitivity at 10-4 p.a. probability of exceedance (again, having regard not just to the pga hazard)

· score each decision for each assessment against best practice (which, for consistency, is taken to be the practice currently used in the UK nuclear industry)

· produce an aggregate score for each assessment.

In the light of what has already been said concerning the use of expert judgement (Section 3.1 above) and hazard sensitivity (Section 3.5 above), the ordered list of primary factors involved in the decisions which have to be scored is as follows:

Factor (1) the attention that is paid, overall, and the treatment that is given to the uncertainties involved (as manifest in the results)

Factor (2) the zonation(s) that has been adopted, including the involvement, if any, of fault-specific sources of seismicity

Factor (3) the strong motion attenuation relation(s) that have been adopted, and their usage in this particular instance

Factor (4) the parameter values assigned to all the modelled sources

Factor (5) the attention that has been paid to relevant non-seismological data and the methodologies that have been used for treating such data

Factor (6) the calculational route that has been employed (i.e. the programs and algorithms that have been used)

Factor (7) the representation of the hazard results

This last point is of particular significance in this appraisal, where the hazard results are presented only in contoured graphical form, because, for example, the calculated result at a point could be much closer in value to an unmapped contour than it is to the nearest contour that happens to appear on the original map.

In some parts of the world, serious attention would also have to be paid to the reliability of the earthquake catalogue that has been used. Here, given all the research that has been carried out on NW European earthquakes, this ought not to be a problem. Although some concern has been expressed about the manipulations that have been made to the magnitudes assigned to the events in Reports 2 and 4, this probably does not have very serious overall implications for the hazard results. (In any case, without a good deal more work, it would be difficult to decide which report had committed the greater indiscretion in this regard.)

In making judgements on Factor (2), the pre-occupation is with the overall principles that have been employed to construct the seismic source model, rather than the resulting zonations themselves. (As has already been mentioned, the local details of a source model are likely to result in varying measures of approval being thought appropriate to different parts of that

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model.) In this case, a certain amount of additional judgement has had to be exercised as none of the reports under consideration gives an entirely comprehensive account of the principles they have employed (see Appendix 1).

In terms of the parameterisation of the sources (Factor [4] in the above list), the decisions which have to be assessed are shown in the following list, approximately in order of their descending importance for typical hazard studies:

· the minimum magnitude allowed for

· the method by which the activity rate(s) has been calculated

· the b-value(s) used

· the focal depth distribution(s) used

· the maximum magnitude(s) allowed for

In this particular case, however, the order in which the last two of these decisions appear probably needs to be reversed because, although focal depth often has a significant effect on hazard levels, two of the three reports under consideration here do not use focal depth in their calculational routines. Also, whilst maximum magnitude has very little effect on the 10-4 p.a. pga hazard, it does have some influence on the long period hazard at this same probability.

Therefore, the formal hierarchy within Factor (4) in the list of primary factors used here for rating the decisions made in the three reports is as follows:

Factor (4a) the minimum magnitude allowed for

Factor (4b) the method by which the activity rate(s) has been calculated

Factor (4c) the b-value(s) used

Factor (4d) the maximum magnitude(s) allowed for

Factor (4e) the focal depth distribution(s) used

One final, overall, consideration which could, in theory, play a major part in judging the adequacy of these assessments would be a comparison between their results (in total, or in part) and those from other, methodologically sound, hazard studies. Unfortunately, however, tests of this kind are unlikely to be fruitful in this instance. In later sections of this appraisal, mention is made of comparisons with the punctual results from onland site-specific studies but, as has been discussed in Section 3.3 above, where such comparisons are concerned only with the results involved, they are unlikely to be very informative (except, perhaps, in the unlikely instance that a mapped hazard level was found to be higher than one from a site-specific study).

Otherwise, there are not too many comparisons that can sensibly be made: it would be pointless, for example, to use the results of very early studies, for example Ove Arup and Partners (1980), for this purpose. Although both Report 2 and Report 4 have used seismic source models which cover the whole of the landmass of Britain, they do not show any onland

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hazard results, thus precluding any straightforward comparisons with the one or two recent onland hazard mapping studies that have been carried out.

Accepting this absence of useful comparisons, and notwithstanding the obvious frailties and over-simplifications introduced by breaking down the merits or otherwise of each study into the eleven factors introduced above (i.e. Factors [1] to [3] inclusive, Factors [4a] to [4e] inclusive and Factors [5] to [7] inclusive), the conclusion that has been reached is as follows:

compared with the preferred current practices for estimating the 10-4 p.a. probability of exceedance earthquake ground motion, the overall aggregate ratings assigned to each of the reports are:

Report 1: 48% Report 2: 61% Report 4: 77%

In fairness to the earlier reports, it should be stressed that a fair proportion of the margin by which they have apparently been down-rated is due to the development of more sophisticated methodologies in the time that has elapsed since they were drafted: Report 2, in particular, reveals an awareness of improvements that might be made. (It is also the case that allowances might be due, not just for the technical constraints of the time, but also for procedural or contractual constraints imposed on the projects.)

With these relative degrees of faith in the results of the three reports, weights can, consequently, be assigned in a logic-tree formulation for constructing a hybrid hazard map, as follows:

for areas where all three reports provide results, the weights are:

Report 1: 0.26 Report 2: 0.33 Report 4: 0.41

for areas where just two reports provide results, the weights are:

Report 2: 0.45 Report 4: 0.55

As has already been discussed, for the large area where only Report 2 provides results, those results have, faute de mieux, to be assigned a weight of 1.0.

N.B. strictly, this still leaves outstanding some very small peripheral areas of the UKCS which are not covered by any of the reports.

4.2.4 The hybrid hazard maps

The weights derived above can now be used, in their appropriate combinations, to compile a hybrid map of the whole of the offshore UKCS which reflects the judgement here placed on the likelihood that each of the individual reports has, overall, made an estimate of the 10-4 p.a. probability of exceedance pga hazard which is both reliable and defensible œ it would, of course, be wrong in this context to invoke any suggestion of a 'correct' estimate.

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As has been explained, for those areas where all three reports provide hazard results, the results are weighted in accordance with their individual aggregate scores: at the other extreme, where only one report provides an estimate, that result has to be given a full weight of unity.

The resulting hybrid hazard contour maps are shown in Figures 8 and 9 for the 10-2 and 10-4

p.a. probability of exceedance pga respectively for the area of the UKCS to the east of 10oW. Clearly, the most interesting part of these maps is the North Sea where three opinions on hazard levels are represented and it is instructive to compare Figure 9 with Figures 2, 4a and 6.

For the area to the west of 10oW, the hazard results remain, perforce, as shown on Figures 3b and 4b for the 10-2 and 10-4 p.a. probability of exceedance pga respectively.

4.3 DISCUSSION

4.3.1 Preamble

As an introduction to this general discussion on the reliability and usefulness of the hybrid hazard maps that have been produced, it is only proper to record the procedural shortcomings involved in their production even though it is felt that these represent only minor weaknesses

First, it is important to stress that the method by which the hybrid hazard maps have been derived is quite basic and does not allow, for example, any correction to be made of errors in any of the reports which could have had an effect on their results (such as the 1971/07/20 explosion, which appears in Report 4 as an earthquake).

Then, their construction could be criticised on the grounds that the ratings that have been assigned to the reports represent the views of just three specialists. Possibly, it might also be criticised because the factors that have influenced those ratings are biased towards issues which tend only to become significant at the lowest probability levels of interest. Also, by showing only smoothed contours, these maps, like the ones in the original reports, can be criticised for not revealing local maximal and minimal values of hazard.

Finally, some brief comment is necessary concerning the practical complications involved in drawing these maps and the unavoidable artefacts that have been introduced by that process:

- Inevitably, some difficulty is experienced in combining contoured results from a variety of sources when the behaviour of those contours is not defined beyond the arbitrary boundaries of the relevant study areas. Such difficulties become exacerbated where different studies produce divergent results at common boundaries or where an individual study has rapidly changing results close to its boundary. In the present case, the English Channel stands out in this regard (see Figure 9).

- Artefacts in the contours of the hybrid maps are manifest also in the Southwest Approaches, for instance, where Report 4 is limited to covering points north of about 49.3ºN, leaving only Report 2 to provide results further south. This

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circumstance results in artificial step-changes in the hazard levels shown on the hybrid maps that are solely a function of which datasets are available and where.

- Along the margins of the northwest parts of the mapped contour areas, the UKCS Designated Area, as it is currently defined, extends outside the coverage of any of the existing hazard studies. In those localities, the present exercise has had to take into account estimates of pga hazard that are derived from deliberations of what the minimal level of that hazard should be, given catalogue completeness and other technical considerations.

This last consideration is of major importance when contemplating the reliability of the hybrid hazard maps, and is a theme which is developed further in the discussion below where the focus of attention naturally rests on the 10-4 p.a. probability of exceedance pga hybrid hazard map (Figure 9), rather than on the 10-2 p.a. probability of exceedance pga hybrid hazard map (Figure 8).

4.3.2 Confidence level

It is necessary now to assess the confidence that can be vested in these hybrid maps (in part, at least, as a response to the recognised shortcomings mentioned above).

In terms of qualitative considerations, the point has already been made that the most interesting sector of these maps is the North Sea where three independent, differing, sets of hazard estimates have been synthesised. Inevitably, the hybrid contours are never so high or so low as the highest or lowest of the three inputs. It could be claimed, therefore, that the effects of any extreme positions, whether generic or local, that were adopted in hazard modelling have been smoothed out. This would, most likely, be the view taken by those who believe in the value of taking on board the opinions of as many specialists as possible. As a consequence, protagonists of that view would, presumably, argue that the hybrid maps in that area have some degree of enhanced reliability over the original hazard maps.

Elsewhere on Figure 9, where the input comes only from Reports 2 and 4, the nearly even-handed weights that have been employed in the synthesis (0.45 for Report 2 and 0.55 for Report 4) would make the hybrid results far more prey to the effects of any such extreme views. For the area to the west of 10oW, the reliability of the hybrid hazard maps rests entirely with the hazard modelling decisions that were made in Report 2.

In terms of quantified confidence levels, it might be thought possible to make some use of this last situation: where the hybrid maps depend on hazard estimates from just one report with a rating of 61%, those maps might be thought to have a local 'confidence level' of 61%. However, any such figure would be devoid of formal statistical meaning, and the ratings which have been assigned to these reports should not be regarded as corresponding to confidence levels.

Any attempt at quantifying meaningful confidence levels on a more sophisticated basis is confounded by the constraints imposed by the input material. Although the results put forward in Report 4 can be presumed to be the statistical mean of the output from the logic-tree formulation, unlike Report 3, that report provides no information whatsoever concerning the spread of its output at different confidence levels. In this context, the situation with the other two reports is even worse: because of the methods they have used, it would have been impossible for them to have supplied meaningful confidence bands on their results.

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Therefore, even if it had been possible, within the scope of the present appraisal, to construct a method for handling three sets of statistically comprehensive probabilistic hazard results, there is no information to work with.

Given this situation, the most appropriate method for assessing the robustness of the hybrid maps is to test the finished products (rather than their constituent parts) against 'external' independent evidence. Various approaches could be adopted in this regard but, within the scope of this appraisal, it is possible to pursue just two lines of enquiry:

(i) to compare the hybrid maps with the punctual results from site-specific hazard studies for coastal locations, and

(ii) more importantly, through defining a 'default' hazard condition, to examine critically the hazard levels assigned to areas where there are no recorded earthquakes.

These topics are now discussed:

(i) Comparison with site-specific hazard results

For the area to the east of 10oW, Figure 10 shows the comparison between the 10-4

p.a. probability of exceedance pga hybrid hazard map produced as described above and the results from a number of site-specific studies.

There are ten locations shown on Figure 10 for which full site-specific seismic assessments have been undertaken. These are listed on Table 4.1 with their expected 10-4 p.a. probability of exceedance pga hazard values and, for comparison, the nearest contour level or contour band on the hybrid offshore hazard map shown on Figure 9.

Table 4-1 Comparison of offshore hazard contours with on-land sitespecific hazard estimates

Location Site-specific hazard Nearest offshore pga contour

Devon 0.23g 0.1g → 0.15g

Somerset 0.22g 0.15g

Kent 0.21g 0.15g

Pas de Calais 0.23g 0.15g?

Essex 0.26g 0.1g → 0.15g

Suffolk 0.14g 0.1g → 0.15g

Anglesey 0.19g 0.175g

Lancashire 0.24g 0.2g → 0.225g

Cumbria 0.26g 0.2g

Teeside 0.18g 0.15g

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As has already been discussed in Section 3.3 above, unless there are serious methodological failings in one or other of the two, the expectation is that site-specific studies will always give higher results for the estimated hazard exposure than hazard maps, and this is the case here.

The particular location where the difference is most conspicuous is the one in Essex. However, that particular site-specific seismic source model includes a nearby fault source which has been assigned a fairly high active-status reflecting some possible association with the 1884 Colchester earthquake. (It may be noted that it is this location that provides the example shown in Table 3.2 of the effect on hazard levels of a nearby fault source.)

Even if this particular detailed circumstance is neglected, and even allowing for the fact that the site-specific studies would, in any case, be expected to produce the higher hazard results, the overall gist of the comparisons shown on Figure 10 and listed in Table 4.1 is such as to raise concern as to the adequacy of the hazard levels shown in some areas of the hybrid map.

This is especially true of the area from Essex round into the English Channel where a zone of enhanced seismicity, containing a number of very significant earthquakes (including the events of 1382 and 1580 in the Dover Strait), heads approximately westwards from Belgium towards Britain. (It should be stressed that this comment is made without undue emphasis being placed on the result from the French site which is somewhat outside the mapped areas presented in Reports 2 and 4, and well away from the area covered by Report 1.)

As a result of these comparisons, therefore, there has to be a concern (which cannot easily be quantified) that the hazard levels shown in some areas of the hybrid maps are lower than they should be. Although this is an issue which might not have existed had the alternative approach of drawing the hybrid map to represent always the highest local result given by any of the three studies, there can be no assurance that this is necessarily the case.

