exploring methods to assess paleoliquefaction in the canterbury area

GNS Science Consultancy Report 2014/183October 2014

Exploring methods to assess paleoliquefactionin the Canterbury area

P. VillamorR. LangridgeP. BarkerJ. Howarth

M. Giona-BucciK. ClarkF. MartinM. Quigley

P. AlmondW. RiesS. Bastin

M. TuttleM. VandergoesM. Watson

Project Number 430W1483-00

DISCLAIMER

This report has been prepared by the Institute of Geological and Nuclear Sciences Limited (GNS Science) exclusively for and under contract to EQC. Unless otherwise agreed in writing by GNS Science, GNS Science accepts no responsibility for any use of or reliance on any contents of this Report by any person other than EQC and shall not be liable to any person other than EQC, on any ground, for any loss, damage or expense arising from such use or reliance.

The data presented in this Report are available to GNS Science for other use from publication.

BIBLIOGRAPHIC REFERENCE

Villamor, P.; Giona-Bucci, M.; Almond, P.; Tuttle, M.; Langridge, R.; Clark, K.; Ries, W.; Vandergoes, M.; Barker, P.; Martin, F.; Bastin, S.; Watson, M.; Howarth, J.; Quigley, M. 2014. Exploring methods to assess paleoliquefaction in the Canterbury area, GNS Science Consultancy Report 2014/183. 142 p.

Peter Almond and Monica Giona-Bucci, Department of Soil and Physical Sciences, Faculty of Agriculture and Life Sciences, PO Box 85084, Lincoln University, Lincoln 7647, Canterbury

Martitia Tuttle, M. Tuttle & Associates, 128 Tibbetts Lane, Georgetown, ME 04548, USA

Fidel Martin, Departamento de Biología y Geología, ESCET- Departamental 2, Universidad Rey Juan Carlos, C/ Tulipán s/n, 28933 Mostoles, Madrid, SPAIN

Mark Quigley and Sarah Bastin, Department of Geological Sciences,University of Canterbury, Private Bag 4800, Christchurch

Matt Watson, ScanTec Ltd, PO Box 999, Whangarei, New Zealand

P. Villamor; R. Langridge; K. Clark; W. Ries; M. Vandergoes; P. Barker; J. Howarth, GNS, PO Box 30368, Lower Hutt 5040, New Zealand

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CONTENTS

EXECUTIVE SUMMARY ....................................................................................................... V

1.0 INTRODUCTION ........................................................................................................ 1

1.1 OBJECTIVES ...................................................................................................... 1 1.2 BACKGROUND AND MOTIVATION FOR THE STUDY ................................................. 2 1.3 SCOPE OF WORK ............................................................................................... 5 1.4 SELECTION OF STUDY SITES ............................................................................... 8

2.0 METHODS .................................................................................................................. 9

2.1 EXAMINATION AND INTERPRETATION OF AERIAL PHOTOGRAPHY AND LIDAR FOR MAPPING OF THE GEOMORPHOLOGY AND LIQUEFACTION EJECTA ................... 9

2.2 EXCAVATION OF TRENCHES TO STUDY LIQUEFACTION FEATURES ........................ 10 2.3 HAND PISTON CORING, CORE SCANNING AND X-RAY IMAGING ............................ 11 2.4 RADIOCARBON DATING OF SEDIMENTS .............................................................. 14 2.5 GROUND PENETRATING RADAR......................................................................... 15

3.0 THE EFFECTIVENESS OF SELECTED EXPLORATORY METHODS TO ASSES PALEOLIQUEFACTION IN THE CANTERBURY AREA ............................. 17

3.1 GEOMORPHIC MAPPING ................................................................................... 17 3.1.1 Literature review on the geomorphology and sedimentary environments

of the Canterbury plains ...................................................................................17 3.1.2 Correlation between existing geomorphic maps and the occurrence of

liquefaction ejecta in the Canterbury Plains .....................................................20 3.1.3 Detailed geomorphic map and association between 2010–2011

liquefaction and geomorphic features in the Lincoln area ...............................21 3.1.4 Results: How effective is detailed geomorphic mapping at identifying

sites that may have undergone paleo-liquefaction? ........................................23 3.2 REVIEW OF OLD AERIAL PHOTOGRAPHY ............................................................ 23

3.2.1 Results: How effective is aerial photo review in identifying sites that may have undergone paleoliquefaction? .................................................................24

3.3 PALEOSEISMIC TRENCHING .............................................................................. 24 3.3.1 Types and sizes of liquefaction features observed in paleoseismic

trenches at the Hardwick and Marchand sites .................................................24 3.3.2 Results: how effective is paleoseismic trenching to identify and

characterise paleoliquefaction features? .........................................................29 3.4 SHALLOW GEOPHYSICS: GROUND PENETRATING RADAR (GPR) .......................... 30

3.4.1 Can GPR image the 2010–2011 liquefaction features? ...................................30 3.4.2 Result: how effective is GPR to identify and characterise

paleoliquefaction features? ..............................................................................33 3.5 HAND PISTON CORING ..................................................................................... 33

3.5.1 Can paleoliquefaction features be observed in the cores? ..............................34 3.5.2 Can the source layers of liquefaction features be identified in the cores? .......37 3.5.3 Results: how effective is coring for identifying and characterising

paleoliquefaction features? ..............................................................................37

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4.0 AGE OF PALEOLIQUEFACTION FEATURES AT LINCOLN, CHRISTCHURCH AND CORRELATION WITH KNOWN PALEO-EARTHQUAKES .............................. 39

4.1.1 Age of paleoliquefaction features .....................................................................39 4.1.2 Correlation of paleoliquefaction features with known paleoearthquakes .........41

5.0 CONCLUSIONS ....................................................................................................... 43

6.0 RECOMMENDATIONS: FUTURE RESEARCH ....................................................... 45

7.0 ACKNOWLEDGEMENTS ......................................................................................... 47

8.0 REFERENCES ......................................................................................................... 47

FIGURES

Figure 1.1 A, Active faults of the central part of the South Island .................................................................. 3 Figure 2.1 Examples of: A, DEM from LiDAR data (blue = low and red = high elevation); B,

orthophotos; and C, old aerial photos at the same site. ............................................................. 10 Figure 2.2 HWK 6 trench at the Hardwick site. ............................................................................................ 11 Figure 2.3 Photo of piston coring with a 5 cm diameter round-rod piston corer, fitted with a core

catcher for water saturated loose sands. .................................................................................... 12 Figure 2.4 Multi-sensor core logger (MSCL) at the University of Otago. ..................................................... 14 Figure 2.5 GPR systems used to image liquefaction features: Upper photo, MALÅ Ground

Penetrating Radar system (250 MHz antenna); lower photo, a GSSI SIR10A+ Ground Penetrating Radar system (400 MHz antenna). ......................................................................... 16

Figure 3.1 A, Summary of the main landforms that occur in the Canterbury Plains (modified from David Barrell, Pers. Comm.) B, Detailed map of the study area. ................................................ 18

Figure 3.2 Cross-section through the northern portion of the Canterbury Plains near Christchurch to the edge of the continental shelf showing the stratigraphy of alternating lowstand fluvial gravels and sands, and highstand sand, silt, clay, and peat ...................................................... 19

Figure 3.3 A, Detailed geomorphic map of the study area and B, location of liquefaction ejecta from 2010–2011 events on a LiDAR DEM basemap. ......................................................................... 22

Figure 3.4 Examples of 2010–2011 liquefaction features. ........................................................................... 25 Figure 3.5 Comparative plots of dike and sand blow sizes for the 2010–2011 liquefaction and for the

paleoliquefaction features at the Hardwick and Marchand sites. ................................................ 27 Figure 3.6 Details of a MAR 3 trench wall. Note the similarity in the irregular-shaped dike margins

for the 2010–2011 and the paleoliquefaction dikes. ................................................................... 29 Figure 3.7 Example of GPR profiles and their correlation to liquefaction features. ...................................... 32 Figure 3.8 Photographs of selected hand piston core samples and their corresponding X-ray

images. ....................................................................................................................................... 35 Figure 4.1 Paleoliquefaction sand blow and dikes observed in the HWK 6 trench. ..................................... 40 Figure 4.2 Paleoliquefaction observed in the MAR 3 trench. Note dikes appear subhorizontal in the

north trench wall because the wall is subparallel to the dike strikes (Appendix 3). ..................... 40 Figure 4.3 Timing of major historic earthquakes, prehistoric fault ruptures and large landslides in the

wider region. ............................................................................................................................... 42

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TABLES

Table 3.1 Main Landforms on the Canterbury Plains: proportion of total area (%). .................................... 20 Table 3.2 Contribution of each landform to the total liquefied area, not considering natural water

bodies, Banks Peninsula and Lake Ellesmere. ........................................................................... 21

APPENDICES

APPENDIX 1: GEOMORPHIC AND PALEOSEISMIC TRENCH ANALYSIS AT HARDWICK AND MARCHAND SITES ....................................................... 59

A1.1 GEOMORPHIC MAP .......................................................................................... 59 A1.2 THE HARDWICK SITE ........................................................................................ 61

A1.2.1 Geomorphology of the site and the surface expression of the 2010–2011 liquefaction ...................................................................................................... 61

A1.2.2 2010–2011 Liquefaction and paleoliquefaction features exposed in the trenches .......................................................................................................... 62

A1.3 THE MARCHAND SITE ....................................................................................... 72 A1.3.1 Geomorphology of the site and the surface expression of the 2010–2011

liquefaction ...................................................................................................... 72 A1.3.2 2010–2011 Liquefaction and paleoliquefaction features exposed in the

trenches .......................................................................................................... 73 A1.4 THE SOURCE OF LIQUEFIED EJECTA AND ITS ASSOCIATION WITH

GEOMORPHIC FEATURES .................................................................................. 78 A1.5 REFERENCES .................................................................................................. 80

APPENDIX 2: TRENCH LOGS AND SEDIMENTARY UNIT DESCRIPTIONS .................. 81

A2.1 HARDWICK (HWK) 1 TRENCH WEST WALL ........................................................ 81 A2.2 HARDWICK (HWK) 2 TRENCH ........................................................................... 83 A2.3 HARDWICK (HWK) 3 TRENCH WEST (W) AND EAST (E) WALLS ........................... 85 A2.4 HARDWICK (HWK) 4A TRENCH ........................................................................ 88 A2.5 HARDWICK (HWK) 4BW TRENCH WEST WALL ................................................... 89 A2.6 HARDWICK (HWK) 5 TRENCH , EAST (E, E1, E2A, E2B) AND WEST (W)

WALLS ............................................................................................................ 93 A2.7 HARDWICK (HWK) 6 TRENCH, WEST (W) AND EASY (E) WALLS ......................... 97 A2.8 MARCHAND (MAR) 1 TRENCH, NORTHEAST (NE) AND SOUTHWEST (SW)

WALLS .......................................................................................................... 101 A2.9 MARCHAND (MAR) 3 TRENCH, WEST, NORTH AND EAST WALLS ...................... 105 A2.10 MARCHAND (MAR) 4 TRENCH, NORTH (N) AND SOUTH (S) WALLS ................... 108

APPENDIX 3: PARAMETERS OF LIQUEFACTION FEATURES .................................... 111

A3.1 SAND BLOW AND DIKE SIZES ........................................................................... 111 A3.2 DIKE STRIKES ................................................................................................ 113 A3.3 DIKE STRIKES-STEREONETS ........................................................................... 115

APPENDIX 4: GROUND PENETRATING RADAR .......................................................... 117

APPENDIX 5: CORES ..................................................................................................... 121

APPENDIX 6: RADIOCARBON DATES AND OXCAL ANALYSIS ................................. 139

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APPENDIX FIGURES

Figure A1 A, Orthophoto and B, DEM from LiDAR data of the study area, collected after the 2010–2011 liquefaction events. ............................................................................................................ 59

Figure A2 A, Geomorphic map of the wider study area. B, DEM from LiDAR with the locations of the 2010–2011 liquefaction sand blows (black polygons). ......................................................... 60

Figure A3 A, Orthophoto and B, DEM from the LiDAR data of the Hardwick site showing the location of the trenches. ............................................................................................................. 61

Figure A4 Photographs of sand blows at Hardwick site taken on September 4 2010 ~12 hours after the Darfield Earthquake. ............................................................................................................. 62

Figure A5 HWK 6 trench west wall.............................................................................................................. 65 Figure A6 HWK 6 trench east wall. ............................................................................................................. 66 Figure A7 Comparative plots of dike and sand blows sizes for the 2010–2011 liquefaction and for

the paleoliquefaction features observed at the Hardwick and Marchand trenches. .................... 67 Figure A8 HWK 5 trench east wall. ............................................................................................................. 69 Figure A9 HWK 5 trench west wall.............................................................................................................. 69 Figure A10 HWK 3 trench west wall.............................................................................................................. 71 Figure A11 A, Orthophoto and B, DEM from LiDAR data of the Marchand site showing the location

of the trenches. ........................................................................................................................... 72 Figure A12 MAR 3 trench combined west-north-east walls. ......................................................................... 74 Figure A13 MAR 4 trench east wall............................................................................................................... 76 Figure A14 Comparison of particle size with thresholds for liquefiable sands based on Tsuchida

(1970). ........................................................................................................................................ 78

APPENDIX TABLES

Table A6.1 Radiocarbon Dates ................................................................................................................... 139

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EXECUTIVE SUMMARY

Extensive liquefaction occurred during the Canterbury Earthquake Sequence (2010–2012) in New Zealand. GNS Science in conjunction with Lincoln University and Tuttle and Associates, USA, proposed the following study to exploit the wealth of information provided by this event on the effects of earthquake-induced liquefaction and to characterise the sedimentary environments prone to liquefaction. The study has been funded by the Earthquake Commission and GNS Science Strategic Development Funds.

The overall aim of this project is to determine if paleoliquefaction features are preserved in the sedimentary environments of the Christchurch area and, if so, to estimate the age of those features and thus the causative earthquake(s). By studying the 2010–2011 liquefaction features with a range of methods that have been successfully applied elsewhere, we aimed to determine which of these methods perform best at identifying and dating paleoliquefaction features in the Christchurch region.

The methods and disciplines tested here were: a) geomorphic mapping and analysis of 1940s, 1960s, and modern aerial photographs, satellite images and high precision (decimetre) Light Detecting and Ranging (LiDAR) topographic data; b) conduct Ground Penetrating Radar (GPR) surveys; c) trenching the 2010–2011 liquefaction features; d) hand piston coring; e) analysis of cores using high resolution photography and X-ray; and f) radiocarbon dating.

Trenching and coring of liquefaction features that formed during the 2010–2011 events at two locations in Lincoln (Hardwick and Marchand sites) together with geomorphic mapping resulted in a wealth of information, much of which was analysed within this study. The main results from that exercise are: • Comparison of liquefaction and landform maps suggest liquefaction in the 2010–2012

Canterbury Earthquake Sequence mainly occurred in association with alluvial and sedimentary environments that are younger than ~2500 years in the Canterbury area. Liquefaction occurred in association with point bar deposits at the Marchand site and in association with crevasse splay deposits (although the liquefied sand was sourced from deeper sediments) at the Hardwick site.

• The sizes of the 2010–2011 liquefaction features are similar in the Hardwick and Marchand sites, with sand blows ranging 10 to 40 cm in height and 200 to 400 cm in width, and dike widths range from 1 to 3 cm. These features are of a scale similar to those in eastern Christchurch City and to other international examples such as the M 5.9 1988 Saguenay, Quebec, Canada earthquake. In contrast, sand blows found in the New Madrid Seismic Zone, eastern USA, (earthquakes typically of Mw 6.8–8) are commonly 1–2 m high, 10s of metres wide, 10s–100s of metres long, and associated with sand dikes 10s–100s of centimetres wide. Apart from the characteristics of the ground shaking (Peak Ground Acceleration -PGA-, frequency content, and shaking duration), other factors that appear to control the size of liquefaction features are the liquefaction susceptibility of the sediments and the size of liquefiable sediment bodies (e.g., smaller point bars in rivers in Christchurch region than in New Madrid).

• At the Hardwick and Marchand sites we dated paleoliquefaction features to between AD 1019 and AD 1337. Large earthquakes on the Alpine Fault and Porters Pass Fault occurred during the same time span as the paleoliquefaction event observed at the

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Hardwick and Marchand study sites. Other nearby known and unknown regional faults could also be earthquake sources responsible for the paleoliquefaction documented at these sites.

With respect to advantages and disadvantages of each technique tested during this project we conclude: • Geomorphic mapping, aided by Digital Elevation Models (DEMs) developed from

LiDAR data, is useful to select areas, such as point bar environments, that may be prone to liquefaction. High resolution DEMs and aerial photographs are required to identify these subtle geomorphic features in areas of very low relief.

• In the Canterbury region, analysis of 1940s and 1960s aerial photos was useful in identifying areas with sandy soils (well drained, hence lighter surfaces) that may be indicative of paleoliquefaction features or sand blows.

• Paleoseismic trenching is the best technique for identifying, measuring, and dating paleoliquefaction features, but can be limited by high water tables (e.g., water tables are often within 1.5 m of the surface in the Christchurch region). The depth to which trenches can be excavated limits the stratigraphic record exposed in the trench.

• GPR profiling is a useful technique for non-invasive reconnaissance of sites, but currently is not able to resolve sand dikes ~<10 cm wide and sand blows ~<10 cm thick (depending on ground and surface conditions). The GPR signal is also limited by the depth of the water table.

• Hand piston coring is a useful technique for identifying some liquefaction features, such as high-angle dipping sand dikes containing rip-up clasts and possible liquefaction source beds exhibiting soft-sediment deformation structures. However, within core samples it is difficult to distinguish liquefaction-related sand blows and sills from sand layers deposited by fluvial processes.

• X-ray analysis of core samples is a promising technique for identifying flow structures within sand dikes and soft-sediment deformation. However, the X-ray scanning resolution used in this study was not of high enough resolution to detect features that could suggest liquefaction.

On-going studies will also look at the effectiveness of using other techniques such as: i) high resolution X-ray; ii) Computed Tomography (CT) scanning; iii) other physical properties of cores; and iv) grain magnetisation orientation. We will also continue investigating the correlation between certain fluvial geomorphic elements (point bars, crevasse splays, etc.) and presence of liquefaction, and will look into other sedimentary environments (e.g., coastal sands).

We recommend taking advantage of other 2010–2011 liquefaction sites search for paleoliquefaction features, given that in our study paleoliquefaction features have been found in the same locations that liquefied during the Canterbury Earthquake Sequence. Additional paleoliquefaction studies will help to further evaluate the timing, source area, and magnitude of the paleoearthquake that induced liquefaction between AD 1019 and AD 1337, and to develop the paleoearthquake record in the Christchurch region.

Finally, we recommend that a national database of liquefaction features (historic and paleo-features) be created to help understand ground shaking levels and recurrence, and liquefaction recurrence in areas of New Zealand with liquefaction susceptible soils.

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1.0 INTRODUCTION

Extensive liquefaction occurred during the Canterbury Earthquake Sequence (2010–2012) in New Zealand. GNS Science in conjunction with Lincoln University and Tuttle and Associates, USA, proposed the following study to exploit the wealth of information provided by this event on the effects of earthquake-induced liquefaction and the sedimentary environments prone to liquefaction. The study has been funded by Earthquake Commission (EQC) and GNS Science Strategic Development Funds (SDF).

1.1 OBJECTIVES

The overall aim of this project was to determine if paleoliquefaction features are preserved in the sedimentary environments of the Christchurch area and, if so, to estimate the age of those features and thus the causative earthquake(s). By studying the 2010–2011 liquefaction features with a range of methods that have been successfully applied elsewhere, we aimed to determine which of these methods perform best at identifying and dating paleoliquefaction features in the Christchurch region.

The variety of liquefaction features and related ground failures (sand volcanoes, blisters, sand dikes, seismites, lateral displacements, local subsidence and uplift, etc.) resulting from the various of the 2010 and 2011 earthquakes are used to characterise the types of paleoliquefaction features and related ground failures that formed in different sedimentary environments (e.g., floodplain, fan delta, estuary). Based on our analysis of modern liquefaction, we undertook paleoliquefaction studies in selected areas with well-established methods that are best suited to each particular environment.

The successful application of the paleoliquefaction methodology is providing:

• A strategy to compile a paleoliquefaction record of the Canterbury region. This record will in turn help develop an understanding of the relationship between paleoearthquakes, as revealed in paleoseismic studies, and the occurrence of liquefaction in the Christchurch area. The understanding and data gained will be used to calibrate current earthquake hazard models of the area.

• A record of paleoliquefaction events in areas where the surface expression of active fault scarps is obscured by high erosion or high deposition rates. This record can provide information on the paleoearthquake record of the region and help calibrate the definition of the maximum magnitude of earthquakes in the distributed seismicity component of seismic hazard models.

• A tool to assess previous liquefaction events in areas of future development. In the Canterbury region, development of new urban areas will occur as a consequence of the liquefaction-induced damage and the need for relocation of whole city suburbs as a consequence of the Canterbury Earthquake Sequence. Current projects funded by Local Authorities and the Ministry of Business, Innovation and Employment (MBIE) will produce liquefaction probability maps of the region to help with the relocation and re-building efforts. If any of the methods tested in this project are successful, and a paleoliquefaction record for the Canterbury area is obtained in future studies, the probabilistic liquefaction maps can be calibrated with the pre-historic record of liquefaction, thereby increasing the reliability of the maps. Also, dating

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paleoliquefaction features will help to constrain the recurrence times of earthquakes in the region. This type of study can also be extended to other similar sedimentary and geomorphic settings in New Zealand.