(ii) Appropriate minimum hazard levels

In the discussion, in Section 3.2 above, of the derivation of activity rates, the importance and influence of the magnitude completeness thresholds assigned to the local earthquake catalogue is highlighted. Indeed, examples are there given (in Table 3.1) of the hazard results that correspond to various typical scenarios in terms of catalogue completeness.

Given any set of completeness thresholds and definitions of the geographical areas over which they are valid, this approach can be employed to construct hazard maps for the circumstance where there are no recorded earthquakes. By definition, such maps provide an indication of minimum 'default' hazard levels across the whole of the area they cover.

Because of the concern that exists about some of the hazard levels shown on Figure 9, a default hazard assessment of this type has been carried out specifically for the present appraisal. Based mainly on work previously carried out by the SHWP, Figure

36

11 shows the macroseismic magnitude completeness thresholds that have been assumed for this purpose.

Figure 11 deals with two magnitude levels:

- the blocks of colour (which form regular rectangles of latitude and longitude) indicate the date at which it is here considered that the local earthquake record became complete at magnitude 5MS, and

- the black lines (whether solid or dotted), which separate cross-hatched areas (outside) from non-cross-hatched areas (inside), denote the limits of the areas within which it is here considered that the earthquake record is complete at magnitude 4MS since 1800.

As has been mentioned, Figure 11 is, to a large extent, based on material that has been published and used in site-specific hazard assessments by the SHWP. In the interests of traceability, however, it needs to be recorded that it has here been necessary to extend the area covered by the figure beyond its previous confines. Thus, those parts of the limit of the magnitude 4MS since 1800 completeness envelope that are shown dotted have been added specifically for the present appraisal. Similarly, it has been necessary to add here a number of magnitude 5MS completeness blocks around the edge of the figure.

The significance of all this is that the additions that have been made are used to illustrate a principle: they have not been subject to the rigorous scrutiny that was applied to the thresholds in the central part of the figure. They would, therefore, need to be revisited and, possibly, revised before they could be used as evidence in any serious formal decision-making exercise: this would apply particularly to the area around the west coast of Ireland.

As has been said, Figure 11 relates specifically to the macroseismic earthquake record. Beyond this, some weight clearly has to be given to the instrumental evidence on earthquake occurrence (or, more precisely, in this context, non-occurrence). For the purposes of this appraisal, it has been decided simply to presume that the instrumental catalogue across the whole area of Figure 11 is complete at magnitude 4.5MS since 1980. As has been noted in Section 3.2 above, the principle which has to be borne in mind in assigning completeness thresholds - whether this is being done to a whole map for the purposes of zonation, or to a single zone for the purposes of calculating activity rates - is that those thresholds must reflect conditions in that part of the area of interest where the circumstances are least favourable for recording earthquakes.

With this single instrumental completeness threshold and the information shown on Figure 11, it is possible to construct the required default hazard map, and this is shown in Figure 12 for the 10-4 p.a. probability of exceedance pga.

The importance of this map is that, wherever any contours on the hybrid map which has been produced as a working synthesis of the existing hazard reports (or, indeed, any of the contours in those reports themselves) drop below those shown on Figure 12, there would be grounds for concern. Visual comparison of Figure 12 with Figure 9 shows that there are, indeed, areas where the default hazard map is indicating higher

37

hazard levels. This immediately raises questions as to the confidence that should be vested in the hybrid map because it implies:

· doubts about the local completeness thresholds that have been used in one or other of the source documents, and/or

· more seriously, doubts overall about the way activity rates have been treated in one or other of the source documents.

In either case, this means that (at least, locally) any of the hazard levels shown on the hybrid map could be suspect although the significance of this problem would, normally, be expected to diminish with increasing numbers of earthquakes in a zone.

Figure 13 shows the departures of the contours on Figure 9 below the default (or minimal) expected 10-4 p.a. probability of exceedance pga hazard, as provisionally defined by Figure 12. These departures are readily apparent in the more remote offshore areas, occurring most notably in the central parts of the North Sea (where the deficit is at least œ0.05g below the default hazard), in the SW Approaches, and west of the Hebrides, towards Rockall (where the deficit is about œ0.1g). The most extreme deficits on Figure 13, at -0.15g, appear along the NW margins of the UKCS, but these arise because none of the reports provided estimates in those areas.

On the other hand, as is to be expected, the hazard contours on the hybrid map exceed the default level close inshore and in seismically active areas. (As has already been mentioned, the issue in such areas is whether the hybrid hazard levels are higher than the default levels by a sufficient amount.)

To make best use of all the available information, Figure 14 conflates the hybrid hazard map (Figure 9) with the default hazard map (Figure 12) in such a way that the hazard levels it shows are everywhere the higher of the figures given by the two maps. In doing this, the presumption is that the provisional default hazard map can, for the present, be relied upon to define the minimum hazard (in terms of the 10-4 p.a. probability of exceedance pga) that ought to appear anywhere in the offshore areas around the British Isles.

As has been explained, the default hazard values shown on Figure 12 depend on the magnitude completeness thresholds that have been used in their derivation. These thresholds have been based on generic historiographical arguments, coupled with cautious judgements concerning the implications of instrumental information. (Reflecting the significant advances made in modern instrumental coverage, the presumption is that an earthquake of magnitude 4.5MS, or greater, occurring anywhere on the map since about 1980, would have been certain to be detected and reported.)

In much the same way as the macroseismic magnitude completeness thresholds used here, this last criterion is no more than a fairly crude approximation to what is, in fact, a complex situation. However, compared with the macroseismic thresholds, there is much more potential for refinement. In terms of the monitoring capability across the area of interest, a marked improvement in the British instrumental network in the 1970‘s has been followed by continued evolution ever since, so that detection thresholds have changed significantly in space and time over the last thirty years. By March 2000, the national network monitoring was such that detection thresholds were considered to be better than 3.5ML for anywhere in the UKCS east of approximately

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10ºW, and better than 2.0ML in inshore waters (see, for example, Figure 3 of BGS [2000]).

Such variations and improvements through time, with their consequent implications for catalogue completeness, can be expected, if properly treated, to have a significant influence on the estimation of seismic activity rate and, hence, on hazard levels (or default hazard levels).

An illustration of the sensitivity of hazard levels to this effect is provided by the comparison between Scenarios 4 and 6 of Table 3.1. Scenario 6 conjoins a low recent instrumental threshold (3.5MS from 1990-2000) with historical and instrumental thresholds (5MS 1900-1970; 4MS 1970-1990) typical of some fairly remote offshore areas. Using, perforce, different b-values for the seismicity above and below magnitude 4MS (b = 1.28 for MS ≥ 4; b = 0.8 for MS < 4), the addition of this recent instrumental completeness threshold results in a reduction of the 10-4 p.a. pga default hazard from 12.9%g to 11.3%g.

The argument for utilising such information is further reinforced when it is considered that the stringent thresholds which are invoked in Scenario 2 of Table 3.1, and which have actually been used for deriving the default hazard in the most remote parts of Figure 12 (5MS 1900-1980; 4.5MS 1980-2000), result in a 10-4 p.a. pga default hazard of 19.7%g. Thus, a relatively short complete sample of instrumental data, obtained with the modern sensitive detection capabilities, can indeed have a marked effect on the calculated pga hazard.

The ramifications of this issue are profound, and its proper consideration would require more detailed, and time-consuming, examination than can be undertaken within the present exercise (because, for example, there would obviously be quite marked variations in modern completeness thresholds across the UKCS and, even, across the area covered by Figure 12). For the moment, all that can be said is that it is likely that in some parts of Figure 12 the representation of minimal hazard levels is somewhat cautious.

However, the confidence that can be vested in Figure 14 depends also on the reliability, in those areas where there have been earthquakes, of the hazard levels which emerge from the construction of the hybrid hazard maps and attention has already been drawn to the issues that remain unresolved in this regard.

To sum up, given the necessary information, it might have been possible to enumerate properly the statistical confidence level(s) associated with the hybrid hazard levels depicted on Figure 9. In fact, the information is either not available or does not exist.

In any case, the concerns regarding the minimal hazard levels appearing in some areas of Figure 9 have lead to the need for a further map (Figure 14) where some provisional adjustment has been made to the hazard levels in those areas. Given the additional complications introduced by these adjustments (even though they, at least, could be statistically defined), it is inconceivable that any kind of meaningful confidence level could be attached to Figure 14.

All that can be said is that, while the provisional compensating adjustments that have been made here are probably conservative, concern remains that some of the other hazard levels shown on Figure 14 are not high enough.

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4.3.3 The effect on hazard levels of a new earthquake

The point has been made that none of the existing reports addresses the issue of the stability of their hazard results against the occurrence of a new earthquake.

In fact, of course, there should only be any effect if the new earthquake - that is, the one which might occur tomorrow - is larger than the relevant current zonal magnitude completeness threshold. Some indications of the magnitude of this effect at three probability levels are given in Table 3.1 (these are the differences between the hazard figures shown for Scenarios 2 and 3 and those shown for Scenarios 4 and 5).

Both of these comparisons relate to the occurrence of a single earthquake in a zone where, previously, there had been none. Given the nature of the calculation of the distribution of potential long-term average activity rates (see Section 3.2), these comparisons quantify an extreme effect: that is to say the occurrence of more and more earthquakes would produce a progressively diminishing increase in the estimated hazard.

Where the scenarios identified in the Table 3.1 match those that have been employed here to construct the default hazard map (see Figure 12), this provides a direct indication of the likely effects on the mapped hazard levels of the occurrence of a new earthquake.

4.3.4 Special circumstances

Within the scope of this appraisal, it is clearly not possible to pay close attention to the hazard results that have been derived for those locations where special considerations might be thought to apply. Obvious examples of such locations are in the vicinity of significant large earthquakes, such as 1927/01/24, 1931/06/07, etc., or where there is geophysical evidence of neotectonic fault movement.

The outcomes appearing on the hazard maps derived here for such locations are, inevitably, just products of the way they were represented (in terms of local source modelling, hazard contouring, etc.) in the three original reports and the rating procedure used to combine the individual results.

In terms of the hazard results that have been used as inputs to the hybrid maps, the following points are worthy of note:

- the source model in Report 1 pays no special attention to this issue

- the source model in Report 2 allows for a very tight zone around the 1931 event and for a fault-specific source at the epicentral location of the 1884 Colchester earthquake

- the source models in Report 4 assign that seismicity which includes the 1931 event to a fairly broad zone

Thus, only one of the three source documents has taken any measures at all, in constructing its seismic source model, to reflect the particular circumstances which might exist around the locations of major earthquakes. Beyond this, because all the hazard results presented in the three reports appear only in the form of smoothed contours, it is impossible to know what the hazard estimate actually is at any precise location.

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It would wise, therefore, to conclude that the hazard levels at any such special location have not, so far, been properly enumerated by anyone. Given the complications and uncertainties that are likely to exist in these locations, it is probable that the best approach is to call for a site-specific hazard assessment.

When this is done, it may be found necessary to allow in the seismic source model for the possibility that a local fault is active. Some indication of the effect that a nearby fault-specific source can have on hazard results is provided by the material shown on Table 3.2: for more general guidance on this particular aspect of hazard sensitivity, reference can be made to Mallard and Woo (1991).

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5 CHARACTERISING EARTHQUAKE GROUND MOTION FOR THE UKCS

5.1 INTRODUCTION

The discussion in Section 4 above is concerned exclusively with the earthquake ground motion hazard expressed solely in terms of pga. It is necessary now to consider a fuller characterisation of that ground motion, in terms of all the frequencies involved.

Before discussing the options that are available to HSE OSD in this regard, however, certain fundamental principles need to be aired.

For engineering purposes, it is the usual practice to depict earthquake ground motion in the form of response spectra which represent the maximum amplitudes of response of an ensemble of simple damped harmonic oscillators subject to external transient excitation. Response spectra are often plotted at various damping values on a tripartite log-log scale, allowing the spectral velocity, acceleration and displacement to be read simultaneously, along with estimates of peak acceleration and displacement at the high and low frequency asymptotes.

While response spectra are largely of benefit to engineers, they can also usefully be employed in an engineering seismological context which is the primary concern here. With the response spectra for a selected population of individual earthquakes, the trends which exist in that population can be analysed in ways that would, otherwise, be impossible. By this means, it is possible, for example, to compound all the individual frequency-dependent characteristics into an enveloping response spectrum or to derive frequency-specific spectral velocity attenuation relations, both of which manipulations are relevant to the present discussion. (For such purposes, the individual response spectra are usually calculated for 5% critical damping.)

Given that most characterisations of earthquake ground motion take the form of response spectra, it is now necessary to discuss briefly an issue noted in Section 1.2 above, namely the distinction that needs to be made between the ground motion hazard and the design basis ground motion.

The position taken here is that term “hazard” is best reserved just for probabilistic estimates of the severity of the free-field ground motion at the surface or, in this case, at the sea bed. In contrast, a design basis ground motion, whether it is to be used for the design of a new facility or as the input motion for the safety assessment of an existing facility, is functionally required to characterise all the earthquakes which might reasonably be presumed to have the potential to hit that facility during its lifetime. As they should, therefore, not reflect the peaks and troughs associated with individual strong motion records, design basis spectra are typically specified as smooth, slowly-varying, curves, or as piecewise-linear segments.

Several relationships are possible between the ground motion hazard at a site and the design basis ground motion, i.e. the seismic action which needs to be used for the satisfactory design of a particular structure on that particular site. Most commonly, one or other of the following options will be adopted:

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- there is no direct relationship (for example, where a design is adopted which uses a standard ground motion characterisation as its design basis);

- the design basis ground motion is a standard ground motion characterisation which is simply scaled to fit, in some specified fashion, the local hazard exposure as determined by a probabilistic hazard study (while the scaling is most commonly done in terms of pga, this is not always the case), and

- the design basis ground motion is a ground motion characterisation which is taken in its entirety from a probabilistic hazard study.