1.2 BACKGROUND AND MOTIVATION FOR THE STUDY

During ground shaking, pore-water pressure builds up in loose, water-saturated, sandy sediment. If the pore-water pressure exceeds the overburden pressure, the sediment liquefies and can be mobilised, flowing through cracks, fissures and other zones of weakness towards the ground surface (e.g., Seed and Idriss, 1982; Youd, 1984; Tuttle, 2001). This phenomenon is capable of producing a large amount of damage to the built environment, as shown in the recent Canterbury Earthquake Sequence (e.g., Cubrinosky et al., 2011). These deformation features can be preserved in the landscape for tens to thousands of years and can thus be used as evidence for past earthquake-induced strong ground shaking.

Paleoliquefaction studies have been used widely to develop the paleoearthquake record in areas of the central and eastern US and south eastern Canada. A prehistoric record of large earthquakes has been developed for the New Madrid seismic zone, eastern US, on the basis of paleoliquefaction features. At least three series of clustered events similar to the 1811– 1812 New Madrid earthquakes have been identified during the past 4,500 years (e.g., Tuttle et al., 2002, 2005). Paleoliquefaction studies have postulated that several large paleoearthquakes between 4.5 and 10.1 ka were centred southwest of the New Madrid seismic zone near the southern end of the Reelfoot Rift (Tuttle et al., 2006; Al-Shukri et al., 2009). In the Wabash Valley seismic zone in the central US (e.g., Obermeier et al., 1991; Munson and Munson, 1996), in the Charleston seismic zone in the southeastern US (e.g., Talwani and Schaeffer, 2001), and in the Charlevoix seismic zone in southeastern Canada (Tuttle and Atkinson, 2010), paleoliquefaction studies have extended the historical record of earthquakes by 5–12 kyr. Results of paleoliquefaction studies have been used to develop national seismic hazard maps (Petersen et al., 2008) and have been incorporated into a paleoliquefaction database that was used to characterise seismic sources in the central and eastern US (Tuttle and Hartleb, 2012; EPRI, 2012). Other regions where paleoliquefaction features have been identified and used to assess earthquake hazard include the Himalayan front (Sukhija et al., 1999), southeastern Spain (Alfaro et al., 2001) and Cascadia in western US and Canada (Atwater et al., 1995; Clague et al., 1997), among others.

Liquefaction induced by the 4 September 2010 Darfield and the 22 February 2011 Christchurch earthquakes and other aftershocks (Figure 1.1) produced sand volcanoes or blows and related ground failures, including fissures, lateral displacements, subsidence and uplift, on the order of tens of centimetres to meters, and concentrations of sand blow over large areas (from individual sand blows to sand blow fields up to ~1 km2, within a wider area of 40 x 20 km2) (Cubrinovski & Green, 2010; Cubrinovski et al., 2011; Ward et al., 2011; Almond et al., 2012; Brackley, 2012; Kaiser et al., 2012; Reid et al., 2012; Bastin et al., 2013, in review; Quigley et al., 2013). As many as ten distinct liquefaction episodes were reported from some highly susceptible sites in eastern Christchurch (Quigley et al., 2013). Liquefaction-induced lateral spreading and subsidence, exceeding tens of cm, increased flood and marine inundation hazard in parts of eastern Christchurch (Hughes et al., 2015), highlighting the potential for liquefaction to generate longer term hazard cascades with large economic and societal implications.

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Figure 1.1 A, Active faults of the central part of the South Island (Active fault database, http://data.gns.cri.nz/af/; Forsyth et al., 2008; Barnes et al., 2011). B, General geology and active faults of the Canterbury Region. Black Stars are epicentres of historic earthquakes: 1. 1929 Arthur’s Pass (Mw 7.1), 2. 1888 Amuri (Mw 7.1), 3. 2010 Darfield (Mw 7.1), 4. 1870 Lake Ellesmere (Mw 5.8), 5. 1869 Christchurch (Mw 4.9), 6. February 2011 Christchurch (Mw 6.2),7. December 2011 (Mw 5.9), 8. June 2011 (Mw 6.0). Small black dots are our study sites (M, Marchand site; H, Hardwick site).

Five damaging historic earthquakes occurred within ~150 km of Christchurch between 1869 and 1922 (Downes & Yetton, 2012), however no liquefaction was reported in Christchurch following any of these events. Liquefaction was reported in the residential areas of Kaiapoi and Belfast, north of Christchurch (Figure 1.1) following the 1901 moment magnitude (Mw) 6.8 Cheviot earthquake (Berrill et al., 1994). MMI 7 shaking and infrastructure damage (including collapse of the Christchurch cathedral spire) was reported in the 1869 ~Mw 4.8 Christchurch earthquake (Downes & Yetton, 2012). Following this earthquake it was observed by a local resident that ‘the tide runs higher up the Heathcote River than formerly’, suggesting that settlement potentially induced by liquefaction may have occurred (Downes & Yetton, 2012). The 1870 ~Mw 5.7 Lake Ellesmere, 1888 ~Mw 7.2 Hope Fault, 1901 Cheviot, and 1922 ~Mw 6.4 Motunau, North Canterbury earthquakes all caused strong shaking and infrastructure damage in Christchurch and other areas of Canterbury (Downes & Yetton, 2012).

During the last 1,000 years, several large earthquakes, identified with paleoseismic studies, were caused by rupture of the Alpine, Hope, Poulter and Porters Pass faults (Figure 1.1; Alpine Fault: Wells et al., 1999; Sutherland et al., 2007; Berryman et al., 2012; Howarth et al., 2012; 2014; Hope Fault: Cowan and McGlone, 1991; Langridge et al., 2003; 2013; Khajavi et al., in review; Poulter Fault: Berryman and Villamor, 2004; Porters Pass Fault: Howard et al., 2005; Ashley Fault: Sisson et al., 2001). Some of those could have caused liquefaction in the Canterbury region. The approximate epicentral locations and shaking intensities generated in Christchurch for these events are poorly constrained given the broad constraints on earthquake Mw for these events.

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While paleoliquefaction studies have contributed to the understanding of earthquake potential in many other countries (see below), New Zealand researchers have not taken full advantage of paleoliquefaction as a tool to help constrain paleoearthquake records. Preliminary studies of paleoliquefaction in eastern Christchurch have provided evidence for pre-2010 liquefaction at post 1660 AD to pre ca. 1905 AD at one site, and post 1435 AD to pre ca. 1910 AD at another (Bastin et al., 2013; in review), although the interpretation of these paleoliquefaction events is limited by the absence of a precise chronology. On-fault paleoseismic studies can be hampered in settings where faults have not ruptured to the surface or are difficult to map due to burial or erosion, and where scarce recent Holocene sediments along the fault limit ability to date and measure displacements. Paleoliquefaction studies can provide information about the timing, source area, ground-shaking intensity and magnitude, and recurrence time of moderate to large earthquakes, if suitable sedimentary and ground water conditions existed at the time of the earthquakes and if adequate exposure is available to find and study the paleoliquefaction features. For example, paleoliquefaction studies in the central US determined that earthquake sequences including the M ≥7.6 earthquakes centred in the New Madrid seismic zone in AD 1450 ± 150 yr and in AD 900 ± 100 yr, have an average recurrence time of 500 years for the past 1,200 years (Tuttle et al., 2002). An earlier New Madrid-type event in BC 2350 ± 150 was also recognised from recent statistical assessment of the age data of paleoliquefaction features (Tuttle et al., 2005).

In New Zealand, the frequency of fault ruptures is such that often a good record is preserved on the fault trace despite the obscuring effects of landscape processes (erosion and deposition). This is why sound paleoearthquake records from on-fault paleoseismic data have been developed (e.g., Wellington Fault: Langridge et al., 2011; Wairau Fault: Zachariasen et al., 2006; Ohariu Fault: Litchfield et al., 2006; Wairarapa Fault: Little et al., 2009; Rangipo Fault: Villamor et al., 2007). For those faults in locations where on-fault studies are not appropriate, or for offshore faults, researchers have conducted off-fault paleoearthquake studies. For example, high deposition rates and thick vegetation impedes on-fault studies along most of the Alpine Fault. Off-fault efforts to develop paleoearthquake chronologies of the Alpine Fault include studies of earthquake-induced rock falls (Bull, 1996), landsliding and terrace aggradation (Yetton, 2000; Cullen et al., 2003; Wells et al., 1998, 1999), marine dune formation (Wells and Goff, 2007), sag pond sedimentary sequence alternation (Cooper and Norris, 1990; Berryman et al., 2012), and lake records (Howarth et al., 2012; 2014). Paleoearthquakes on offshore faults along the Hikurangi margin also have been constrained from off-fault studies of uplift and subsidence of the coast (Berryman et al., 2011; Cochran et al., 2006; Wilson et al., 2007) and river terraces (Litchfield et al., 2010). Mackey and Quigley (2014) used cosmogenic 3He dating of prehistoric rock falls that suggest that previous earthquakes of similar shaking intensity to that of the 22 February 2011 and 13 June 2011 Christchurch earthquakes could have occurred at ca. 8–6 ka, and possibly ca. 14–13 ka, and were likely sourced from active faults proximal to Christchurch.

Most of the off-fault paleoearthquake studies in New Zealand are undertaken with the intention of characterising a known, mapped fault. However, the 2010–2012 Canterbury Earthquake Sequence has shown that despite efforts to understand active faulting in New Zealand, rupture of hidden, but slow-moving, active faults can be as devastating as rupture of many of the well-known large, fast-moving faults. In New Zealand, there is clearly a need to utilize existing methods of studying records of strong ground shaking in areas of low to moderate deformation where active faults are scarce, slow, and/or hidden.

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1.3 SCOPE OF WORK

The original project proposal aimed to undertake the following tasks or approaches to assess the most appropriate method to identify and document paleoliquefaction in the Christchurch region:

1. Investigate different sedimentary environments within the Christchurch region to determine the past and current liquefaction potential. Most liquefaction in the 2010 and 2011 earthquakes was associated with silty and sandy sediment of the Christchurch Formation in former flood channels of the Waimakariri River and related tributaries (Almond et al., 2010; Bastin et al., in review) and in the Avon-Heathcote estuary (Reid et al., 2012) on the lower Canterbury Plains. Widespread liquefaction occurred along a former distributary channel of the Waimakariri River, now occupied by the Halswell River (Almond et al., 2010). Soil maps allow a correlation of the sediments in this paleochannel to a phase of sedimentation 700–2400 yr BP. This sediment package is old enough to have experienced at least one major episode of shaking from an earthquake on the Porters Pass Fault and likely, significant shaking from at least three Alpine Fault earthquakes. We chose the Lincoln area because of the good potential for preservation of paleoliquefaction and because of the presence of liquefaction susceptible sediments, which will allow us to test the techniques proposed under ideal conditions. Furthermore, the rural setting increases our chances of finding relatively undisturbed surface ground conditions, and our excavations will cause limited disruption.

In our study area and prior to this study, we had conducted a preliminary survey of the sediments using hand augering and Swedish Weight Sounding. Those results demonstrated the widespread occurrence of liquefaction susceptible sediments at depth, but a more limited occurrence of surface expression of liquefaction (sand blows, lateral spreading etc.). This appears to be related to: the presence of surface caps, i.e., thick layers of silt or clay that resist the stresses exerted by the liquefied sediment; variation in sediment density, which determines the potential for sediment to express the contractive behaviour characteristic of liquefiable materials; and/or the distance from the site to a free face such as a river bank. Our preliminary mapping in this area had also shown that areas of surface expression of liquefaction are associated with subtle topographic highs (0.5–1 m). This topography is likely related to depositional processes and units along paleochannels of the Waimakariri River.

In our search for paleoliquefaction the following items were proposed:

a. Review existing literature on the geomorphology and sedimentary environments of the area.

b. Analyse 1940s, 1960s, and modern aerial photographs, satellite images and high precision (decimetre) LIDAR topographic data to identify areas with surficial features related to modern and ancient sand blows, and identify environments similar to those that liquefied in 2010 and 2011.

c. Undertake field reconnaissance of riverbank exposures during period of low water levels and tides to search for paleoliquefaction features.

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2. Use the existing information on the 2010–2011 liquefaction features to identify sites for testing different methods to detect paleoliquefaction. As well as areas that liquefied in 2010 and 2011, we proposed to study sites that did not liquefy during the 2010–2011 events but that have signs of potential paleoliquefaction. We also proposed to search for possible signatures of earlier events. For this purpose we proposed to:

a. Conduct geophysical surveys at sites of modern (2010–2011) liquefaction. Geophysical surveying such as ground penetrating radar and/or resistivity will help to target sites for trenching, maximising results while minimising disturbance. With these tools we can map surficial sand blows and identify the location of related sand dikes. We need to limit our work to times when the water table is lowest (late summer) in order to maximise the efficacy of the geophysical techniques and minimise the problems associated with interference of groundwater with geophysical signals.

b. Trench promising targets of paleoliquefaction. If liquefaction has occurred, our trenches may expose both modern and ancient liquefaction features; the features that formed during the 2010 and 2011 earthquakes, whose locations and magnitudes are well known, will help to calibrate and interpret paleoliquefaction features. We expected to be able to identify buried sand volcanoes and sand dikes (feeder dikes of the sand volcanoes), injected sills, and possibly source layers of liquefied sediment if we are able to trench deep enough.

c. Where these methods are inappropriate, e.g., if the water table is too high, we proposed to use a set of closely spaced cores in an attempt to identify the same kinds of features.

d. Analyse sediments recovered for magnetisation orientation. We predict that liquefied sediment injected into overlying layers will be randomly magnetised (and hence lack a magnetisation direction), whereas the surrounding un-liquefied sediment will retain a detrital magnetisation direction inherited from the time of sedimentation.

e. Analyse cores using high resolution photography, X-ray and magnetic susceptibility in order to pick up subtle variations in sedimentary properties to augment visual assessment.

f. Radiocarbon date any collected buried organic material. The high water table conditions of the Canterbury Plains and episodic flood sedimentation have contributed to the burial and preservation of organic layers. If we can reliably identify paleoliquefaction, there is good potential for using radiocarbon dating of the buried organics collected during this project to constrain the timing of the prehistoric liquefaction events and estimate recurrence times of moderate to large earthquakes in the Christchurch region.

3. Analyse the data collected and assess preservation of paleoliquefaction in the Christchurch region. We proposed to digitise and analyse the trench logs, core logs, geophysical information and radiocarbon dates to assess the timing and severity of liquefaction. If these methods are successful, we should be able to identify and date pre-2010 liquefaction events.

4. Present results at a conference (4a) and compile final report (4b) with recommendations for future work.

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Given several circumstances such as: a) the numerous liquefaction studies that developed in parallel to the one presented here; b) the successful application by the authors for further funds for a PhD graduate fellowship (funded by the National Hazards Research Platform; student Monica Giona-Bucci) to continue this project; and c) the wealth of information obtained at the two selected sites in Lincoln, a few of the bullet points above were not undertaken (partially covered by other studies) or have been delayed (preliminary analysis will be reported here) for the PhD student to fully develop, These are:

• Analysis of estuary sedimentary environments in the Canterbury area (part of 1a) is covered by Reid et al. (2011) and Cochran et al., 2014 (EQC Grant).

• River bank exploration for paleoliquefaction features (1c) will be undertaken by PhD study for new liquefaction sites.

• Magnetisation orientation to identified sand that has liquefied (part of 2d) will be undertaken by the PhD study.

• Analysis of cores using high resolution photography, X-ray and magnetic susceptibility (2e). Although this information has been obtained, it has not been fully evaluated during this study. Full analysis of these data will be undertaken within the PhD study and only preliminary results are presented here.

In this report we present results from tasks (1) to (4) (except for those parts mentioned above). Note that from the datasets of this study, and in conjunction with Natural Hazards Research Platform Funding, we are currently producing a paper to be submitted to an international journal. Sections of that paper are presented in Appendix 1. While that publication has a slightly different focus to this report, it explains important aspects that can clarify or add to the goals of his study, and thus it is referred to in numerous occasions within this report. We recommend that Appendix 1 is read prior to Section 3 of this report to facilitate the understanding of the main results presented here. Also note that various datasets are presented in successive appendices.

Other outcomes of task 4 have been previously reported and are:

• Two conference presentations (poster and oral) to the 2012 Geological Society of America Annual Meeting in Charlotte, North Carolina (Almond et al., 2012; Tuttle et al., 2012).

• One conference presentation (oral) at NZ Geosciences Annual Meeting in Auckland (Villamor et al., 2012).

• Assistance with, and peer review of, other on-going paleoliquefaction studies in the Christchurch area (e.g., Bastin et al., 2013; in review).

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1.4 SELECTION OF STUDY SITES

The study area is located close to Lincoln township, 16 km to the southwest of Christchurch City. The area lies on a late Holocene, low-lying flood plain-delta complex formed by old distributary channels of the Waimakairiri River, which now flows to the Pacific Ocean via an outlet north of Christchurch (Figure 1.1). We have chosen the area of Lincoln, because:

a. liquefaction occurred here during both the Darfield (4 September 2010) and Christchurch (22 February 2011) earthquakes, and minor liquefaction occurred during the 13 June 2011 earthquake. The levels of shaking were different for each event in the area: a) the epicentre of the September 2010 Mw 7.1 earthquake was 29 km away (closest distance to the Greendale fault was ~15 km) and produced 0.44 g peak ground acceleration (PGA) in Lincoln (Cousins and McVerry, 2010); b) the epicentre of the February 2011 Mw 6.2 earthquake was 17 km away and generated 0.12 g PGA (Bradley et al., 2014); and c) the epicentre of the June 2011 earthquake was 22 km away and generated 0.06 g (Bradley et al., 2014);

b. the study sites are located at the same distance from the Greendale Fault (source for the 4 September 2010 earthquake) and from the 22 February 2011 earthquake, and thus can provide insights into feature type and size for distal sites of liquefaction as well as the relationship between distance of surface expression of liquefaction and earthquake magnitude. This study complements other studies that have focused on liquefaction features in Christchurch City and closer to the epicentre of the 22 February 2011 earthquake (e.g., Bastin et al., 2013, in review);

c. the area is farmland with few buildings and the water table varies from 1.5 to 2 m below the surface during the dry season, both of which are favourable for conducting ground investigations as well as using aerial photography and LiDAR for mapping surface features; and

d. the study sites were visited during the post-earthquake reconnaissance by some authors of this report, and are supplemented by excellent photographic and observational records by the land owners.

Two properties, Hardwick and Marchand, were selected for detailed analysis in the area of Lincoln. (Figure 1.1)

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2.0 METHODS

The methods used to identify and date prehistoric liquefaction features depend mainly on the sedimentary environment (e.g., Tuttle et al., 1999; Tuttle, 2001; Tuttle and Hartleb, 2012; Wolf et al., 1998; Liu and Li, 2001; Obermeier, 1996; Obermeier and Dickenson, 2000; Al-Shukri et al., 2005, 2009). In this study we have mainly concentrated on an alluvial setting and we have used the following approaches.

2.1 EXAMINATION AND INTERPRETATION OF AERIAL PHOTOGRAPHY AND LIDAR FOR MAPPING OF THE GEOMORPHOLOGY AND LIQUEFACTION EJECTA

Firstly, we reviewed Quaternary formations in the Canterbury plains in the context of the distribution of the 2010–2011 liquefaction ejecta. We undertook a Geographic Information Systems (GIS) analysis with existing geomorphic mapping and assessed which formations were associated with the occurrence of sand blows and other surface effects of liquefaction that formed in the 2010–2011 earthquakes. The data sources used for this analysis are:

a. Maps of the distribution of sand blows produced by a consortium of institutions (GNS Science, Tonkin and Taylor, Lincoln University, Beca, University of Canterbury and Geotech consulting immediately after the event (Brackley, 2012).

b. Canterbury Landform map from GNS Science 2013, confidential (Pers. Comm. David Barrell). At the time of this analysis, only a portion of the map was available.

c. Digital elevation model (DEM) from light detection and ranging (LiDAR) survey from Canterbury Geotechnical Database (CGD). Airborne LiDAR is an active remote sensing technique that measures reflectance values of laser light to obtain distance from the surface of the earth to a sensor mounted on an aircraft. These data can be used to produce precise elevation models of the Earth’s surface and surface features.

d. Aerial Photography: modern aerial imagery from Land Information New Zealand (LINZ), Landcare Research, Google Earth, CGD, Koordinates.

Secondly, for a selected area that comprises the two field sites, we produced a detailed geomorphic map and updated the existing map of the occurrence of sand blows and other surface effects of liquefaction (David Barrell Pers. Comm.). We analysed the DEM derived from LiDAR and aerial photos taken after the events (CGD; Figure 2.1). LiDAR surveys and aerial photography were acquired prior (2003) to the Darfield earthquake and after each of the largest earthquakes in the Canterbury Earthquake Sequence (September 4, 2010, February 22, 2011, June 13, 2011; Dec 23, 2011). The Marchand and Hardwick sites were covered by a survey flown between the 8th and 10th of March 2011. The survey was conducted by New Zealand Aerial Mapping (NZAM) using an Optech Gemini (07SEN211) instrument flown from a light plane at a typical altitude of c. 900m above ground level (AGL). Sensor settings of 100 kHz of pulse repetition frequency (PRF), 48 Hz scan frequency and 40 degrees “field of view” were used. To support the georeferencing of the sensor, a Global Navigational Satellite System (GNSS) base station receiver was operated at a temporary survey mark that NZAM established at Christchurch Airport. Independent of this work, GNS Science staff field surveyed LiDAR control sites. These were used to correct the LiDAR dataset to the post-earthquake 22 February 2011 geodetic system (Palmer, 2011). Positioning and orientation (POS) data were combined with LiDAR range files and processed into LiDAR point clouds by NZAM. The resulting point cloud is within the manufacture specifications of a vertical accuracy of +/- 0.1 m. The point cloud was classified into ground,

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first and intermediate returns using automated routines and manually edited to increase quality. Bare-earth DEMs and hillshade models with a ground resolution of 1 m were generated from the ground returns using inverse distance weighting (IDW) interpolation with a minimum of 12 points and a search radius of 20 m.