The last of these options calls for the availability of so-called “uniform risk spectra”(URS), in which all the spectral ordinates are calculated from a seismic source model, using frequencyspecific attenuation relations, and have the same annual probability of exceedance. N.B. in such cases, it is the individual ordinates which have the selected probability of exceedance, NOT the overall spectrum. The all-important distinction between this form of characterisation and the standard spectral shapes employed for design purposes in the second option on the above list is that, even though the latter are attached to some probabilistically-derived anchor point, there will be considerable variation in the probabilities associated with the ordinates at the other frequencies of motion.

As the primary concern here is with the ground motion hazard, strictly, it is only URS-type characterisations that are of interest. However, although such characterisations can be tied to site type (hard, medium or soft, as appropriate), they are not necessarily acceptable or suitable for use directly as the design basis input motion for large structures.

Acceptability

Whilst, for example, URS may be used for the safety assessment of existing nuclear power plants in Britain, they are not yet sanctioned by the regulatory authority for the design of new plants.

Suitability

In some circumstances, depending on the design procedures being used, a URS characterisation, or any other form of free-field ground motion spectrum, may be unsuitable simply because it represents the motion at the surface: this is most likely to be the case where soil-structure interaction (SSI) is significant. In analysing SSI, the first step is usually a site response analysis in which a position (commonly called the 'control point') has to be specified where an input motion is applied to a model which is intended to represent the dynamic behaviour of the site stratigraphy

For rock sites, where SSI is rarely a major consideration, the specification of the input motion is straightforward: unless some control point other than the surface is selected, the motion can be any suitable design basis ground motion characterisation, including a uniform risk characterisation where this is allowable. (The use of a control point at some other position is only likely where, for engineering analysis purposes, it has to correspond to the base of an embedded foundation: in such situations, the motion at that level has to be estimated using the 'deconvolution' technique.)

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For other sites, the input motion and the control point for SSI analysis may be chosen in several different ways: medium or soft ground spectra can be used at the surface; hard ground spectra can be used at a nearby rock outcrop, or hard ground spectra, suitably modified to remove the free-surface effect, may be used at the level of bedrock. This last approach again involves some form of deconvolution in order to calculate the motion at the control point. (It should, perhaps, be pointed out that deconvolution is a procedure which is known to have several shortcomings.)

The relevance to the present appraisal of the above remarks is that they explain the concerns raised by some of the proposals concerning ground motion characterisations that are made in Reports 2, 3 and 4. (Although 'standard' piecewise-linear response spectra were already in existence for onland Britain at the time of its writing, these are not mentioned in Report 1 and it appears that this subject was not seen as part of the scope of that study.)

As the two complementary reports (Reports 3 and 4) adopt the same approach and can, therefore, be treated as a single entity for the purposes of this discussion, just two proposals need to be taken into account. These two alternative suggestions are summarised in the following paragraphs and, then, compared with each other, with existing standard spectra (including the ones mentioned above, which were derived for the UK nuclear industry), and with some uniform risk spectra. For convenience, the terms 'hazard spectra' and 'design spectra' are used, as appropriate, in this discussion to make the necessary distinction.

For completeness, this section concludes with a brief discussion of the use of spectrum-matching time histories in the design process.

5.2 THE GROUND MOTION CHARACTERISATIONS PROPOSED IN THE EXISTING REPORTS

Report 2

Hazard spectra results, at 5% of critical damping, are presented at three probability levels for five example locations. These best-estimate shapes have been calculated using the frequency-specific attenuation relations due to Dahle et al. (1990) although, as is noted in Appendix 1, manipulated values have been used for the sigma values associated with these relations (rather than the values actually derived by Dahle et al.).

These example hazard spectra are compared with the design spectrum for hard sites recommended in API RP 2A, one of the relevant industry standards in play at the time.

Interestingly, and significantly, Report 2 notes a change in the shape of their calculated hazard spectra for locations where the larger earthquakes occurring out in the mid-Atlantic exercise an influence.

Reports 3 and 4

Using the frequency-specific attenuation relations due to Ambraseys et al. (1996) and Toro et al. (1997) as equally-weighted alternatives in a logic-tree formulation, normalised uniform hazard spectral shapes at three probability levels for four sites are presented, the calculations

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having been carried out at just six frequencies. These normalised hazard spectra correspond to the expected (i.e. mean) confidence level ordinates and Report 3 draws attention to the spread of results at differing confidence levels. Based on the similarity of these twelve spectral shapes, a single normalised spectral shape, which is closely equivalent to a uniform risk spectrum is proposed as a —bedrock“ spectrum for all sites, whether onshore or offshore, and for all probability levels.

This single normalised spectral shape is then extended to frequencies lower than those covered by the selected attenuation relations, i.e. below 0.5Hz. This is done, with no explicit consideration of probability levels, by taking the ordinate at 0.2Hz as being the mean of that which would be given by the assumption of constant spectral velocity below 0.5Hz and that which would be given by the assumption of constant spectral displacement below 0.5Hz.

In Report 3, this extended normalised spectral shape is compared with a bedrock design spectrum from Eurocode EC8. It is also compared with the spectra from three previous Norwegian studies into offshore ground motion characteristics, two of which (due to the NORSAR Safety Offshore study and to the ELOCS study) give every impression of being piecewise-linear design spectra. In considering these comparisons, it may be noted that, notwithstanding the newer work presented in Report 3, the ELOCS spectrum appears to have continuing currency as a preferred Norwegian design criterion, according to the most recent version of NORSOK Standard N-003.

In Report 4, the extended normalised spectral shape is compared with the Principia Mechanica Ltd. (1981) hard ground piecewise-linear design spectrum and with a normalised hard ground hazard spectrum shape derived using the frequency-specific attenuation relations due to Principia Mechanica Ltd. (1988), both of which spectral shapes are discussed below. It may be noted, however, that, in the latter case, Report 4 provides no explanation as to what has actually been plotted (in terms of the source model used, or the probability or confidence levels associated with the normalised shape that is shown). Report 4 also mentions, but does not illustrate, a comparison with a design spectrum from Eurocode EC8.

5.3 THE ADEQUACY OF THE GROUND MOTION CHARACTERISATIONS PROPOSED IN THE EXISTING REPORTS

Given the uncertainties involved, assessing whether or not a proposed ground motion characterisation is suitable inevitably calls for comparisons with potential alternatives and with established practice. As noted in Section 5.2 above, several such comparisons have been made in Reports 2, 3 and 4 and, before widening the net to include additional comparisons, the following comments may be made in the context of the present appraisal (taking into account the difference between hazard spectra and design spectra):

- for this appraisal, there seems to be no very good reason why any comparison should be made with a design spectrum from Eurocode EC8, which relates most appropriately to higher probability levels than those which are of most concern here and which, of necessity, has to allow for the larger earthquakes that occur in southern Europe. Doubtless, the comparison is included in Report 3 because the brief adopted by that document is very wide and, seemingly, covers (like the Eurocode documents) conventional building standards

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- the Principia Mechanica Ltd. [PML] (1981) piecewise-linear design spectra were derived for the UK nuclear industry and remain, to this day, the accepted standard in that industry for the design of new facilities

- the frequency-specific attenuation relations derived by Principia Mechanica Ltd. [PML] (1988), also commissioned by the UK nuclear industry, allow the calculation of uniform probability spectral ordinates direct from the ground motion hazard model: in that industry, they provide acceptable input motions for probabilistic seismic safety assessments and for the periodic safety reviews of existing facilities

- while the API RP 2A (1993) design spectra are directly relevant to the structures that are of concern here, in an environment like most of the offshore UKCS it is likely to be over-conservative since, like any worldwide standard, it has to cover a range of seismotectonic settings including those that are far more energetic: in any case, this document is now, perhaps, somewhat dated

In this last connection, it needs to be pointed out that the most recent 'standard' recommendations concerning suitable spectra for offshore facilities are those contained in the draft ISO document (ISO TC 67/SC 7 N 278A, dated 21 January 2001), mentioned in Section 1.1 above, which is referred to throughout this appraisal as ISO (2001). Whilst that document would not have been available to the authors of any of the existing hazard studies, it is clearly desirable that some attention is paid here to the import of these new guidelines, even though they only exist in draft form.

With regard to the spectra that are actually derived in Reports 2 and 3 (and 4), the following points need to be noted:

Report 2

- as noted above, the hard site hazard spectra derived in this report depend entirely on the attenuation relations put forward by Dahle et al. (1990). These relations are based on a database that included mixed magnitude types, which is one reason why their sigma values are high (0.83 for pga, for example). The authors themselves identify the global selection of data, and diverse ground conditions as other factors contributing to their high sigma value, which, inevitably, lead to conservative hazard results. Presumably, this issue was the reason why the authors of Report 2 made their (somewhat questionable) adjustments to the original sigma values

- since other, preferable, relations are now available (and since the Dahle et al. relations appear to have been eschewed by their Norwegian colleagues who prepared Report 3), there seems to be little advantage in pursuing the hard site spectral shape recommended by Report 2

- this being the case, there is no need to discuss in any detail the approach this report appears to suggest for handling soft sites, which involves site response calculations using the hard site hazard spectrum as the input motion.

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Report 3 (and 4)

- these reports also derive their proposed universal bedrock spectral shape more or less directly from their hazard model, albeit using better-constrained attenuation relations than Report 2

- for soft sites, rather than repeat the process, they recommend using this bedrock hazard spectrum in combination with amplification factors that have, apparently, been derived from a number of site response analyses to arrive at the motion at the sea bed

- for soft sites, therefore, the recommended approach combines a probabilistic input motion with a deterministic calculation of the amplification at different frequencies. Whilst a ground motion characterisation derived in this way may be considered acceptable as a design spectrum, it should NOT be confused with an estimate of the ground motion hazard

- because of this, it follows that, although comments on the bedrock spectral shape proposed in these reports can be made here (see below), no detailed opinion can be expressed on the acceptability or otherwise of the proposed treatment of soft sites because this involves policy considerations, as well as technical issues which are outwith the scope of this appraisal

- a final comment which needs to be made here is that, unlike Report 2, Report 4 makes no mention of the likely need to allow for different spectral characteristics out to the west of the United Kingdom towards the mid-Atlantic Ridge.

It is necessary now to extend the comparisons that are made with the spectra derived in these reports. In doing this, there is considerable merit in maintaining a distinction between hard and soft sites. (While there is little evidence of systematic variation in pga hazard due to site type, all the available evidence suggests this is not true at low frequencies, particularly at annual exceedance probabilities of about 10-4.) The implications of all the work that has been carried out, thus far, on behalf of safety-related projects in Britain suggest that, for any given value of pga, the severity of the motion at low frequencies will be higher on soft sites than on hard sites. This tendency is clearly embodied, for example, in the hard- and soft-site piecewise-linear design spectra derived by PML (1981), see Figure 15, and also observed in the uniform probability hazard spectra that are calculated using the PML (1988) attenuation relations

In the case of standard spectra which, in application, are anchored to some pga value by simply normalising the spectra to a common pga value of 1g, comparing their shapes is very straightforward as they can simply be normalised to a common pga value of 1g, as exemplified by Figure 15.

Experience in Britain shows that it is reasonably reliable to presume that many uniform risk spectra can also be scaled wholesale simply in terms of pga. Indeed, in essence, this is what is proposed by Reports 3 and 4. (For any given confidence level, the shape of a URS is most likely to vary when there is a change in the size-distance pairings of the contributing earthquakes, as noted by Report 2.)

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It is nowhere near so easy, however, to make diagrammatic comparisons with the ISO (2001) spectra which, effectively, are anchored not to a pga (or a ”zero period acceleration‘ - ZPA), but, rather, at 1Hz and 5Hz. Where estimates of the hazard at these frequencies are not available, the ISO document provides low-resolution default hazard maps for various offshore parts of the world.

Therefore, in order to involve the ISO (2001) spectra in these comparisons, some form of manipulation is necessary and the method used here is explained below in the specific context of the two plots that have been produced.

The first of these plots (Figure 16) shows the (bedrock) hazard spectra taken from Reports 2 and 3 (and 4) compared with the generic design spectra for hard sites given by PML (1981) and API (1993), and a version of the ISO (2001) design spectrum, all anchored to 1g ZPA (40Hz). Also shown in this normalised form on Figure 16 is the scaled expected 10-4 p.a. probability of exceedance uniform risk spectrum from a recent site-specific hazard assessment for a typical British hard site.

The second plot (Figure 17) shows a similar variety of spectral shapes for soft site conditions. The first point to make concerning this plot is that no attempt has been made to incorporate any input from Report 2. As a consequence of the change in site category, the standard design spectra taken from PML (1981) and API (1993), together with the two versions of the ISO (2001) design spectra shown on this figure differ from those shown on Figure 16. It may be noted that the spectrum from API (1993) which is shown on Figure 17 is the one corresponding to their soil type C (—competent sands, silts and stiff clays with thicknesses in excess of about 61m [200ft] and overlying rock-like materials“). The scaled expected 10-4

p.a. probability of exceedance uniform risk spectrum which appears on Figure 17 is that for a typical soft site (in this case, a North Sea coastal location in France). Against this background, Figure 17 shows the band of spectral shapes which result from the range of amplification factors suggested in Figure 14 of Appendix A of Report 3 for the 10-4 p.a. input bedrock motion on —soft“ sites.

As has already been said, because of their obvious importance, it was considered essential that some representation of the spectral shapes recommended in ISO (2001) should be included in the comparisons shown in tri-partite form on Figures 16 and 17. The method by which these representations have been constructed is now described, starting with a brief summary as to how the spectral shapes recommended in this relatively new document are intended to be defined.