Figure 2.1 Examples of: A, DEM from LiDAR data (blue = low and red = high elevation); B, orthophotos; and C, old aerial photos at the same site.

For the Hardwick and Marchand study sites, we also reviewed pre-2010 aerial photos. The aerial photo stereo pairs were taken in the 1940s and 1960s and have a scale of 1:16.000. This review aimed to assess whether we could identify evidence of liquefaction prior to 2010. In other areas such as the Mississippi River, USA, aerial photo analysis has proven to be effective at revealing paleoliquefaction (Tuttle, 1999; Tuttle, 2011).

2.2 EXCAVATION OF TRENCHES TO STUDY LIQUEFACTION FEATURES

We excavated a total of 11 paleoseismic trenches at selected sites within the Hardwick and Marchand properties. Nine trenches were excavated across the 2010–2011 liquefaction features to further document their geometry and the properties of sediments hosting the liquefaction features. Two trenches were excavated in an area with no liquefaction, primarily to assess whether the sedimentary units are different from the sites that liquefied. The trenches were excavated down to the water table (~1.2 to 1.5 m deep). The walls and floor of 9 of the 11 trenches were smoothed and cleaned, and a square grid with a spacing of 1 m was placed along the walls and on parts of the floor. In areas that required more detailed study a 0.5 m grid was used. After gridding, the walls and parts of the floors were photographed, logged and figures of the stratigraphic units and the liquefaction features were produced, including a description of the sediments and locations of organic samples collected for radiocarbon dating.

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Figure 2.2 HWK 6 trench at the Hardwick site. Note the 1 x 1 m grid on the walls used to map sedimentary units and liquefaction features.

2.3 HAND PISTON CORING, CORE SCANNING AND X-RAY IMAGING

The main purpose of the acquisition and analysis of cores was to:

• Assess whether we could identify liquefaction features, such as dikes intruding younger stratigraphic units and source layers of sand dikes and sand blows at depths below the trenches and in areas where trenching is not possible (e.g., high water table);

• Complement the information on the deep sedimentary sequence from the trenches, for example by obtaining information from deeper sediment to check if we could find the liquefaction source layer.

We extracted ~18 m of intact core (in sections of 0.7–1 m length) with a 5 cm diameter round-rod piston corer, fitted with a core catcher that can retrieve water-saturated sands.

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Figure 2.3 Photo of piston coring with a 5 cm diameter round-rod piston corer, fitted with a core catcher for water saturated loose sands. Note that some cores were obtained from the bottom of the trenches; Trench HWK 6 in the photo.

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The sedimentary sequence within the cores was studied with the following techniques:

a. Visual inspection for liquefaction features and description of sedimentary units;

b. Acquisition of X-ray measurements. X-ray image and X-ray attenuation measurements were acquired by scanning each core in a Smiths Detection EagleFA scanner at the National Isotope Centre, GNS Science, New Zealand;

c. Acquisition of optical images and physical properties. Cores were imaged using a Geoscan camera on a multi-sensor core logger (MSCL) at the University of Otago, New Zealand. Core images were acquired on split cores at across- and down-core resolution of 50 μm. The physical properties were analysed using a GEOTEK MSCL operating in split-core mode. Gamma density, point magnetic susceptibility, and colour spectrometry were collected at a 5 mm down-core resolution. In this report we will only discuss the high resolution images. Measurements of other parameters and their applicability to liquefaction studies are currently under investigation in the PhD study.

Core images were acquired using a camera with three 2048 pixel charged-coupled detector (CCD) arrays that measured light in the red, green and blue (RGB) bands. Lens effects, uneven lighting and pixel-to-pixel variations were accommodated using a gain correction that was calibrated by imaging black and white reference tiles before image analysis. The camera acquisition apertures were optimised for cores and kept constant to allow quantitative comparison between images. Imaging was completed on split cores at across- and down-core resolution of 50 μm. During imaging, the surfaces of split half cores were illuminated using two high-frequency lamps on either side of the image acquisition line. This flooded illumination minimised shadow effects caused by microtopography on the core surface. RGB data were collected from the middle third of the core to avoid depth averaging associated with the down core deformation of lamina at the core margins. The data outputs included high-resolution .tiff files of the core image and .xls files containing the RGB data for each 50 μm depth increment. Tiffs were displayed in the graphics program Adobe Photoshop.

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Figure 2.4 Multi-sensor core logger (MSCL) at the University of Otago.

2.4 RADIOCARBON DATING OF SEDIMENTS

Absolute dating of sediments was obtained through radiocarbon dating. Samples were dated at the Rafter Radiocarbon Laboratory, GNS Science. Radiocarbon ages were calibrated, and stratigraphic sequences analysed, with the Bayesian analysis tools of Oxcal v 4.2.4 (Bronk Ramsey, 2009), using the Southern Hemisphere calibration curve SHCal13 (Hogg et al., 2013). Uncertainties are quoted throughout this report at 2 sigma and that calibrated ages are quoted in AD/BC format.

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2.5 GROUND PENETRATING RADAR

We used Ground Penetrating Radar (GPR) to image subsurface liquefaction features. GPR is a generally quick and cheap tool that can be used to locate sites that may have liquefied in the past. The technique is based on the measurements of the subsurface response to high frequency (typically 100–1000 MHz) electromagnetic (EM) waves (Reynolds, 1997; Neal, 2004; Jol, 2009). A transmitting antenna on the ground surface emits EM waves into the ground in distinct pulses that propagate, reflect and/or diffract at interfaces where the dielectric permittivity of the subsurface change. Reflections of EM waves are usually generated by changes in the electrical properties of rocks, variations in water content, and changes in bulk density at stratigraphic interfaces. EM wave velocity data allows conversion to estimate the depth. The mean velocity value used in this study was a representative value selected from the literature (Jol and Smith, 1991; Neal, 2004; McClymont et al., 2009a,b).

The penetration depth and resolution of the reflection data are a function of both the wavelength and the dielectric constant values, mainly controlled by the water content of materials. Resolution increases with frequency but, in turn, the depth to which the waves can penetrate decreases.

GPR is widely used in geological investigations to define lithological contacts (e.g., Pratt and Miall, 1993), to locate faults (e.g., Rashed et al., 2003; Slater and Niemi, 2003; Gross et al., 2004; McClymont et al., 2009a,b) and in soil studies (Doolitle and Collins, 1995). GPR surveying has also been successful in imaging paleoliquefaction features (Liu and Li, 2001; Al-Shukri et al., 2005; 2006). Recently, Nobes et al. (2013) used a variety of geophysical techniques including GPR to image disruption of subsurface stratigraphy beneath liquefaction features induced by the 22 February 2011 Christchurch earthquake, in particular sediment mounding beneath sand volcanoes.

In our study, we used two different systems with two different types of antennas (Figure 2.5). Prior to trenching we used antenna MALÅ GPR with a 100 MHz sunshield antenna and 250 MHz shield antenna. The control unit was a MALÅ ProEx Control Unit. Imaging disruption and deformation with this initial GPR survey had the objective of identifying potential trenching sites. In this first attempt were aimed at penetrating as deep as possible, because no constraints on the dimensions (depths) of paleoliquefaction deposits existed.

After the trenching exercise we used a SIR-3000 GPR with 400 MHz and 1.5 GHz antennas. GPR scans were recorded at intervals of 1 cm (100 scans/metre) which was achieved by using a digital survey wheel for pulse triggering. A track log was recorded at the same time using differential GNSS. Data were processed in RADAN 6.7 software which involved frequency filtering, horizontal background removal, predictive deconvolution, and gain adjustments. Topographic corrections were applied by using elevations extracted from the LiDAR data.

Unfortunately, we did not have a GPR system available during the trenching exercise, which would have been useful to try to image and correlate the trench walls.

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Figure 2.5 GPR systems used to image liquefaction features: Upper photo, MALÅ Ground Penetrating Radar system (250 MHz antenna); lower photo, a GSSI SIR10A+ Ground Penetrating Radar system (400 MHz antenna).

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3.0 THE EFFECTIVENESS OF SELECTED EXPLORATORY METHODS TO ASSES PALEOLIQUEFACTION IN THE CANTERBURY AREA

As mentioned above, we recommend that Appendix 1 be read before proceeding with the remainder of this report.

3.1 GEOMORPHIC MAPPING

In this study, we focus on the alluvial environment of the ancestral Waimakariri River in the Lincoln area. Other studies have focused on the Avon-Heathcote estuary environment (liquefaction and subsidence; Reid et al., 2012; Cochran et al., 2014), the alluvial environment of the Avon River (Quigley et al., 2013; Bastin et al., 2013, in review) and Holocene near-coastal sediments in the Kaiapoi area (Bastin et al., in review). Here we report results of the Lincoln area and discuss the results from other published studies.

3.1.1 Literature review on the geomorphology and sedimentary environments of the Canterbury plains

The Canterbury Plains are underlain by Quaternary fluvio-glacial outwash and marine formations (Figure 3.1 and Figure 3.2). Eustatic sea level changes in association with glacial-interglacial cycles have led to a complex interfingering of terrestrial and marine deposits (Brown et al., 1988; Brown and Weeber, 1992; Field et al., 1989; Browne and Naish, 2003) (Figure 3.1 and Figure 3.2). Due to the geological and stratigraphic context most of the sediment at and near the coast in the Canterbury area is fine grained (sand, silt, clay and peat), water-saturated due to a high water table, and most are characterised by high to moderate liquefaction susceptibility (Elder et al., 1991).

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Figure 3.1 A, Summary of the main landforms that occur in the Canterbury Plains (modified from David Barrell, Pers. Comm.) B, Detailed map of the study area. Most landforms are associated with fluvial deposits except for the Christchurch and Coastal Sand Dunes surfaces.

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Figure 3.2 Cross-section through the northern portion of the Canterbury Plains near Christchurch to the edge of the continental shelf showing the stratigraphy of alternating lowstand fluvial gravels and sands, and highstand sand, silt, clay, and peat (modified from Brown and Weeber, 1992). Numbers refer to inferred marine isotope stages.

The majority of the Quaternary geological formations in the vicinity of our study area are associated with the Waimakariri River and have been named the Burnham and Springston formations (Figure 3.1 and Figure 3.2). The Burnham Formation (~18–25 ka) represents the last phase of fluvio-glacial aggradation and coastal extension of the Canterbury Plains at the time of the Last Glacial Maximum, when sea level was about 120 m below its current level. Landforms on this formation include the Burnham and Darfield surfaces (Table 3.1).

Latest Pleistocene to Holocene sedimentary successions in the study area include the Springston and Christchurch formations (Table 3.1). The more recent of the two, the Christchurch Formation, is inferred to have started accumulating at the end of the Otira Glaciation about 14,000 years ago (Brown et al., 1988). It is a marine formation that comprises beach, estuarine, lagoonal, dune and coastal swamp deposits. The Springston Formation represents fluviatile deposition that followed the glacial retreat at the end of the Otira Glaciation. It comprises several members of different age, listed here in order of decreasing age: Bleak House Member, Riverview Member, Courtnay Member, Halkett Member and the Yaldhurst Member (Suggate, 1958). There are several landforms associated with the Yaldhurst Member that are present in our study are and which have been defined as the Yaldhurst 1 Surface of Waimakariri soil age (~700 to 2400 yr BP), and the Yaldhurst 2 Surface of Selwyn soil age (<300 yr BP) (Cox and Mead, 1963).

The extent of each landform within the Canterbury Region varies (Table 3.1).The Darfield surface on the Burnham Formation is the most extensive in the wider Canterbury region; however in the area analysed (area encompassing the 2010–2011 liquefaction) it only represents 10% of the total area (Table 3.1). The Yaldhurst and Halkett surfaces occupy the largest percentage of the total (Table 3.1).

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Table 3.1 Main Landforms on the Canterbury Plains: proportion of total area (%).

Landform %

Darfield Surface 9.2

Christchurch estuarine flat 1.6

Coastal Sand Dunes 4.0

Darfield Surface 9.8

Halkett Surface 19.8

Human 1.5

Undifferentiated surfaces on Springston Formation 1.5

Yaldhurst Surface (undifferentiated) 28.4

Yaldhurst 1 Surface 11.8



3.1.2 Correlation between existing geomorphic maps and the occurrence of liquefaction ejecta in the Canterbury Plains

The total area of mapped liquefaction from September 2010 and February 2011 (Brackley, 2012) was overlaid and compared with the Canterbury geomorphic map (David Barrell, Pers. Comm.) in a GIS framework. Contribution of each landform to the total area of sand blow occurrence was assessed. The area studied is bounded to the north by the Waimakariri River and the Avon-Heathcote estuary and to the south by the Rakaia River. This area excluded Banks Peninsula to the east and Lake Ellesmere to the southwest, and includes the Darfield area on the west. It is important to note that detailed mapping of landforms of the western portion of the study area was not complete at the time of this study. Therefore, the total area that suffered liquefied in 2010–2011 may be underestimated. This lack of information in the Darfield area mostly comprises liquefaction that occurred during the 2010 September earthquake. The 22 February 2011 Christchurch earthquake is not known to have induced liquefaction in the Darfield area.

The contribution of each landform to the total liquefied area is presented in Table 3.2. The Yaldhurst and Halkett members of the Springston Formation were most strongly affected by liquefaction due to their material properties (see below) and high water table (together they represent 50% of the total liquefied area; Table 3.2). The Halkett Member comprises younger deposition of the main channel of the Waimakariri River and is capped by numerous sand dunes, especially near the coast. In contrast, the Yaldhurst Member represents flood and overflow deposits originating from the Waimakariri River when it flooded into the Avon, Heathcote and Halswell rivers. The Yaldhurst Member includes fewer dunes than on the Halkett Surface (Brown et al., 1988). The second-most important geomorphic features are the coastal sand dunes, which represent 10% of the total liquefied area (Table 3.2). These features consist of water-saturated, loose sediments which experienced substantial liquefaction (Reid et al., 2012). In contrast, despite its large area, the Darfield surface had rare liquefaction effects when compared with the Holocene surfaces.

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Table 3.2 Contribution of each landform to the total liquefied area, not considering natural water bodies, Banks Peninsula and Lake Ellesmere.

Landform %

Yaldhurst raised channel 14

Yaldhurst flood basin 12

Yaldhurst 1 Surface interchannel/alluvial flat 11

Coastal Sand Dunes 10

Yaldhurst 1 Surface 5

Yaldhurst 2 Surface 4

Lake Ellesmere bed 4

Halkett Surface 4

Darfield Surface 4

3.1.3 Detailed geomorphic map and association between 2010–2011 liquefaction and geomorphic features in the Lincoln area

A preliminary version of a geomorphic map of the Christchurch City area and surroundings based on the LiDAR data, geological (Qmap; Forsyth et al., 2008) and soil maps of the region (Webb, 2010) was provided by David Barrell (GNS, Pers. Comm.). This map was used as a basis for establishing first-order geomorphic units. A more detailed geomorphic map was produced for a section of the Lincoln-Taitapu area with emphasis on the selected study sites (Figure 3.3). The detailed map presents landforms associated with different parts of the alluvial system (e.g., point bars, channels, levees, etc.). The detailed maps produced in this study are described in Appendix 1.

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Figure 3.3 A, Detailed geomorphic map of the study area and B, location of liquefaction ejecta from 2010–2011 events on a LiDAR DEM basemap.

There is a strong correlation between particular sedimentary sub-environments within the alluvial system and the occurrence of the 2010–2011 liquefaction ejecta (Figure 3.3). The most prominent correlation is between the higher elevation areas within the point bars (scroll bars) and occurrence of liquefaction (Figure 3.3 and Appendix 1). This correlation is clearly observed in several areas along the river and also at the Marchand site. At the Marchand site, excavations also exposed paleoliquefaction dikes at the same location as the 2010–2011 liquefaction (see section 3.3 below and Appendix 1).

Liquefaction associated with meander scroll bars has been widely reported in New Zealand (Quigley et al., 2013; Bastin et al., 2013) and elsewhere (e.g., Youd & Hoose, 1977; Tuttle, 2001; Holzer et al., 2011). The source layer for the liquefied sands was not exposed at either study site but liquefaction of the trench floor of the MAR2 trench at <1 m depth during excavation of paleoseismic trenches (caused by the vibration of the digger) suggests that this layer could be quite shallow. The water table at the time of excavations was ~1.5 m deep at the Marchand site. Liquefaction ejecta is more prevalent along the point bar ridges and is absent at the intervening swales. This alternation could suggest a correlation of the presence of liquefaction with coarser grained sediment (in this case medium sand) beneath the ridges as opposed to finer grained materials (silt and clay) below the swales (Pranter et al., 2007; Van de Lageweg et al., 2014) (see also trench and core sections below).

There is also a correlation between 2010–2011 liquefaction and the higher points of the crevasse splay that fills an old alluvial channel at the Hardwick site (Figure 3.3 and Appendix 1). In this particular case, more studies are currently being undertaken to understand why this area was so susceptible to liquefaction. Our preliminary results suggest

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that the source layer for the liquefied sands was a silty fine sand to very fine sand at 4+ m depth (see section A1.4 of Appendix 1).That layer is likely to consist of channel or point bar deposits associated with the abandoned channel. Channel fill deposits may have influenced the formation of liquefaction features as has been observed in similar deposits in the New Madrid seismic zone (Tuttle and Barstow, 1996). However at the Hardwick site, liquefaction only occurred where the channel or point deposits are covered by the loamy silt and very fine sand of the crevasse splay, and not in the section of the meander channel to the south where the channel is not covered by the crevasse splay deposits. The grain size, morphology, and stratigraphic relationships of sedimentary deposits all factor into whether or not liquefaction occurs at depth and where liquefaction features are expressed at the ground surface.

3.1.4 Results: How effective is detailed geomorphic mapping at identifying sites that may have undergone paleo-liquefaction?

The strong spatial correlations between liquefaction and some sedimentary environments (e.g., point bar deposits) presented above suggest that detailed mapping of geologic deposits from their geomorphic expression is a useful approach for identifying potential sites for paleoliquefaction studies. However, in areas with subtle topography, such as the Halswell River area and similar settings, high resolution topographic maps from LiDAR and from detailed aerial photographs are useful for mapping point bar deposits, crevasse splays, etc.

3.2 REVIEW OF OLD AERIAL PHOTOGRAPHY

Aerial photos from the 1940s and 1960s at a scale of 1:16,000 were analysed to assess whether there was evidence of liquefaction prior to 2010 at the sites that liquefied in 2010–2011. Liquefaction has been documented to occur in close spatial association with sites that have liquefied previously (e.g., Tuttle and Seeber, 1991; Tuttle et al., 1992; Quigley et al., 2013; see also Appendix 1). For that reason, locations with historic liquefaction are preferred targets for excavating trenches to assess liquefaction recurrence and timing of pre-historic events.

Our goal was to identify locations that may have liquefied previously through aerial photo analysis. In other regions where large earthquakes have occurred in close proximity to liquefiable sediment, such as the 1811–1812 New Madrid earthquakes centred in the lower Mississippi River Valley, sand blows are reflected on aerial photos as large (in the tens of meter scale) circular, elliptical, and linear areas or domains that are light in colour compared to surrounding soils. The difference in colour is related to their grain-size, moisture-holding capacity, and ability to sustain plant growth compared to the surrounding soil developed in silty and clayey floodplain deposits (Tuttle, 1999, 2011).

In our study, we have identified lighter grey areas on the 1940s aerial photos of the Marchand site. The lighter areas correspond with mapped scroll bars (Figure 2.1). Also, in the trenches we have found that at least one of the two scroll bars located at the Marchand site had liquefied prior to 2010. The old dikes intrude into the uppermost layer (below the top soil; see section 3.3 below and Appendix 1) suggesting that the paleoliquefaction features may have reached the ground surface at the time. Light grey areas to the south of the Marchand site coincide with other scrolls bars that liquefied in 2010–2011 (see Figure 2.1C and Figure 3.3). However, at the Hardwick site where the older paleoliquefaction features reached the surface but were subsequently buried by fluvial deposits (see section 3.3 and Appendix 1), the 1940s aerial photos do not show lighter areas in the locations that liquefied in 2010–2011.

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3.2.1 Results: How effective is aerial photo review in identifying sites that may have undergone paleoliquefaction?

While we have not undertaken an exhaustive analysis of pre-2010 aerial photos in the wider area, this preliminary result suggests that aerial photos could be useful in identifying paleoliquefaction features across the Canterbury area, including features at sites where historical liquefaction has not occurred. This realization is different from what we originally thought given the amount of land modification related to intensive farming and the high rate of soil forming processes in the temperate humid climate. Although anthropogenic and natural erosional processes may obscure surface liquefaction features within weeks of the formation in some locations, (Quigley et al., 2013) other sites may enable sand blows to have longer lifespans as recognizable features. Buried sand blows and sand dikes below the depth of farming and soil formation may be permanently preserved in the geologic record (i.e., >10 Ma; Loope et al., 2013). Further analysis may help the understanding of the potential of aerial photos to be used for paleoliquefaction studies, both in Canterbury and elsewhere.

3.3 PALEOSEISMIC TRENCHING

For the purposes of this project the excavation of trenches had three aims:

a. to expose and characterise the shape, size, and composition of the 2010–2011 liquefaction features;

b. to assess whether paleoliquefaction features could be found in areas that liquefied in 2010–2011 (with application to other areas with historical liquefaction) earthquakes; and

c. to better understand the techniques required to explore for paleoliquefaction features in this area.

The trench locations and descriptions are contained in Appendix 1 and the logs of all trenches are presented in Appendix 2.