The ISO (2001) response spectrum is usually drawn in terms of spectral acceleration: in the plots shown here, spectral acceleration has been converted to pseudo-spectral velocity so that it can be drawn on the usual tri-partite plot. The basic shape of the spectrum is defined by two or by three anchor ordinates, depending on whether or not a certain spectral amplitude ratio is exceeded:

- the two main anchoring periods are equivalent to the 0.2 sec and 1.0 sec SDOF oscillator responses which define the shoulder- or break-points in what is, effectively, a piecewise-linear shape on a tri-partite plot. The spectral acceleration amplitudes at these periods, Sa(0.2) and Sa(1.0), are used to determine the height of the spectral shape within different period ranges, using formulations given in the document;

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- as mentioned above, there may also need to be third break-point (TS) in the mid-period part of the spectrum. This is deemed to be required if the

( .aratio S 10 S

) exceeds 0.2 and, in practice, is inserted at the period ( .0 2)a

(measured in seconds) which is numerically equal to the value of this spectral ratio. That is, if, for example, the ratio S ( .a 10

S) is found to

( .0 2)a

be 0.3, then the TS break-point is added at 0.3 sec period (3.3Hz). It is not immediately clear how this construction is justified but it has the effect of introducing a change in spectral behaviour associated with intermediate or soft site conditions. For the bedrock conditions represented in Figure 16, all the datasets or spectral representations suggest that the ratio S ( .a 10

S) is less than 0.2, implying that there is

( .0 2)a

no need for the addition of this extra break-point;

- finally, ISO (2001) allows for a further, optional, break-point at 4 sec period, with its spectral amplitude being related linearly to the Sa(1.0) spectral acceleration. This introduces a change of slope for very low frequency motions in excess of 4 sec period.

In order to make the comparisons required for the present exercise, and with a little legerdemain, it is possible to construct ISO spectral shapes in two different ways:

- the first is to use the normalised pga value (usually 1g) to estimate the Sa(0.2) value, given that the fall-off shape of the ISO spectrum is linear at frequencies above 0.2sec period. This is equivalent to anchoring the high-frequency shape of the spectrum at the pga end. Then, a corresponding Sa(1.0) value can be estimated by assuming a value for the

( .aratio S 10 S

) 0 2

, and the rest of the spectral shape can be calculated ( . )a

from the given formulae. In this way, effective —anchoring“ to the normalised pga value (or, if required, to any other selected pga value) is achieved;

- for drawing Figures 16 and 17, just two options are currently appropriate (and available) for providing a value for the ratio S ( .a 10

S) : a typical

( .0 2)a

value suggested by the default maps in ISO (2001) could be used, or a value from some independent source could be imported. In this case, for convenience, the values used have been calculated from the single 10-4

p.a. probability of exceedance URS shown on the same plot. Inevitably, this introduces some bias into the comparisons shown on Figures 15 and 16 but there is no readily available, locally valid, alternative: by inspection, the information given on the default maps presented in the ISO document itself is unlikely to have such local validity;

- the alternative approach is to import or assume values for the individual spectral acceleration values Sa(0.2) and Sa(1.0) and straightforwardly construct the spectral shape according to the procedure described in the ISO document and summarised above, including, where necessary, the secondary [TS] break-point. The problem with this approach is, of course, that the resulting high frequency spectral behaviour is unlikely to

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converge to the desired ZPA;

- the individual spectral acceleration values Sa(0.2) and Sa(1.0) used to compile this version of the ISO spectrum shown on Figure 16 have been taken from the 10-4 p.a. probability of exceedance soft site URS results. It may be noted that the lack of convergence to the ZPA is, in this case, manifest in only a small discrepancy. No attempt has been made to include the results of using this second (Sa-anchored) approach on the bedrock comparison plot, Figure 16, because: (i) that figure is already over-congested, and (ii) as Figure 17 demonstrates, the differences between the results of applying the two approaches are not great.

It has to be acknowledged that these are somewhat tortuous routes for constructing ISO-like design spectra. However, there is no sensible alternative if meaningful comparisons with the other spectra are to be made.

Examination of the variety of spectral shapes which appear on Figures 16 and 17 is instructive in drawing attention to the following features:

- from the comparison shown on Figure 16, and from the comments which have already been made, there is little point in pursuing the bedrock hazard spectrum put forward by Report 2;

- the universal spectral shape at bedrock put forward by Reports 3 and 4 compares quite closely, as one might expect, with the a typical 10-4 p.a. probability of exceedance hard site uniform probability hazard spectrum, and not too badly with the contrived ISO (2001) design spectrum for hard sites;

- the maximal values of the very wide range of design spectral ordinates between 0.2 and 5Hz suggested by Reports 3 and 4 for soft sites is such as to exceed even the PML (1981) design spectrum at frequencies less than about 2Hz;

- the API (1993) design spectra for hard and soft sites appears to be unwarrantedly over-conservative for British conditions at lower frequencies (i.e. below about 5Hz for hard sites and below about 2Hz for soft sites);

- probably for similar reasons to do with their intended generic applicability, the ISO (2001) design spectra for both hard and soft sites appear to be marginally over-cautious in the very low frequency range (i.e. below about 0.4Hz) although this might not be the case for those parts of the UKCS where large earthquakes associated with the mid-Atlantic ridge make a significant contribution to the hazard, particularly at a soft site.

In terms of the design spectra which have been imported, for comparison purposes, from the nuclear industry, it is not surprising that, for both categories of sites, with the exception at lower frequencies of the API (1993) design spectra and the maximal soft site values put forward in Reports 3 and 4, the most conservative of all the spectral shapes shown on Figures 16 and 17 are those derived by PML (1981).

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In considering the PML (1981) spectra, it may well be felt that these would be unnecessarily conservative for the purposes of the design of offshore structures, having been developed for nuclear facilities with all that that entails in terms of regulatory requirements. A sense of the context in which they are used is most simply conveyed by recording the current regulatory stance, as expressed in HSE (1992). This explicitly requires a conservative estimate of the 10-4 p.a. probability of exceedance ground motion hazard with the additional proviso that —It should be shown that there will not be a disproportionate increase in risk from an appropriate range of events which are more severe than the design basis event“.

It follows from this that the comparison which gives rise to most doubts regarding the proposals made in Reports 3 and 4 is the one on Figure 17 with the PML (1981) soft site design spectrum. For most of the offshore UKCS, their recommended maximal amplification factors appear to lead to an unnecessarily over-conservative design basis ground motion characterisation for frequencies less than about 2Hz.

As has been noted, it would not be appropriate to come to any firm conclusion at this stage as to which of the various spectra discussed here should be adopted for design purposes. The very widely spread proposals for soft site conditions made in Reports 3 and 4 are based on work which has not been seen and, in any case, their acceptance would require HSE OSD to sanction the process that has been used in their derivation. Also, the versions of the ISO (2001) design spectra that have been plotted here are, at best, contrived approximations and, with the passage of time, the current ISO draft recommendations may be changed as a result of the consultation process. Even so, it might be unwise if HSE OSD were to promulgate the use of a design spectrum which is much less conservative than that current draft.

As with several other components of this appraisal, therefore, it is only possible to conclude here that more work is required. In this case, what is needed is a single coherent hazard assessment providing results, for at least two categories of sites (hard and soft), at a selected range of frequencies (including 1Hz and 5Hz), as well as for pga, so that all the potential options remain open. In terms of design spectra relating to the ground motion characteristics at the surface, these options are:

- being able to use standard design spectra anchored to pga;

- if appropriate, being able to use some form of uniform probability hazard spectrum, or

- being able to adopt, with local validity, the draft ISO (2001) design spectrum.

Obviously, such an exercise would need to be limited as to the number of frequencies that could be selected: practically, it would need to follow the precedent set by Reports 3 and 4 (six frequencies, in addition to pga), rather than that set by PML (1988) for site-specific studies (twenty-one frequencies).

Also, it would be necessary to decide in advance just how much work should be expended on addressing the rather special circumstances that obtain towards the mid-Atlantic ridge.

If, alternatively, the use of site response analyses is, indeed, the preferred policy, it then becomes necessary to decide whether such manipulations should be done on a site-specific basis or whether some properly-validated 'standard' amplification factors would suffice.

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To conclude this discussion, it is worth noting that an additional benefit of producing Figures 16 and 17 in their present form is that, potentially, they provide a means of estimating, albeit crudely, how the 10-4 p.a. probability of exceedance 1Hz ground motion hazard varies around the UK. This topic is discussed in Section 7 below.

5.4 TIME HISTORIES

The usual practice in analysing the dynamic response of a structure is to work in the time domain, using acceleration time histories. This requires that appropriate time histories are available which match some specified characteristic design basis ground motion which is usually expressed in the form of a response spectrum, as discussed above.

Most commonly, the time histories that are used are artificially generated, using standard software, to match directly the so-called target spectrum with only a minimal number of undershoots across the whole range of the spectrum. In terms of the earthquakes that are likely to affect the structures of interest, these artificial time histories introduce a major conservatism into the design process. By the very nature of broadband design spectra, no single earthquake is ever likely to exercise all of the frequencies involved to their peak values. (Therefore, the total energy that is present in an artificial time history - which is no more than a construct - would be singularly unlikely to be present in any real earthquake.)

Unfortunately, conventional methods of dynamic analysis being what they are (i.e. based on peak elastic responses), no significant reduction in conservatism is achieved by adopting either of the other two, more rational, approaches to the selection of appropriate time histories, namely:

- using a suite of imported real time histories from appropriately sized and located earthquakes which, between them, match the target design spectrum, or

- using a suite of synthetic time histories, which again overall match the target spectrum, and which have been generated from local data using the empirical Green‘s Function approach.

Because of the problems associated with the present conventional methods of analysis, however, the only current applications for these techniques are where significant non-linearity is anticipated and has to be accommodated: under such circumstances, it is becoming generally accepted that the use of artificial time histories is to be avoided.

(It may be noted, nevertheless, that formalised methods have been developed, and used, by the authors of this appraisal for selecting suites of real time histories to match target spectra and that, currently, a method is being developed for achieving the same outcome with synthetic time histories.)

The account given in Report 2 of the process by which three real time histories were selected and, then, scaled so as to match two target spectra (one of which is considered to represent conditions in the westernmost part of the UKCS) is not at all clear. In fairness, however, the records are only used in Report 2 to provide some illustrative routine examples of site response effects.

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In contrast, Report 3 appears to go so far as to recommend three real three-component time history records for dynamic analyses: again, however, the description of how the records have been selected is wanting. From what is said in Report 4, it would seem that the user simply has to scale the recommended three sets of records (which are in the public domain) by pga, but it would be useful to have this confirmed. Also, it should not be forgotten that the spectral shape (for soft or hard sites) to which the proposed time histories have been matched may well be inadequate for the westernmost parts of the UKCS.

These reservations, and related remarks on earlier topics, highlight several technical issues that need resolving before an acceptable generic approach can be formulated for the selection of suitable time histories to represent adequately conditions across the whole area of the UKCS. Unfortunately, this challenge cannot be addressed within the scope of the present study.

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6 VERTICAL MOTION

As has been noted, this topic is not dealt with in either Report 1 or Report 2. Whilst it is discussed (albeit, very briefly) in Report 3, its companion document, Report 4, is mute on the subject. It is not clear if this omission results from a simple editorial error in the latter report or from a difference of opinion. If possible, this matter should be resolved as soon as possible.

In fact, Report 3 recommends the adoption of the findings of research work reported in an earlier document which has not been made available for this appraisal. Apparently, this earlier study found that the ratio between the vertical motion and the larger of the two horizontal components of motion is frequency dependent such that the overall spectrum can be divided into three sections (less than 1Hz, between 1Hz and 3Hz, and more than 3Hz), with different figures being used for the ratio of vertical to horizontal motion in the three sections, as follows:

· less than 1Hz: V/H = 0.9

· between 1Hz and 3Hz: V/H = 0.9 - 0.42 log (f)

· more than 3Hz: V/H = 0.7

Without seeing any details concerning the derivation of these figures (such as the database used and the analytical procedures employed, etc.), it is not possible to make any sensible comment on these recommendations. It would be interesting to know, for example, what extent of compatibility exists between this work and the derivation of the horizontal spectral shape that is proposed in the same report.

Until now, the convention in Britain has been to use a V/H ratio of two-thirds at all frequencies when using the piecewise-linear PML (1981) spectral shape and, of course, to calculate the vertical spectral shape directly when using uniform risk spectra. (In the latter case, for the expected 10-4 p.a. ground motion, typical values for V/H of 0.6 to 0.65 emerge at high frequencies, which drop to around 0.4 to 0.5 at 1Hz, with the lower values, as might be expected, being associated with soft sites.)

Compared with these existing approaches, the proposal made in Report 3 is obviously more conservative. For Britain, it may even be somewhat over-conservative, given the fact that higher ratios tend to be associated with thrust earthquakes which, from the evidence of recent moderate and low magnitude earthquakes, do not appear to be all that common in this country

This said, there is increasing recognition (in recent versions of Eurocode EC8, for example) that using the same spectral shape for horizontal and vertical might no longer be reasonable. Recent guidelines (ASCE, 1998) aimed specifically at nuclear facilities in the United States are worthy of mention in this context. These suggest that, for conditions in that country, the vertical component can still be obtained by scaling the corresponding ordinates of the horizontal component by two-thirds throughout the entire frequency range where distant earthquakes control the design spectra.

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However, where near-field earthquakes (defined as source to site distances less than about 15km) are dominant, the minimum recommended ratios of vertical to horizontal motion are as follows:

above 5Hz: V/H = 1

at 3Hz and below: V/H = 2/3

with a transition being constructed between these two circumstances, unless a site-specific evaluation of the ratio is available.

Thus, ASCE (1998) supports Report 3 in proposing some measure of frequency dependency. However, it is not at all clear why the variation of the ratio V/H with frequency suggested in Report 3 should run counter to the trend suggested for near-field earthquakes in the American code (and also counter to the trend implied by the British URS).

It is probably fair to say that the behaviour of the ratio between vertical and horizontal motion is a topic that has lacked recent analytical attention in this country, even though sufficient empirical data must now exist to explore properly the seismological dependencies. This situation has been brought about, no doubt, by the wide acceptance of a factor (2/3) which was demonstrably conservative when it was first proposed. However, it might now be timely for some further work on this aspect to be carried out.