3.3.1 Types and sizes of liquefaction features observed in paleoseismic trenches at the Hardwick and Marchand sites

Four main types of features have been found in the paleoseismic trenches in the Lincoln area (Figure 3.4):

• Sand blows that formed singular and multiple, coalescing sand blows along linear fissures (Figure 3.4A, B, and F; and logs from trenches HWK 1, HWK 3, HWK 5, HWK 6 and MAR 3 in Appendix 2). Some sand blows contain different types of sand (Figure 3.4F), which implies that they come from different sources.

• Blisters or injections of subhorizontal dikes into the top soil that have not fully ejected all the way to the surface. These injections inflate the ground surface producing a sub-rounded mound, like a blister (Figure 3.4C and D; and logs from trenches HWK 4b and MAR 4 in Appendix 2). Occasionally, part of the injection reaches the surface as ejecta via rupture of the blister (see logs of trenches HWK 4b and MAR 4 in Appendix 2).

• Dikes related to the blows and blisters (Figure 3.4C and D; also in most trench logs in Appendix 2).

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• Collapse structure similar to a tectonic graben associated with dikes, where some of the “faults” are intruded by dikes (Figure 3.4E; log of MAR 4 in Appendix 2). This structure occurs close to the river bank and thus it is likely to be associated with lateral spreading but not always.

Figure 3.4 Examples of 2010–2011 liquefaction features. A, Photo of the Hardwick site immediately after the 4 September 2010 Darfield earthquake. Coalescent sand blows that formed along ground fissures occupy a great percentage of some paddocks. Photo from Caroline Hardwick. B, Example of sand blow with sand from different source layers as expressed in the different colours. Photo from Caroline Hardwick. C. Blister exposed in the MAR 4 trench (see trench log in Appendix 2 and Figure 3.7). Vertical dikes of relatively small size (up to 2 cm max) fed into 0.1–0.3 m wide dikes with dips

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between 45° to subhorizontal. These latter dikes formed the blister. Note the “blister” or mound on the surface and the raised topsoil (~0.5 m). Minimum trench grid square is 0.5 m. D. Detail of the sill at MAR 4 trench (see trench log in Appendix 2 and Figure 3.7). Flow structures are subparallel to the dike and sill. Also there are numerous clusters of rip-up clasts of silt derived from the host deposit. E, Collapse structure exposed in the MAR 4 trench. The structure looks like a tectonic graben where some faults have steep dips. Some faults toppled and display a reverse sense of movement. Dikes intrude along the faults. F, Example of coalescent sand blows at the Hardwick site after the September 2010, February 2010 and June 2011 earthquakes. Note that the smooth part of the sand surface corresponds to the 13 June 2011 liquefaction event ejecta covering part of the February sand blows (with larger extent and eroded surface). Photo taken on 14 June 2011 by Caroline Hardwick.

We compared the size of the sand blows, blisters and dikes with other sites in Christchurch and with international examples. Figure 3.4 and Appendix 3 show that sand blow sizes range from 10–40 cm height and >2–4 m width, and dike widths range from 1 to 3 cm for both our study sites. These values are likely to represent the composite sand blows for the three events that caused liquefaction at our study sites. While we have not been able to quantify the sizes for each of the three events at our studied sites, photographic evidence after the June 2011 event shows that the width of dikes formed during the September earthquake (PGA ~0.44 g) may represent the largest widths measured here, and that the June 2011 (PGA 0.06 g) sand blows are about a third to half of that width (see photos E and I in Figure 3.4).

In eastern Christchurch, feeder dikes formed during strong shaking (PGA >0.4 g) in the 22 February 2011 Christchurch earthquake reached widths of 25 cm in highly susceptible sediments with severe lateral spreading. Maximum sand blow heights (thickness) locally exceeded 30 cm in eastern Christchurch (Quigley et al., 2013). Sand blow thicknesses and dike widths do not easily correlate because other factors such as the rates and magnitudes of lateral spreading, and material properties of the overburden, are also important. Paleoliquefaction features identified by Bastin et al. (in review) in eastern Christchurch and those found at our sites (Figure 3.4) have similar dimensions to modern liquefaction features. Some dikes are wider when they are in close proximity to a free face such as the edge of a river channel (likely filled lateral spreading fissures).

Sand blows that formed during the M 5.9 1988 Saguenay, Quebec earthquake are of a similar size to the ones described for the 2010–11 events (Tuttle et al., 1990,1992). There the sand blows generally range from 7.5–22 cm thick, 30–500 cm wide, and 30–1000 cm long, with associated feeder dikes ranging from 1–30 cm wide. At the liquefaction site closest to the Saguenay epicentre, the sand blow is 10 cm thick, 1000 cm wide, and 1500 cm long, in association with two sand dikes 2 cm and 50 cm wide. Sand dikes in the Charlevoix seismic zone, south-eastern Canada, range from 0.5 to 15 cm wide (Tuttle and Atkinson, 2010). In contrast, sand blows in the New Madrid Seismic Zone are commonly 1–2 m thick, 10s of metres wide, 10s–100s of metres long, and associated with sand dikes 10s–100s cm wide (Tuttle et al., 2006; Tuttle and Hartleb, 2012).

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Figure 3.5 Comparative plots of dike and sand blow sizes for the 2010–2011 liquefaction and for the paleoliquefaction features at the Hardwick and Marchand sites. Top, Average dike widths for modern (L1) and paleodikes (L2). Middle, Average sand blow width. Bottom, Average sand blow height (thickness). See Appendix 3 for data.

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The difference in size and area of occurrence between the 2010–2011 liquefaction features in the Christchurch area and the New Madrid seismic zone is likely related to: a) the liquefaction susceptibility of the sediments; (b) the characteristics of the geomorphic elements such as size of the braided stream and meandering stream belts of the two river systems, slopes, and local topography; and c) the characteristics of ground shaking (PGA, frequency content and shaking duration). In general, sediment is moderately dense and liquefiable in the New Madrid region; whereas, sediment is loose and highly liquefiable in the Christchurch region (Saucier, 1994; Obermeier, 1996). The 1811–1812 New Madrid sequence included three mainshocks of M 6.8 to M 8.0 and one aftershock of M 6.3–7.2 that induced liquefaction (Tuttle and Hartleb, 2012), whereas the Canterbury Earthquake Sequence included one earthquake of M 7.1 and at least seven (and up to 11) earthquakes of M 5.8 to M 6.4 that induced liquefaction (Quigley et al., 2013; GEER, 2010). Also the size of the geomorphic elements in the Mississippi alluvial system is larger than those of the Waimakariri-Halswell alluvial system. For example some point bars in the Mississippi River are tens of kilometres long and hundreds of meters wide, while the point bars in the Waimakariri system area usually less than 1 km long and only a few tens of metres wide.

We have not analysed sand blows and dike sizes from the 2010–2011 earthquake sequence to the extent where robust comparisons between the morphology of liquefaction features can be compared to earthquake shaking intensity, epicentral distance, and/or sediment type. However, from our preliminary observations it seems that the liquefaction features close to the epicentres are of a similar size, or only slightly larger than, those found at distant areas (i.e., our study area, 17 km away from the 2011 February epicentre). This suggests that, in addition to earthquake shaking intensity and epicentral distance, the morphology of the liquefaction features is strongly influenced by geologic and geomorphic site characteristics.

Quigley et al. (2013) developed power law empirical relationships between magnitude-weighted PGA and sand blow thicknesses and areal extents, indicating that liquefaction surface ejecta volumes (but not feeder dike widths) correspond to shaking intensity at a given site. However, spatial and temporal changes in site characteristics (e.g., water table fluctuations, sediment densification and cementation, stream channel migration, etc.) and the susceptibility of feeder dikes to reactivation make deduction of earthquake shaking intensity from both surface and subsurface paleoliquefaction features challenging. Nonetheless, it is highly important to document the morphology of the 2010–2011 liquefaction features in order to compare their dimensions to paleoliquefaction features and to speculate on paleo-earthquake shaking intensities, to inform future geophysical investigations of paleoliquefaction at sites that have (and have not) had historic liquefaction, and to predict the likely locations of future liquefaction under different intensities of seismic shaking.

The sizes of liquefaction and paleoliquefaction features described in the Christchurch area are relevant when assessing the methods that will be most useful for identifying the presence of paleoliquefaction in alluvial system similar to the Waimakariri River. For example, size will have important implications for the type of subsurface techniques used to image potential paleoliquefaction features, as is discussed below.

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3.3.2 Results: how effective is paleoseismic trenching to identify and characterise paleoliquefaction features?

Paleoseismic trenching is the best exploratory method to analyse paleoliquefaction in the Canterbury region and other regions in New Zealand with alluvial environments with similar scale of geomorphic and sedimentary features. Trenching exposes liquefaction features regardless of their size. For example, the distinction between a paleoliquefaction sill (or a dike parallel to bedding, in this area usually with sub horizontal dip) and a sedimentary layer is possible if a significant portion of the layer and the sill are exposed. For example, in MAR 3 trench the distinction between these two types of features is obvious because of the extensive exposure (Figure 3.6). The boundaries are clearly exposed, showing an irregular shape for the paleodikes that cannot be explained by primary sedimentary processes; contacts between primary sedimentary layers are subhorizontal and much smoother. The paleodikes also change from a subhorizontal orientation (sill) to a vertical one cutting through the sedimentary boundaries. The connections between the vertical and the subhorizontal sections can only be observed because of the large exposure.

Figure 3.6 Details of a MAR 3 trench wall. Note the similarity in the irregular-shaped dike margins for the 2010–2011 and the paleoliquefaction dikes. Also note that paleodikes can be distinguished from 2010–2011 dikes by the weathering features (e.g., mottles) that penetrate across their margins. The trench log for the MAR 3 trench is presented in Figure 4.2 and Appendix 2. Boxes A and B are used to evaluate how modern and paleodikes will appear if a core had been extracted at that location in section 3.5.1. Note the wall shown on the figure is subparallel to the dike trends, and thus apparent dip is subhorizontal (true dips were variable along individual dikes and ranged from 26° to 90°; Appendix 3).

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Exposures provided by trenches allow modern liquefaction to be distinguished from paleoliquefaction by observing crosscutting relationships and their different degree of weathering. The main characteristic that we have used to distinguish between the two is the presence of redox segregations (concentrations of Fe oxides resulting from sequential reduction-oxidation processes, or mottling). These features overprint the older liquefaction dikes but are themselves cut by the modern liquefaction dikes and blisters. Such relationships confirm that the liquefaction and emplacement of dikes relates to at least two distinct events separated by the minimum time taken for redox segregations to form (years). In fortuitous circumstances similar relationships may be visible in a core, for example, but the chances of finding clear and definite cases are small relative to a trench exposure.

With trench analysis, we are also able to clearly analyse the relative timing between old dikes, new dikes and sedimentary deposition, as well as to collect samples for radiocarbon dating to establish the absolute timing of the geologic events.

3.4 SHALLOW GEOPHYSICS: GROUND PENETRATING RADAR (GPR)

The GPR survey was undertaken for two reasons:

1. Assess whether GPR can image the 2010–2011 liquefaction features, and thus can be used to explore areas for potential presence of paleoliquefaction features. This is potentially a quick and relatively inexpensive tool to constrain areas to excavate trenches.