In the particular context of the present appraisal, one other factor needs to be borne in mind in connection with vertical earthquake ground motion. It may well be that the ratio between vertical and horizontal motions represents the most significant difference between onland and offshore situations due to the influence of hydrostatic pressure exerted by the water column, particularly in deeper waters.

Although there is little empirical evidence available to confidently constrain this effect, some recent work has been published by Boore and Smith (1999). Their results indicate that seafloor responses in the Santa Barbara Channel, off California, have very low vertical motions compared with those that are typical of onshore sites, particularly at higher frequencies. Theoretical considerations indicate that the water layer will have little effect on the horizontal component of motion but should produce a marked spectral low at the resonant frequency of P-waves in the water column. For offshore sites, Boore and Smith find that V/H ratios at frequencies less than about one-half the resonant frequency of P-waves in the water column are similar to those for a few, selected, onshore sites which are underlain by strata with low shear wave velocities. This is taken to suggest that the shear wave velocity beneath a site is more important than the water column in determining the character of seabed motions at lower frequencies.

Until such time as further consideration of this whole issue is possible, the conservative decision would, clearly, be to accept the recommendations made in Report 3. Although it has not been possible to scrutinise the report on which these recommendations are based, it is understood that empirical observations from the North Sea were used in that earlier study (although the range, and hence validity, of the database that was taken into account is not known). As has been discussed, whilst this option might be unnecessarily over-conservative, further work would be necessary to resolve satisfactorily whether or not this is the case.

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7 TOWARDS ESTIMATING THE 1HZ HAZARD ACROSS THE UKCS

As has been mentioned, none of the reports considered in this appraisal actually provides an indication of the variability of the ground motion measure that is of greatest importance to most offshore structures. This said, it is understandable that all of the reports have concentrated on the pga hazard, this being the measure which is more or less universally adopted as the shorthand indicator of the severity of earthquake ground motion. As a result of this widespread usage, comparisons between the results of different hazard studies are usually framed in terms of pga (which also has the advantage of being more directly comparable with other hazard loadings).

Nevertheless, it is a requirement of the present appraisal that some guidance is given, based on the information that is readily available, as to how the likely levels of 1Hz hazard and their potential variability across the UKCS can be estimated.

The only way this is possible, within the scope of the present project, is to invoke amplification factors for hard and soft site conditions which could be applied to Figure 14, or any other pga hazard map.

These amplification factors are taken directly from the spectral comparisons discussed in Section 5 above, see Figures 16 and 17. From these figures, the amplification factors (i.e. 1Hz spectral acceleration / pga acceleration) that have, for the present, been selected are:

· for hard sites: 0.35 · for soft sites: 0.7

These figures are based on selected spectral velocities at 1Hz that are approximately central within the spreads appearing on Figures 16 and 17, laying emphasis on the hazard spectra shown there, rather than the design spectra. For the record, the actual values are 54.8 cm/sec and 109.95 cm/sec respectively.

While there is clearly some uncertainty attached to the precise values of these ratios, a good measure of support for there being a significant difference between their values on hard and soft sites is provided by simple deterministic calculations of the ground motions that would be experience in typical British earthquakes. This can be shown by using a variety of attenuation relations, including most of the ones involved in this appraisal.

In this connection, it may be noted that the smallest differences between the amplification ratios for hard and soft sites are predicted by the Ambraseys et al. (1996) attenuation relations used in Reports 3 and 4. (Unfortunately, it has, in the time available, proved impossible to track down the source for the soft site modification factors quoted by Toro et al. [1997] which, presumably, were also used in Reports 3 and 4.)

Such simple deterministic calculations are, of course, unable to provide support for the precise numerical values that have here been derived for the amplitude ratios. This is because the selected values most closely correspond to the ones suggested by uniform risk spectra which, by their very nature, involve a whole range of earthquakes with different magnitude-distance pairings making varying contributions to the overall hazard at different frequencies.

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For a preliminary indication of the 1Hz ground motion hazard on hard or soft sites, therefore, it is recommended that the two ratios derived above can be applied to any preferred estimate of the 10-4 p.a. probability of exceedance pga hazard.

Given the database developed for the present appraisal, it would be a trivial matter for this information to be presented in the form of contoured hazard maps for hard and soft sites. In theory, it might then be possible to compare any such maps that are produced with the maps presented in ISO (2001). However, given the fundamental difference that that document is concerned with the characterisation of the 1Hz motion for design purposes whereas the present discussion is concerned with hazard levels, any such attempt would largely be pointless (and very difficult, given the nature of the map-plotting in that document).

While this is an extremely rudimentary approach to the problem, it does provide at least some insight into an issue which is not at all represented in any of the existing reports. This said, it should not be forgotten that some form of modification to the amplification factors would, almost certainly, be required for locations in the westernmost part of the UKCS.

Thus, for this topic also, more work remains to be done on several fronts: once again, it would be better by far to have a hazard assessment in which the 1Hz motions are coherently derived, with the pga motions, from a single source model using attenuation relations which reflect properly the influence of the site category.

Until such work has been done, the amplification ratios presented here can be regarded as provisional expedients for guiding decisions on spectral factors, insofar as hazard considerations are concerned. Even so, for any particularly sensitive location or special structure, a site-specific analysis of its exposure to low frequency earthquake ground motion should probably be undertaken.

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8 CONCLUDING REMARKS AND RECOMMENDATIONS

Every effort has been made in this appraisal to develop, from disparate existing estimates, a logical and robust, yet not over-conservative, synthesis of seismic hazard levels across the offshore areas of the UKCS. The intention is that this synthesis can be used for giving advice until such time as a single coherent assessment is completed which covers the whole area uniformly and provides all the information that is needed, or until such time as some other policy can be agreed. In doing this, the appraisal has, of necessity, concentrated on the lower end of the range of probabilities that are of concern in assessing the safety of offshore structures.

The need to make such a synthesis is brought about by the fact that: (a) the results given by the three reports which address the area of interest are different, and (b) for various reasons (which vary from one to another), none of the studies is regarded as being sufficiently satisfactory that its results can be accepted wholesale. Further complications arise because the reports present hazard results which cover different, albeit overlapping, areas, with only one set of results (those from Report 2) extending over more or less the whole of the area that is of interest to HSE OSD.

8.1 THE GROUND MOTION HAZARD IN TERMS OF PGA

Using just the information provided by the three reports, the hybrid synthesis that has been arrived at for the 10-4 p.a. probability of exceedance pga hazard is shown in Figure 9. Under critical examination, however, that synthesis is demonstrably suspect, primarily because the estimated hazard levels it defines for some areas where there are no known earthquakes appear to be too low. This conclusion is based on a 'default' hazard map (Figure 12) which has been used here to define provisional, and somewhat crude, minimum allowable hazard levels.

Although there are some quite serious local concerns (through the English Channel in particular), the undershooting by the hybrid map of acceptable minimal hazard levels is considered to be a more significant general failing than the fact that the predicted hazard levels are everywhere lower than those reported in site-specific hazard studies for adjacent coastal locations. When adjustments are made to address this primary concern, the resulting hazard map is as shown in Figure 14 and, for the present, this is considered to be just about the best map that can be drawn in the circumstance when attention is paid to all the opinions that have been expressed.

Two points need to be borne in mind when considering Figure 14:

- first, it has to be remembered that the adjustments that have been made to counteract situations where the minimal hazard levels are too low represent the findings of a simple preliminary analysis (suggestions as to how a more sophisticated - and, locally, less conservative - representation of the default hazard could be constructed are discussed in some detail in Section 4.3.2), and

- second, the very fact that some minimal hazard levels are too low implies that there may be complementary deficiencies elsewhere on Figure 9 where there has not been a total absence of recorded earthquakes. (The strongest hint that this may be the case comes from the area around the Strait of Dover.)

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Clearly, it is necessary to come to some form of conclusion on this matter, even if that conclusion can only sensibly be regarded as an interim position. It has to be recognised that, in some areas, the hazard estimates appearing on Figure 14 are lower than those provided by Report 4 which, of course, is the most recent assessment available to HSE OSD. The comparison between the hazard levels given by Figure 14 and Figure 6 is depicted on Figure 18 in the form of contoured differences. This shows that, whereas the former indicates higher hazard levels in the more remote parts of the area of interest, the reverse is true for certain inshore areas, particularly down the west coast between Britain and Ireland. This last circumstance occurs because there are just two reports covering the Irish Sea and a significant weight (as high as 0.45) has, in consequence, been assigned to the results from Report 2 which imply generally much lower hazard levels for those areas than those proposed by Report 4.

Given the fact that it would not be prudent to accept wholesale the findings of the present appraisal in preference to the most recent full-scale hazard assessment, it is recommended that, for the present, the hazard level should be set at the higher of the two figures given by Figure 14 and Report 4 (i.e. Figure 6). Thus, the currently preferred 10-4 p.a. pga hazard levels are as shown on Figure 19.

One final point needs to be made concerning pga hazard levels. For the area of the UKCS which lies to the west of about 11oW, the only information that is available comes from Report 2. More work would be necessary to come to a properly considered opinion on the adequacy of the treatment that is given in that report to the changing nature of the seismogenics as one approaches the mid-Atlantic ridge.

8.1.1 The ground motion hazard at 1Hz

In order to overcome a significant deficiency in all the existing hazard estimates (and, therefore, in the information database available to HSE OSD), this appraisal has suggested a simple way of estimating the ground motion hazard at a typical frequency of concern (1Hz) in the design of offshore structures. It is important to recognise that the concern here is with the hazard per se, not with the design basis ground motion (the distinction that is made in this appraisal between these two concepts is discussed in Section 5.1 above).

The suggestion is that simple amplification factors can be used to convert pga hazard levels to a fairly reliable estimate of the hazard exposure at 1Hz. Two figures have been proposed for the ratio 1Hz spectral acceleration/pga, namely:

- for hard sites: 0.35

- for soft sites: 0.7

In proposing these ratios, it has to be stressed that they are provisional values, awaiting further analysis, and that no adjustment has been made for the variations in spectral shape that are likely to be necessary in order to reflect conditions out towards the mid-Atlantic ridge and the bigger earthquakes to be found there.

Nevertheless, for the moment, it is considered that these multipliers can be applied to any of the pga hazard maps that are in play (e.g. Figures 6, 9, 12, 14, or 19) to provide estimates of the 10-4 p.a. probability of exceedance ground motion at 1Hz which are not much less robust than the pga hazard estimates themselves.

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8.1.2 Ground motion characterisation

With regard to the spectral shape that should be used to characterise the ground motions across the UKCS, Section 5 above draws attention to the need to discriminate between characterisations that are intended to represent the probabilistic free-field ground motion hazard and those that are intended for use as design basis ground motions. In terms of the proposals in this regard that are put forward by the reports made available by HSE OSD, the conclusions of this appraisal are:

- Because of the attenuation relations used in their derivation, the spectral shapes put forward in Report 2 should not be pursued for either purpose.

- Based on the comparisons shown on Figure 15, the —bedrock“ spectral shape put forward in Reports 3 and 4 appears to be a not unreasonable characterisation of the ground motion hazard for hard sites. Even for this purpose, however, some reservations concerning its reliability must inevitably persist until the existing doubts regarding the hazard modelling decisions have been resolved. (As well as issues that have been highlighted in this appraisal, these doubts arise from the current absence of some important items of information: no evidence has been provided in Report 4, for example, of the degree of variability around the UK of the calculated spectral shapes or of the extent of the differences in typical spectral ordinates at different confidence levels.)

- Even when all the hazard-related issues have been resolved, the use of this bedrock spectrum as the design spectrum for hard sites might be a cause for concern. Such a decision implies a de facto acceptance of uniform risk-type spectra for this purpose whereas, to-date, it has been more usual to define the design basis ground motions for new safety-critical structures in the form of normalised standard piecewise-linear spectra. (As is shown in Report 3, the proposed spectrum is markedly lower than the 10-4 p.a. ELOCS design spectrum which, it is understood, has previously been used around Norway and is still recommended by the latest relevant NORSOK Standard N-003.)

- For reasons that are discussed in Section 5 above, serious additional concerns are raised by the proposals made in Reports 3 and 4 for defining the design basis ground motion characterisation for soft sites. The combination of probabilism and selective determinism that is involved in the recommended approach, reinforced (again) by the current absence of crucial information and cryptic reporting, makes it difficult, for the present, to recommend acceptance of the proposed generic spectral envelope. (If use is made of only the upper-bound amplification factors, this appears, on present evidence, to be excessively conservative at low frequencies.)

- If HSE OSD are contemplating accepting an approach involving site response analyses for dealing with all soft sites (which definition, no doubt, covers the overwhelming majority of the sites of interest), it would be better, by far, to insist that such analyses be carried out on a site-specific basis.

- In the context of site response analyses, it needs to be pointed out that the traditional methods of computing site response are seismologically simplistic.

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Only vertically-propagating shear waves are considered, without reference to: their true angle of emergence; the potential for interference effects between up-going and down-going waves, or the contribution of surface waves (which are of some significance to all structures, but which are conventionally considered to be of most engineering concern to long period systems). These are but three of the many assumptions which have to be adopted in order to make response analyses tractable.

- Reports 3 and 4 make no suggestions for characterising ground motion, whether for understanding variations in hazard levels or for design purposes, in the different circumstances that prevail in the westernmost part of the offshore UKCS.

A major complication affecting any conclusions that can be reached regarding this element of the appraisal is the uncertain status of the spectrum suggested in the recent draft document issued by the ISO (and the fact that the existing hazard studies do not provide the data that would allow that spectrum to be depicted properly).

8.1.3 Vertical motion

Only one of the reports (Report 3) addresses the subject of vertical motion. Whilst the overall thrust of its recommendations reflects, in some measure, the view which is emerging from other sources, it is not clear that this view is shared by those responsible for its companion report (Report 4). If vertical motion is a serious concern in the design of offshore structures, it is in the interests of HSE OSD that this dilemma is resolved as soon as possible.

It is difficult to comment on the proposals made in this regard by Report 3 as they are based on research that has not been seen. However, although they appear, in matters of detail, to conflict with other recent guidelines and may well, ultimately, be shown to be over-conservative for British conditions, they call for the use of more severe vertical motions than is conventional in the British nuclear industry, for example.

As it is the case that insufficient attention has recently been paid to this topic in Britain, until further work is done the conservative decision is, clearly, to accept the recommendations made in Report 3, provided those recommendations have the support of the authors of Report 4.

8.1.4 Outstanding issues and their resolution

In the foregoing sections, certain topics relating to both policy and technical issues have been touched on which ought briefly to be revisited here.

Policy issues:

1. One policy issue of primary importance is whether it is sensible even to try to produce a map of earthquake ground motion hazard at annual exceedance probabilities as low as 10-4 p.a., particularly for offshore localities. (It may be noted that Report 1 recommends that site-specific studies should be carried

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out wherever the 2 x 10-2 p.a. probability of exceedance pga is not less than 3 %g and that some rather less specific remarks, along the same lines, are made in Report 2.)

Some of the technical arguments, both theoretical and practical, that are involved in this debate have been highlighted in Section 3.3 above. In truth, the policy that should be adopted in this regard depends on the importance that will be attached to 10-4 p.a. probability of exceedance hazard estimates in the design and/or assessment process for offshore structures. Whilst such results are all-important in the nuclear industry where it is the convention to call for site-specific hazard assessments, if, in the offshore industry, they are treated in a more relaxed manner as a desirable target, without such severe regulatory constraints, then some less stringent approach may be appropriate. For example, the use of generic hazard maps (albeit improved from the present ones) might be sanctioned for most sites with site-specific studies only being required under specified exceptional circumstances. Such circumstances would probably have to include:

- sites in the vicinity of major earthquakes (e.g. the 1931 North Sea event);

- sites where there are geological or geophysical anomalies nearby;

- sites where there may be potential for induced, i.e. triggered or provoked, earthquakes to occur (as, perhaps, exemplified by the 2001/05/07 Ekofisk event).

2. Clearly, policy issues need to be resolved concerning the need for reliable hazard estimates for the area of the UKCS which lies to the west of about 11oW. The point has already been made that a fair amount of work would be necessary before being able even to assess properly the estimates that have been made in Report 2. However, given the fact that serious commercial interest in this area has only recently emerged, it has to be anticipated that further fundamental preparatory work would, in any case, be necessary before hazard levels could be estimated to the degree of reliability that is appropriate for safety-critical offshore installations. Such work would need to consider the earthquake catalogue and its completeness thresholds, and the latest thinking on local seismotectonic conditions and correlations.

3. There seems to be a need to develop a properly-considered, coherent, policy regarding the most appropriate approach for dealing with soft sites. The recommendations of Reports 3 and 4 in this regard should properly be seen as relating to design basis ground motions, rather than ground motion hazard, and, whilst theirs may be considered an acceptable route to follow in that context, there has been no opportunity, within the scope of this appraisal, to explore in any detail the reliability of the site response analyses on which those recommendations depend. One of the factors that may have to be considered in formulating such a policy is the possible need for reconciliation with the draft ISO document.

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Technical issues:

1. One of the generic issues highlighted at the outset of this appraisal, was the fact, unrecognised in any of the existing hazard studies, that the concern here is not with conventional free-field conditions but, rather, with the ground motions at the seabed. This matter has received little attention in the discussion so far principally because, for the foreseeable future, it is unlikely that anything can be done to develop attenuation relations particularised for sites on the seabed. In any case, it may well be that the most appropriate, as well as the most practical, approach to addressing the uncertainties introduced by these special circumstances is to concentrate on the relationship between horizontal and vertical motion.

2. Another comment made early on in this appraisal concerned the apparent under-utilisation in the existing reports of geological or geophysical information, basically because appropriate methodologies for its proper employment did not seem to be in play. To be fair, part of the reason for this must relate to the passage of time that has elapsed since some of the reports were written (in recent years, much more detailed geological and geophysical information has become available and much more research has been carried out). This being the case, it is, perhaps, strange that Report 1 seems to contain the most coherent and closely argued geological basis for its source model, which is one that involves only a few zonal sources and no fault sources. (In the other reports, as the number of source zones expands, often beyond sustainability on seismological grounds, so the quoted geological constraints and pointers that are invoked to justify those zones appear to become more and more questionable - if not specious).

3. On the same theme of trying to understand the seismotectonics of the area, there does seem to be unanimity between the reports in considering that the underlying processes consist of interactions between crustal deformations driven by post-glacial rebound and by the current regional tectonics. It may be noted, however, that the assertion in Report 1 of aseismicity in the central North Sea is challenged in Report 2. Further light can only be shed on this last matter by developing the idea put forward here of drawing up properly considered default hazard maps.

4. It is recognised that a necessary bias in this discussion has resulted in insufficient attention being paid to the 10-2 p.a. probability of exceedance ground motion hazard which may well be of considerable importance to HSE OSD. Depending on the decisions which are made concerning the other current deficiencies in the available information on seismic hazards, it might be appropriate to correct this situation: only a small amount of further work would be required to bring the information on the higher probability hazard up to the same standard.

5. In order to establish formally the envelope of applicability for the recommended interim hazard levels, further work would be necessary (building on the methodology developed here for combining the results of different assessments).

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6. The issue of determining proper minimal hazard levels is important in any regulatory context, and needs more detailed consideration than has been possible here. A coherent and structured assessment of macroseismic and instrumental catalogue completeness across the whole of the offshore UKCS is required.

7. The present suggestions concerning spectral amplification factors for ground motion hazard should be seen as a provisional position: there is a need to develop a robust long-term position.

All of the above issues and suggestions emerge directly from material that has been discussed in this appraisal. To move forward on a more sophisticated basis, various other techniques are now available and, perhaps, should be considered. In this context, the following final suggestions are offered:

· an exercise using the latest techniques to examine the disaggregation of pga and spectral hazard (into magnitude, distance and attenuation factors) at a grid of locations across the UKCS area should be considered

· to avoid the limitations introduced by relying on questionable zonations, an evaluation of pga hazard using a zone-free model should be considered as a counterpart to the traditional Cornell-Maguire approach adopted in the four existing reports

· one point emerging from this appraisal is that the time may be considered appropriate for evaluating the conservatisms engendered by the use of the Gutenberg-Richter characterisation of seismicity (i.e. linearity of b-value and related issues)

· given the recent occurrence (in 2001) in the North Sea of an earthquake which is conjectured to have been induced by hydrocarbon extraction, it might be timely to put some effort into studying the general likelihood and potential severity of such phenomena across the whole of the offshore UKCS

Authors’ note

It is recognised as being almost unavoidable that the present document will be perceived as being critical of what has been done before. In general, this was not at all the intention, although due criticism is justified in instances where an existing hazard assessment report has not properly explained itself, for example. Rather, the main thrust of this appraisal has been to highlight issues where the use of expert judgement in the existing reports has resulted in decisions which, with hindsight, could be questioned. (Such questions might arise either as a result of comparisons with one or more of the other reports, or as a result of the existence of an option that has not been taken up in any of the reports.)

None of this should be seen as questioning the probity of any of the assessments that have been carried out: apart from anything else, it is not known what constraints were applied to those who were doing the work. When such large measures of expert judgement are involved, there are bound to be differences of opinion. The aim here has been to identify, from an independent perspective conditioned by experience in other safety-critical industries,

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which of those opinions are likely to have had an adverse effect on the reliability and defensibility of the hazard results.

Finally, it is recommended that, in any future seismic hazard study undertaken on behalf of HSE OSD, the actual calculated results be reported in toto: it should not be acceptable for such results to be presented solely in the form of smoothed contour maps.

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9 REFERENCES

9.1 THE REPORTS FORMING THE INPUT TO THIS APPRAISAL

BGS & Ove Arup and Partners (1997) UK continental shelf seismic hazard. Health and Safety Executive Offshore Technology Report OTH 93 416, HMSO, London; 103pp [REPORT 2]

EQE International Ltd. (2002) Seismic hazard - UK continental shelf. Health and Safety Executive Offshore Technology Report 2002/005; 70pp. [REPORT 4]

NORSAR & NGI (1998) Seismic zonation for Norway. Report for the Norwegian Council for Building Standardization; 162pp. [REPORT 3]

Principia Mechanica Ltd. (1986) North Sea seismicity. UK Dept. of Energy Offshore Technology Report No. OTH 86 219, HMSO, London; 73pp. [REPORT 1]

9.2 REFERENCES QUOTED IN MAIN TEXT

Ambraseys, N.N., Simpson, K.A. and Bommer, J.J. (1996) Prediction of horizontal response spectra in Europe. Earthq. Engng Struct. Dyn., 25, 371-400.

Ambraseys, N.N., Simpson, K.A. and Bommer, J.J. (1996) Prediction of vertical response spectra in Europe. Earthq. Engng Struct. Dyn., 25, 401-412.

Allen, C.R. (1995) Earthquake hazard assessment: has our approach been modified in the light of recent earthquakes? Earthquake Spectra, 11, 357-366.

API (1993) Recommended Practice for Planning, Design and Construction of Fixed Offshore Platforms - Load and Response Factor Design. American Petroleum Institute RP 2A-LRDF 1st Edition, July 1, 1993.

Ove Arup and Partners (1980) Earthquake effects on platforms and pipelines in the UK offshore area. Report 8878/DJD for UK Dept. of Energy under Contract E/5A/CON/596/375.

ASCE (1998) Seismic Analysis of Safety-related Nuclear Structures. American Society of Civil Engineers, Standard 4-98.

Atakan, K. and Havskov, J. (1996) Local site effects in northern North Sea based on single-station spectral ratios of OBS recordings. Terra Nova, 8, 22-33.

Boore, D.M. and Smith, C.E. (1999) Analysis of earthquake recordings obtained from the Seafloor Earthquake Measurement System (SEMS) instruments deployed off the coast of Southern California. Bull. Seism. Soc. Am., 89, 260-274.

Braunmiller, J., Ottemöller, L., Jensen, S.L., Ojeda, A. and Atakan, K. (2001) The May 7, 2001 Earthquake in the Ekofisk Area, North Sea. ORFEUS Newsletter, 3, 10.

66

Cooke, R.M. (1988) Uncertainty in risk assessment: a probabilist`s manifesto. Reliability Engng and System Safety, 23, 277-283.

Dahle, A., Bungum, H. and Kvamme, L.B. (1990) Attenuation models inferred from intraplate earthquake recordings. Earthq. Engng Struct. Dyn., 19, 1125-1141.

HSE (1992) Safety Assessment Principles for Nuclear Power Plants. Health and Safety Executive, HMSO.

IAEA (1991) Earthquakes and Associated Topics in Relation to Nuclear Power Plant Siting. Safety Series No. 50-SG-S1 (Rev. 1), International Atomic Energy Agency, Vienna.

Inoue, H. (1986) A least-squares smooth fitting for irregularly spaced data: finite-element approach using the cubic B-spline basis. Geophysics, 51, 2051-2066.

ISO (2001) Petroleum and natural gas industries — Specific requirements for offshore structures — Part 2: Seismic design procedures and criteria. ISO TC 67/SC 7 N 278A, Date: 2001-01-21; ISO/CD 19901-2 ISO TC 67/SC 7/WG 3; 41pp.

Lomnitz-Adler, J. and Lomnitz, C. (1979) A modified form of the Gutenberg-Richter magnitude-frequency relation. Bull. Seism. Soc. Am., 69, 1209-1214.

Mallard, D.J. (1993) Harmonising seismic hazard assessments for nuclear power plants. In: Proc. I.Mech.E. Conf. NPP Safety Standards: Towards International Harmonisation, I. Mech. E., London, 203-209.

Mallard, D.J., Higginbottom, I.E., Muir Wood, R. and Skipp, B.O. (1991) Recent developments in the methodology of seismic hazard assessment. In: Proc. I.C.E. Conf. Civil Engineering in the Nuclear Industry, Thomas Telford, London, 75-94.

Mallard, D.J. and Woo, G. (1991) The expression of faults in UK seismic hazard assessments. Quart. J. Engng Geol., 24, 347 œ 354.

Principia Mechanica Ltd. (1981) Seismic Ground Motions for UK Design. Report for BNFL and CEGB.

Principia Mechanica Ltd. (1982) British Earthquakes. Report No. 115/82 (for CEGB, SSEB and BNFL).

Principia Mechanica Ltd. (1988) UK Uniform Risk Spectra. Report No. 498/88 (for NNC Ltd.) with Addendum: Smoothing of UK Uniform Risk Spectra.

Toro, G. R., Abrahamson, N.A. and Schneider, J.F. (1997) Model of strong ground motions from earthquakes in central and eastern North America: best estimates and uncertainties. Seism. Res. Lett., 68, 41-57.

Vere-Jones, D. (1987) Statistical aspects of the analysis of historical earthquake catalogues. In (Margottini, C. and Serva, L., eds.) In: Proc. ENEA/IAEA Workshop on Historical Seismicity of the Central-Eastern Mediterranean Region, 271-295.

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9.3 REFERENCES QUOTED IN APPENDICES

Ambraseys, N.N., Simpson, K.A. and Bommer, J.J. (1996) Prediction of horizontal response spectra in Europe. Earthq. Engng Struct. Dyn., 25, 371-400.

Ambraseys, N.N., Simpson, K.A. and Bommer, J.J. (1996) Prediction of vertical response spectra in Europe. Earthq. Engng Struct. Dyn., 25, 401-412.


Dahle, A., Bungum, H. and Kvamme, L.B. (1990) Attenuation models inferred from intraplate earthquake recordings. Earthq. Engng Struct. Dyn., 19, 1125-1141.

Mosar, J., Lewis, G. and Torsvik, T.H. (2002) North Atlantic sea-floor spreading rates: implications for the Tertiary development of inversion structures of the Norwegian-Greenland Sea. J. Geol. Soc. London, 159, 503-515.

Murphy, J.R. and O`Brien, J.A. (1977) The correlation of peak ground acceleration amplitude with seismic intensity and other physical parameters. Bull. Seism. Soc. Am., 67, 877-915.

Toro, G.R., Abrahamson, N.A. and Schneider, J.F. (1997) Model of strong ground motions from earthquakes in central and eastern North America: best estimates and uncertainties. Seism. Res. Lett., 68, 41-57.

9.4 A BRIEF SELECTED BIBLIOGRAPHY

9.4.1 Seismic hazard and seismicity of the North Sea

Ambraseys, N.N. (1985) The seismicity of western Scandinavia. Earthq. Engng and Struct. Dyn., 13, 361-399.

Ambraseys, N. and Melville, C. (1983). Seismicity of the British Isles and the North Sea, Vol. 1. SERC Marine Technology Centre, Imperial College, London.


BGS (1990) Seismic monitoring of the North Sea. Health and Safety Executive Offshore Technology Report OTH 90 323, HMSO, London; 35pp.

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Bungum, H., Lindholm, C.D., Dahle, A., Woo, G., Nadim, F., Holme, J.K., Gudmestad, O.T., Hagberg, T. and Karthigeyan, K. (2000) New seismic zoning maps for Norway, the North Sea, and the United Kingdom. Seism. Res. Lett., 71, 687-697.

Bungum, H, Swearingen, P.H. and Woo, G. (1986) Earthquake hazard assessment in the North Sea. Phys. Earth Planet. Int., 44, 201-210.

Burton, P.W., Main, I.G. and Neilson, G. (1982) Seismic risk and the North Sea. Inst. Geol. Sci. Global Seism. Unit Report No. 163.

Dowrick, D.J. (1981) Earthquake risk and design ground motions in the UK offshore area. Paper 8428, Proc. I.C.E., 71, 305-321 (with discussion).

Kvamme, L.B., Dahle, A., Bungum, H. and Alsaker, A. (1990) Earthquake hazard and loading on the Norwegian continental shelf. In Proc 9th European Conf. Earthq. Engng, 1, 3-12.

Lindholm, C. and Havskov, J. (1989) A detailed study of the seismicity in the northern North Sea. Bergen, Norway, Seismological Observatory, University of Bergen.

Muir Wood, R., Woo, G. and Bungum, H. (1988) The history of earthquakes in the northern North Sea. In: Lee, W.H.K., Meyers, H. and Shimazaki, K. (eds) Historical Seismograms and Earthquakes of the World, Academic Press, San Diego, 297œ 306.

NORSAR and Risk Engineering Inc. (1988) Ground motions from earthquakes on the Norwegian Continental Shelf. Report for Den Norske Stats Oljeselskap a.s., Contract No. T.9704; 96pp.

Ringdal, F. (1983) Seismicity of the North Sea area. In: Ritsema, A. R. and Gürpinar, A. (eds) Proc. NATO Advanced Research Workshop Seismicity and Seismic Risk in the Offshore Area, Reidel, Dordrecht, 53-75.

Frode Ringdal, Eystein S. Husebye, Hilmar Bungum, Svein Mykkeltveit and Ottar A. Sandvin (1983) Earthquake hazard offshore Norway. A study for the NTNF Safety Offshore Committee.

Ritsema, A.R. (1981) Comments on the Ove Arup report Earthquake effects on platforms and pipelines in the UK offshore area. Royal Netherlands Meteorological Institute Report.

Ritsema, A.R. and Gürpinar, A. (eds) (1983) Seismicity and seismic risk in the offshore North Sea area. Proc. NATO Advanced Research Workshop Seismicity and Seismic Risk in the Offshore Area. Reidel, Dordrecht, 420pp.

Selnes, P.B. (1981) Geotechnical problems in offshore earthquake engineering. Norwegian Geotechnical institute Report 40009-6.

Selnes, P.B., Ringdal, F., Jakobsen, B., Hansteen, H. and Hove, K. (1983) Earthquake risk and design of structures offshore Norway. A study in the NTNF Offshore Safety Program.

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9.4.2 Seismic hazard and seismicity of Ireland

Principia Mechanica Ltd. (1986) Seismic Design Criteria for Dams in Ireland. Report for Electricty Supply Board of Ireland.

Richardson, M. (1975) Seismicity of Ireland. Civil Works Department, Electricity Supply Board of Ireland.

9.4.3 Seismic hazard maps of the British mainland

Ove Arup and Partners (1993) Earthquake Hazard and Risk in the UK. Report for UK DOE.

Building Research Establishment (1991) An Engineering Guide to Seismic Risk to Dams in the United Kingdom. Report BR 210.

Lilwall, R.C. (1976) Seismicity and Seismic Hazard in Britain. Inst. Geological Sciences Seismological Bulletin No. 4, HMSO, London.

Long, R.E. (1985) A ground motion probability analysis for Britain based on macroseismic earthquake data. In: Proc. SECED Conf. Earthquake Engineering in Britain, Thomas Telford, London, 169-179.

Musson, R.M.W. and Winter, P. (1996) Seismic Hazard of the UK. AEA Technology Report GNSR(DTI)/P/(96)196: AEA/16423643/Milestone BSH01, for UK DTI.

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10 APPENDIX 1: NOTES ON THE INDIVIDUAL REPORTS

10.1 REPORT 1 – PRINCIPIA MECHANICA LTD. (1986)

10.1.1 General

- this report was published in 1986 and summarised three years of work following up an earlier report (Ove Arup and Partners, 1980)

- all the material presented in this report falls within the compass of the present appraisal

- geographical coverage is North Sea only

- oldest of reports considered: lot of work done since on earthquakes (certainly on instrumental events and probably also on macroseismic events)

- this is the only report to place any real emphasis on crustal deformation trends with their obvious implications for the pattern of seismogenesis

- this area zone seismic source model is the simplest of those considered

- route by which pga attenuation relation is derived is somewhat tortuous and likely to have introduced larger than necessary uncertainties (also, no sigma value is quoted for the scatter about the final relation that has been adopted)

- provides no spectral results of any kind

- gives no description of contouring process used

-5- this report includes a continuous pga hazard curve (down to about 10 p.a.) for

one selected location in the northern North Sea

-2- recommends site-specific studies where the 2 x 10 p.a. ground motion is not less

than 3%g

10.1.2 Geological input

- a brief high level geological appraisal concludes that while isostatic uplift contributes to contemporary seismicity in coastal western Norway, the seismicity in the North Sea reflects long term tectonic processes with some Mesozoic faults in the North Sea still seismically active

- three seismotectonic provinces are identified: west Scandinavian, NW-SE "aseismic" zone from Scotland to Germany and England-Rhine

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- a model is proposed base on uplift and subsidence, consistent with the crustal deformation

- boundaries of uplift are seismic, centres of subsidence aseismic

- both post glacial unloading and late Tertiary and Quaternary regional uplift are invoked as driving processes

- there is substantial discussion of neotectonics and halokinesis is recognised

- the South Hewitt Fault, away from the influence of halokinesis, seems to show displacement of a few metres by normal faulting over the past 100000years

10.2 REPORT 2 – BGS & OVE ARUP AND PARTNERS (1997)

10.2.1 General

- given the scope of the present appraisal, Sections 7 (apart from sub-Section 7.3.1) and 8 are not considered here

- geographical coverage is whole of UK Continental Shelf Designated Area

- various detailed questions exist concerning some of the surface wave magnitude values assigned to catalogued events

- whereas all the other reports considered employ seismic hazard programs intended for site-specific application, this one uses a program specifically designed for hazard computation on a regular grid of points

- however, as it was designed not to use a priori a numerical seismicity model (e.g. Gutenberg-Richter) but, rather, a database of observed earthquakes, there are doubts about its application here, i.e. in a low seismicity region where such a model has to be used just to generate a catalogue of so-called `observed` earthquakes for input to the calculation

- also, it is not clear that the program used was intended to provide reliable results -4

down to 10 annual probability of exceedance (the examples quoted by the authors extend just to the —1000 year return period“)

- uniquely, this source model includes two fault-specific sources of seismicity (however, even though one of these is close to the coast, it appears to have little

-4discernible effect on even the 10 p.a. pga offshore hazard estimates)

- the zonation hardly justifies the claim that it has a seismotectonic basis: it is hard to accept, for example, that true seismotectonic arguments were involved in constructing some of the very large zones (or the very small zone which includes the 1931 North Sea earthquake)

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- the zonation raises a number of other concerns, especially as some areas are explicitly treated as —background“ sources which is a far from appropriate construct when hazard mapping

- the treatment of modelled depth distribution is obscure

- certain concerns exist about the treatment of locational uncertainty

- the pga attenuation relation used has a number of shortcomings (perhaps because of this, it appears that it is not now used by the Norwegians themselves) and the calculated scatter given by the actual database has been eschewed in favour of a poorly justified (arbitrary) curtailment

- gives no details of the grid over which the results have been generated or of the method of contouring

- the results are mapped in a slightly different way from the other reports: the presentation of these results is far from reader friendly

- no continuous hazard curves are presented

- spectral results (5% damping) are presented at three probability levels for five example locations (using manipulated values for the sigma values associated with the six frequency-specific attenuation relations used)

- for one of these locations, a comparison with the API RP 2A spectrum is presented

-4- advises the use of 10%g at 10 p.a. probability of exceedance, as a minimum,

everywhere

-4- recommends that 10 p.a. hazard maps showing 0.5, 1, 2 and 3 second period

motions be produced

- process for selecting the suite of real time histories is extremely poorly explained and raises a number of questions


- this report is notable in that it has extended its study across the UKCS to the Mid-Atlantic Ridge and to 65oN

- under the heading "Tectonics of the study area", a substantial high-level synopsis is given of the structures including transform faults which bound five of the identified source zones

- there is a description of three main crustal types: Oceanic (A); Laurentian-Hebridean Craton (B), and Rockall Grabens (C). Although there are no recorded

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earthquakes on these features, attention is drawn to passive continental margin A/B and the failed rift B/C as possible sites of large earthquakes in the future

- excluding the extreme northwest of Scotland, the British Isles are divided into four crustal divisions: Metamorphic Caledonides; Non-Metamorphic Caledonides; Variscides, and the London Brabant Massif: seismicity is not uniform within these divisions

- major identified structural features include: the Great Glen Fault; the Highland Boundary Fault; the Southern Uplands Fault, and the Iapetus Suture. Only the second of these corresponds with a boundary between the crustal divisions and only the last seems to have demonstrable seismic connotations, appearing to truncate the seismicity associated with the Pennine Ridge

- it is also noted that, whereas the western end of the Variscan Front in Wales seems active, this does not appear to be the case in southern England

- a correlation of seismicity in Scotland with the post-glacial uplift is remarked upon (see comments made above concerning Report 1 in relation to this topic)

- the study area extends into France and broaches upon: the Paris Basin; the Ardennes, and the Ruhr Graben

- for the North Sea, the report introduces an additional crustal type, namely, "North Sea Grabens", describing rifting, sediment deposition and crustal thinning and drawing attention to the activity in the Sole Pit Basin/Dowsing Fault Zone graben

- some attention is paid to Fennoscandia with mention being made of the West Viking Graben Boundary Fault: the comment is made that it is much easier to interpret Norwegian seismicity in terms of structure

- in developing its so-called —seismotectonic“ model and the constituent sources zones, the report annotates 31No. area source zones and "two pseudo faults" with brief notes referring to geological boundaries that have been taken into account in defining the geographical limits of those sources (within the —UK Area“, geology appears to be invoked in defining the boundaries of just three of the seven zones: for the —North Sea and Norwegian Coast“ area, only the Central Graben appears to be defined solely on geological grounds although geological considerations seem to enter into the defining of five of the remaining nine zones).

10.3 REPORT 3 – NORSAR & NGI (1998)

10.3.1 General

- this report describes a complex joint study with several objectives in mind including a contribution to the Structural Eurocode 8: it constitutes the —mother“ document for Report 4

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- given the scope of the present appraisal, where appropriate, consideration is here given to Sections 1 to 8 inclusive, but not, in any detail, to Section 9 or Appendix A

- geographical coverage is Norway and its surrounding seas

- the first in this sequence of reviewed reports to use the favoured logic-tree formulation for addressing uncertainties

- this said, no real explanation is given for the rationale behind the inclusion in the model of the two alternative zonations

- appears to be based on the definitive catalogue of historical Norwegian earthquakes

- this report is notable in the attention it pays to focal depths and faulting processes and its recognition of the significance of the tectonic regime

- further, this report contains several excursions into current scientific thinking, e.g. this is the only one of the four reports which even touches on fault plane solutions as indicators of the crustal stress regime (a newly-established database has been used to help effect zonation using such evidence) and as indicators of the likely earthquake focal mechanisms (which has an effect on ground motion levels)

- however, the incorporation of the latest scientific thinking does not necessarily enhance the reliability of the results of a hazard assessment: most significantly, the excursion into the use of moment magnitude is a major procedural departure for NW Europe

- this use of moment magnitude (which may or may not prove to be a landmark decision), in combination with two (weighted) alternative attenuation relations, one of which employs surface wave magnitude, raises a number of concerns

- eschews the sigma values actually calculated from the databases employed to derive the selected attenuation relation in favour of a three-value weighted distribution which is varied (arbitrarily [?]) according to the frequency under consideration but is taken to be the same for both relations

- while the matter is strictly outwith the scope of the present appraisal, the zonation of Norway involves quite a few very very small zones

-4 - continuous hazard curves (down to 10 p.a.) are presented for four sites at 10Hz

and 1Hz, as well as pga

-4- absolute 10 p.a. uniform hazard spectra (5% damping) for three confidence

levels are presented for two sites

- normalised uniform hazard spectral shapes at three probability levels for four sites are presented (without the confidence level being stated)

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- based on the close similarity of these twelve spectral shapes, a single normalised —bedrock“ spectral shape is proposed: this said, no clear definition is given as to what is meant by —bedrock“

- the basis on which this single spectral shape is then extended to frequencies lower than those covered by the selected attenuation relations, i.e. below 0.5Hz, raises some questions

- the extended normalised spectral shape is then compared with one from EC 8 and others from previous Norwegian studies relating to offshore design requirements

- amplification factors for soft sites, apparently derived from various site response analyses, are discussed in an Appendix but, overall, the report is indifferently written, and, as a result, opaque on this topic, particularly in the context of offshore applications. It would appear that some (unseen) 'standard' amplification factors proposed in 1997 by the Norwegian Petroleum Directorate are considered acceptable but possibly over-conservative at frequencies below 0.25Hz; site-specific soil response studies are recommended for very soft soil profiles

- this is the only report to include discussion of, and recommendations concerning, vertical motion: these recommendations are based on previous (unseen) research and raise a number of concerns

- following brief discussion of: (i) the computation of suitable spectra for other damping levels, and (ii) strong motion duration, a suite of three three-component real time histories is proposed for use in analysis - this important topic raises a number of issues


- geology and tectonics are given a high-level joint introduction in this report with 34 main structural elements being illustrated (in Fig 4.2.21)

- the assembly of the crust is presented, drawn mainly from various publications by Muir Wood

- late period Miocene can be characterised by the uplift of Fennoscandia and rapid subsidence of the continental margin. The development of local depocentres may have led to reactivation of faults at depth yet there is little evidence of faulting in the Quaternary coastal sediments

- offshore Norway is treated as consisting of: Southern Norway (Tornquist Zone, West Viking Graben Boundary Fault); Mid-Norway; Northern Norway, and the Barents Sea

- only the first of these areas is considered here (there is a discussion of the history of the important features in that area)

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- the significance of faulting for seismic hazard assessment is dealt with in some detail with references to current ideas about large earthquakes at continental margins and the correlation between post Miocene depocentres and seismicity along the Norwegian margin (this discussion does not extend to the north and northwest of the British Isles)

- it is argued that fault rupture triggered by small to medium magnitude earthquakes (<6) were associated with deeper (>10km) events. However the absence of folding of Tertiary and Quaternary strata suggests that, if large earthquakes have occurred in the past, then these are unlikely to have been on the known faults

- notwithstanding the value of this report as providing background information, it remains the case that the geological input into the zonation that is concern to the present appraisal only appears in Report 4.

10.4 REPORT 4 – EQE INTERNATIONAL LTD. (2002)

10.4.1 General

- this report apparently constitutes a "follow on" from Report 3, extending the Norwegian work into the UK sector with a large degree of harmonisation

- in assessing this report, only those few paragraphs dealing with site response are outwith the scope of the present appraisal

- geographically, claims to cover the UK Continental Shelf but the results extend out only to 11oW

- also uses the logic-tree formulation

- follows Report 3 in adopting two alternative zonations with no proper explanation of the need for this approach (or of the differences represented by the two)

- follows Report 3 in embarking on the use of moment magnitude (or surrogates thereof), a procedure which raises a number of queries concerning, in particular, the use of one of the two attenuation relations

- includes a known explosion (1971/07/20) in the earthquake catalogue

- unlike Report 3, makes no mention of the information which is nowadays available from the focal mechanisms that have been derived for British earthquakes (while the bulk this evidence, admittedly, relates to onland events, this is not exclusively the case)

- the distribution of, and weights assigned to, the various modelled activity rates is not stated

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- appears to depart from Report 3 in using a single (and different) b-value

- as a minimum, should have followed Report 3 in explaining that focal depth is not particularly significant to this assessment on account of the particular attenuation relations adopted: as it is, there is no mention whatsoever of focal depth

- eschews the sigma values actually calculated from the databases employed to derive the selected attenuation relation in favour of a three-value weighted distribution which is varied (arbitrarily [?]) according to the frequency under consideration but is taken to be the same for both relations

- no continuous hazard curves are presented

- unlike Report 3, gives no indication of the spread of its hazard results at different confidence levels

- presents normalised uniform hazard spectral shapes (at 5% critical damping), without specifying confidence levels, at three probability levels and follows Norwegian argument for adoption of a single normalised response spectrum shape (which is tabulated and which corresponds, more or less, to a uniform risk profile). Although this is far from clear in the wording of the report, it may be presumed that this is intended to correspond to a bedrock spectral shape but, as with Report 3, no explanation is given of what is meant by bedrock

- the selected spectral shape is extended as per Report 3 and then compared with the normalised PML (1981) hard ground spectrum and a normalised hard ground spectral shape derived using PML (1988), although, in the latter case, no explanation is provided as to what has actually been plotted (in terms of the source model involved, or the probability or confidence levels associated with the normalised shape that is shown)

- mentions, but does not illustrate, comparison with Eurocode spectrum and concludes by recommending same real time histories as Report 3

- in discussing soil amplification effects for soft sites, goes over some of the same ground as Report 3 but appears not to make any firm conclusions


- substantial amount of geological background of a rather high level nature is given, together with a cogent justification for doing so

- a map of the structural framework of the UK and the North Sea lists twent-two structural elements and the text addresses the events which have moulded them

- there is a substantial discussion of neotectonics drawing heavily upon the work in the UK of the Seismic Hazard Working Party and a small section on the significance of recent faulting (where it is noted that faults are subsumed into the selected area zone sources appearing in the hazard model)

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- the use of geological data in delineating zones is discussed and two zonations, which are not properly justified but which are said to harmonise with the two source models appearing in Report 3, are described

- the primary zonation has 38No. area zone sources of which 17 are annotated with significant geological comments.

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11 APPENDIX 2: ATTENUATION RELATIONSHIPS

11.1 REPORT 1 – PRINCIPIA MECHANICA LTD. (1986)

Relationship used: customised relationship, derived specifically for the report

Definition of hazard: larger of either recorded horizontal component peak accelerations (where given)

Form of attenuation relation: Ln(A) = C0 + C1 * M + C2 * Ln[R] + C3 * R (cm/s/s)

Magnitude scale: M is in terms of surface wave magnitude (Ms)

Range of magnitude validity: not indicated

Definition of distance variable: R is hypocentral distance (in km)

Range of distance validity: not indicated

Scatter: not indicated

Ground conditions: not specified

Remarks:

1] Peak acceleration attenuation relationship based on a macroseismic intensity attenuation relationship derived from local data, converted to peak acceleration using Murphy-O‘Brien [1977] correlation between intensity and peak acceleration

2] Results using this derived attenuation relationship appear to be indistinguishable from those given by PML [1982]

11.2 REPORT 2 – BGS AND OVE ARUP & PARTNERS (1997)

Relationship used: Dahle et al. [1990]

Definition of hazard: larger of either recorded horizontal component peak accelerations

Form of attenuation relation: Ln (A) = C0 + C1 * M + C2 * R + C3 * Ln[G(R)] (m/s/s)

where G(R) is a factor conditional on distance range

Magnitude scale: where available, M is in terms of surface wave magnitude (Ms), but data on other magnitude scales are used in the regression

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Range of magnitude validity: 3 Ms œ 7 Ms

Definition of distance variable: R is hypocentral distance (in km), adjusted by a control ²factor h (i.e. R = [Rhyp

2 + h2] )

Range of distance validity: 10 km œ 2000 km

Scatter: F = 0.83 on pga (but see remarks)

Ground conditions: rock sites only (data presumed applicable in regression when described as hard rock or firm ground)

Frequencies: six frequencies in the range 0.25 to 10Hz, plus ZPA

Remarks:

1] Dahle et al. [1990] provide regression coefficients for PSV at various frequencies, and the pga attenuation relationship appears to be based on an equivalence to 40Hz PSV results;

2] In terms of scatter on pga, BGS/Arup ignore calculated scatter quoted by Dahle et al. and adopt s = 0.65, following remarks by Bender & Perkins [1993], who advocate s = 0.6, thenadding 0.05 for ”conservatism‘

3] Different ground conditions are dealt with by BGS/Arup separately

11.3 REPORT 3 – NORSAR & NGI (1988)

Relationships used: Ambraseys et al. [1996] and Toro et al. [1997] used as equally-weighted alternatives in a logic-tree formulation

Ambraseys et al. [1996]:

Definition of hazard: larger of either recorded horizontal component peak accelerations

Form of attenuation relation: Log(A) = C0 + C1 * M + C2 * Log[R] + CASA + CSSS (in g)

where SA, SS are factors relating to ground condition

Magnitude scale: M is in terms of surface wave magnitude (Ms)

Range of magnitude validity: 4.0 Ms œ 7.5 Ms

Definition of distance variable: R is Joyner-Boore distance, RJB (distance to nearestprojection of fault to surface, where determined), or

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epicentral distance (in km), adjusted by a control factor h ²(i.e. R = [RJB

2 + h2] )

Range of distance validity: to 200 km

Scatter: s = 0.25 on pga (N.B. Log10 units - but see remarks below)

Ground conditions: four categories of ground condition defined, where available, by shear wave velocity measurements in upper 30m: i.e. rock (Vs>750m/s); stiff soil (360 m/s < Vs < 750 m/s); soft soil (180 m/s < Vs < 360 m/s), and very soft soil (Vs < 180 m/s)

Frequencies: forty-six frequencies in the range 0.5 to 10Hz, plus pga

Remarks:

1] Ambraseys et al. [1996] relationship used by NORSAR/NGI as empirical basis for attenuation

2] Instead of the published s value for this relationship, a weighted range is used by NORSAR/NGI (for both this attenuation relation and that of Toro et al.): 0.5 (wt = 0.3); 0.6 (wt = 0.4); 0.7 (wt = 0.3): not stated whether or not the Ambraseys et al. relationship is recast in terms of Ln

Toro et al. [1997]:

Definition of hazard: average of both horizontal component peak accelerations

Form of attenuation relation:

Ln(A) = C0 + C1*(M œ M0) + C2*(M œ M0)2 + C4*Ln[R] + (C5 - C6)*G(R) + C7*R (in g)

where G(R) is a factor conditional on distance range

Magnitude scale: M is in terms of moment magnitude (Mw) or Love wave magnitude (MLg)

Range of magnitude validity: 5 Mw œ 8 Mw

Definition of distance variable: R is Joyner-Boore distance, RJB (distance to nearest projection of fault to surface, where determined), or epicentral distance (in km), adjusted by a control factor h

²(i.e. R = [RJB2 + h2] )

Range of distance validity: 1 km to 500 km

Scatter: various combinations of estimates of epistemic and aleatory uncertainty are provided by Toro et al., with distance and/or

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magnitude and/or frequency dependences where appropriate (see also remarks)

Ground conditions: rock sites only but adjustment procedures suggested for softer conditions

Frequencies: seven frequencies in the range 0.5 to 35Hz, plus pga

Remarks:

1] Toro et al. [1997] relationship used by NORSAR/NGI as random vibration theoretical basis for attenuation: regression coefficients are provided for different areas of Central and Eastern U.S. (”mid-Continent‘, or ”Gulf‘), for different magnitude scales (Mw or MLg), and at different confidence levels. It is presumed that NORSAR/NGI adopted the combined mid-Continent / Mw / median values but this is not stated

2] NORSAR/NGI increase predictions of Toro et al. attenuation relationship by 13% to allow for difference between the average pga and the larger of the two horizontal component pga`s (following Campbell [1981])

3] Instead of the published s value(s), a range is used by NORSAR/NGI for Toro et al. attenuation relation (as with the Ambraseys et al. relation, see above): 0.5 (wt = 0.3); 0.6 (wt = 0.4); 0.7 (wt = 0.3)

4] NORSAR/NGI claim that the values assigned to earthquake magnitudes in the range of interest on the moment magnitude scale can be presumed to be numerically the same as those on the short-distance surface wave magnitude scale

11.4 REPORT 4 – EQE INTERNATIONAL LTD. (2002)

Uses same approach as Report 3, see Section 11.3 above.

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FIGURES

Figure 1 Report 1: replicated 10-2 p.a. pga contours

Figure 2 Report 1: replicated 10-4 p.a. pga contours

Figure 3a Report 2: replicated 10-2 p.a. pga contours (east of 10° W)

Figure 3b Report 2: replicated 10-2 p.a. pga contours (west of 1° W)

Figure 4a Report 2: replicated 10-4 p.a. pga contours (east of 10° W)

Figure 4b Report 2: replicated 10-4 p.a. pga contours (west of 1° W)

Figure 5 Report 4: replicated 10-2 p.a. pga contours (east of 10° W)

Figure 6 Report 4: replicated 10-4 p.a. pga contours (east of 10° W)

Figure 7 UK Continental Shelf Designations [2001] showing areas covered by existing reports

Figure 8 Hybrid 10-2 p.a. pga hazard contour map (east of 10° W)

Figure 9 Hybrid 10-4 p.a. pga hazard contour group map (east of 10° W)

Figure 10 Hybrid 10-4 p.a. pga hazard contour map compared with onland sitespecific hazard estimates

Figure 11 Macroseismic earthquake magnitude completeness thresholds in and around the British Isles

Figure 12 Minimal 10-4 p.a. pga hazard contour map determined from catalogue completeness considerations

Figure 13 Difference contours for hazards indicated by Figure 9 and Figure 12 (i.e. Figure 9 minus Figure 12)

Figure 14 Combination 10-4 p.a. pga hazard contour map (the hazard levels plotted are the higher of those indicated at each point by Figure 9 or

by Figure 12)

Figure 15 Comparison of PML [1981] spectral shapes for hard and soft sites

Figure 16 Comparison of various spectral shapes for bedrock/hard sites

Figure 17 Comparison of various spectral shapes for soft sites

Figure 18 Difference contours for hazards indicated by Figure 14 and Figure 6 (i.e. Figure 14 minus Figure 6)

Figure 19 Recommended 10-4 p.a. pga hazard contour map (the hazard levels plotted are the higher of those indicated at each point by figure 14

or by figure 6)

ISBN 0-7176-2778-0

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