2. Complement information from trench and cores.

Here we will report on point 1) above. Point 2) is currently being undertaken within the PhD study.

3.4.1 Can GPR image the 2010–2011 liquefaction features?

In this preliminary assessment, we have found that of the four types of antennas used (100, 250, 400, and 1250 MHz) the two with higher frequency show features that can be explained by liquefaction.

The best correlation between the GPR images and the location of the 2010–2011 sand blows and blisters is found with the 1250 MHz antenna at the Marchand site. In Appendix 4 we show three radargrams (GPR profiles 13, 14 and 15), two of which were acquired with the 1250 MHz antenna. The third radargram was acquired with the 400 MHz antenna.

Profile 13 was obtained across the blister exposed in the MAR 4 trench (Figure 3.7 and Appendix 4) and has a NW trend, sub-perpendicular to the scroll bar (N55°E) and to the strike of the dikes and “faults” (N30–70°E; see Appendix 3). The radargram imaged the upper 0.5 m of the ground, which in this area corresponds to thick topsoil (see MAR 4 trench log in Appendix 2). We have overlain an outline of the MAR 4 trench above or on the GPR profile (Figure 3.7 and Appendix 4) to show the coincidence of steep dipping anomalies with the blister dikes (note that the scale is exaggerated in the vertical direction). There is a sharp contrast in the dialectric constant or water-holding capacity and/or conductivity properties of the very firm, organic rich, silt rich top soil and the loose fine to medium sand in the dikes that is reflected in the GPR profile.

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Profile 14 was also acquired with the 1250 MHz antenna and runs NW from a point close to the MAR 3 and 1 trenches (Figure 3.7). It runs sub perpendicular and across the two scroll bars mapped at the Marchand site (Appendix 4). Along this profile we have located a scaled outline of trenches 1 and 3 and marked the positions of sand blows. The sand blows exposed in the trenches and mapped at the surface correlate with strong subvertical reflectors in the radargram. As the radargram of this frequency effectively resolves the upper 0.5 m, the contrast between the dike and the topsoil is strong. Along this profile there are also other subtle subvertical reflectors in areas with no surface expression of liquefaction. It is possible that these are liquefaction dikes that did not reach the surface.

Profile 15 (Appendix 4) was acquired with the 400 MHz antenna. This profile is located at the same position as profile 14 (note that profiles 14 and 15 are displayed differently, i.e., 14 is from SSE to NNW and 15 is form NNW to SSE). The 400 MHz antenna penetrates deeper and images the sediments down to 2 m. We have displayed the outlines of the trenches and marked the location of the sand blows as mapped from the aerial photos taken after the 2010–2011 events. In this profile, strong subhorizontal reflectors delineate sedimentary layers. A section of the profile, from metres 17 to 21, displays strong reflectors that could either represent the backfill of the MAR 1 trench, or the man-made infill found at the northern most part of that trench during excavations (see MAR 1 trench log in Appendix 2).

There is no clear spatial correlation between sand blows, sand dikes observed at the surface, and any features on profile 15 that might indicate the presence of liquefaction. There is perhaps a subtle difference between the continuity and strength of the reflectors; in the low areas with no liquefaction, the reflectors seem more continuous and stronger (better imaged). While this does not seem to be a determining factor in identifying liquefaction features, it can be used as a criterion in siting trenches if no more obvious signature of sand dikes and blows is detected on GPR profiles.

The 250 and 100 MHz antennas were of too low frequency to detect the features found on other profiles, trenches or mapped at the surface. We used these antennas only at the Hardwick site. None of the images yielded clear anomalies in the radargram that could be correlated with sand blows or dikes.

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Figure 3.7 Example of GPR profiles and their correlation to liquefaction features. A., Location of the GPR profiles (red lines) and trenches(short black lines) at the Marchand site. B, MAR 4 trench log showing dikes and sills under the blister. C, Photo of the MAR 4 trench wall. D, GPR image in close proximity to the MAR 4 trench. The complete GPR line can be found in Appendix 4. Note the labels of the upper unit, Ah, in B-D for correlation purposes.

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3.4.2 Result: how effective is GPR to identify and characterise paleoliquefaction features?

The size of the liquefaction features found in the study sites suggests that high resolution subsurface imaging is required to adequately image them. We initially used low frequency antennas that did not delineate the liquefaction features observed by other methods. These first attempts to locate paleoliquefaction before trenching were aimed at penetrating as deep as possible by using MALÅ Ground Penetrating Radar with 100 MHz and 250 MHz antennas. However, the liquefaction features were too small for these frequencies and the initial surveys did not prove effective because the low resolution of the antennas.

After exposing the liquefaction features in the trenches, we were able to quantify their size and observe a lack of sediment deformation in the host sediments. Subsequently, higher frequency antennas were used and the acquired images depicted liquefaction features with differing levels of success.

The 1250 MHz high resolution antenna of the GSSI System is very effective at detecting near-surface liquefaction as it only images the upper few centimetres of the subsurface. However, while resolution increases with frequency, the depth to which radar waves can penetrate decreases. For this reason, this antenna would not be useful to image liquefaction features that are located >0.5 m depth, or where small paleoliquefaction features have been bioturbated or otherwise destroyed by soil forming processes.

It is also important to note that the results shown for the 1250 MHz antenna were those acquired in a field with turf grass that had a very smooth surface. Other profiles obtained in rougher surfaces (e.g., at Hardwick site) with this antenna were not as successful at yielding useful images because the small topographic differences on the ground (in the order of a few to 20 cm) can scatter the radar signal.

The 400 MHz antenna was very effective for determining the sedimentary architecture down to 2 m. Therefore, it is possible that in areas with a clear alternation of sediments with different conductive properties, liquefaction features of the size studied here may be detected. Success will also depend on the type of host sediments. In point bar sediments such as those exposed in Profile 15, the 400 MHz antenna produced mixed results.

3.5 HAND PISTON CORING

We extracted piston cores with several goals in mind, including:

1. To test whether paleoliquefaction can be detected in the area where trenches cannot be excavated due to a high water table.

2. To complement the information of the trenches to identify the source layer of the liquefiable sands.

3. To improve the sedimentary correlation between trenches at closely-spaced sites.

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In this section we will discuss points 1 and 2. Point 3 is briefly discussed in Appendix 1 and will be fully explored by the PhD study. Here, we will only discuss those aspects that are relevant to the main goal of this study, i.e., the effectiveness of using hand piston coring for detecting paleoliquefaction. The location of the cores in relation to the trenches is shown in Appendix 2. A full description of the cores can be found in Appendix 5. High resolution photos from the MCSL can be found in Appendix 5. Figure 3.8 shows a selection of these images, along with the corresponding X-ray images.

3.5.1 Can paleoliquefaction features be observed in the cores?

Some of the 2010–2011 liquefaction features were identified in the cores. Anomalous sand units embedded in the topsoil were identified in the cores extracted close to trench HWK 4b (Figure 3.8). The sediment in the core resembles the sediment of the blister in the HWK 4b trench log (Appendix 2). A 2010–2011 liquefaction sill injected into a peat layer and identified at the base of trench HWK 5 (Appendices 1 and 2) was also identified in the core extracted at that location (Figure 3.8B). The core and the bottom of the trench overlapped which allowed for comparison of the expression of the sill. In both cases, there is a remarkable contrast between the intruded sand and the host sedimentary units which consist of top soil and paleosol (organic silt to peat).

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Figure 3.8 Photographs of selected hand piston core samples and their corresponding X-ray images. A, Sill identified in core HWK 4b. B, Sill identified in core HWK 5. C, Rip-up clasts (square with rip-up clasts is shown at a more detailed scale; the largest rip-up clast is ~ 1 cm long) is shown suggesting forceful intrusion of liquefied sediment (core HWK 5). D, Soft-sediment deformation suggesting layer experienced earthquake-induced liquefaction (core HWK 5).

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During the exploration for paleoliquefaction features, similar relationships to the ones described above (i.e., anomalous sand layers) were found in some cores. In that case, those features could suggest the presence of paleoliquefaction at the site. However, we need to consider some uncertainty in this interpretation:

• If a liquefaction dike is subhorizontal it could be interpreted as a sedimentary unit. For example, an anomalous sand layer found within a peat could be caused by a brief inflow of sand-laden water into a paleo-swamp. In this setting, we can assume a sharp lower contact of the sand layer as the water inflow either erodes the peat or/and deposits material that has very different characteristics (higher energy) from the swamp deposits. Then the nature of the upper contact of the sand layer becomes very important to confirm its origin. In the case of a water inflow, the upper contact of the sand unit is likely to show a gradual change into an organic soil and into peat as the swamp environment is restored. However, if the sand is injected by liquefaction processes, the sill will always have a sharp upper contact.

• If the characteristics of the host sedimentary unit are similar to the injected sand layer, it may be difficult to recognise liquefaction features. Sedimentary processes in higher energy environments than a swamp (example above) can explain a sharp upper contact for the injected sand, by erosion and subsequent deposition above an unconformity. We use the exposure in MAR 3 to explore hypothetical cases of how cores would look if extracted from the excavated sediments. We have drawn two narrow vertical sections (A and B in Figure 3.6) that represent hypothetical cores. In the hypothetical core A, there is a 2010–2011 dike below the top soil and two paleo-dikes below that. In this case, it is likely that the two paleo-dikes could be misidentified as sedimentary layers given their shallow dip and sharp contacts. Also, weathering has reduced the sharpness of the paleo-dike contact so that it resembles a sedimentary one. This would make it even more difficult to identify in a core. However, in hypothetical core B, where several modern and older dikes are exposed, the irregular shape of all dike contacts suggest the presence of dikes or soft sediment deformation, both of which are an indication of liquefaction. It is easier to identify dikes in cores if they have steep dips that cannot be explained by primary sedimentary processes.

In some cores we also identified soft-sediment deformation features in the sand layer tentatively interpreted as the source of the liquefied sands. The observation of these features at a site-reconnaissance stage is here identified as a criterion to warrant further exploration (such as excavation of trenches). This is explored further in section 3.5.2 below.

The core analysis techniques that have been more useful to identify these features were the visual analysis of the cores, complemented by high resolution photography in the MCSL, and the X-ray images. While the X-ray images clearly show a large contrast between the intruded sand (dikes and sills) and the hosting organic soil, the resolution was not detailed enough to identify other features. For example, the soft sediment deformation discussed in the next section had only a subtle expression in the X-ray images. Nevertheless, high resolution X-ray images are a very useful tool for rapid identification of potential liquefaction features.

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3.5.2 Can the source layers of liquefaction features be identified in the cores?

In Appendix 1, we present a discussion of the source layer of the liquefaction features at the Hardwick site. In summary, the source layer was not exposed in the trenches but we interpret it to be a layer at ~3+ m depth in core HWK 5 and at 4+ m depth in core HWK 6 (see Appendix 5 for core descriptions). This interpretation is mainly based on similar particle sizes for sand samples from dikes and sand blows in the trenches and samples from the cores at depths mentioned above (see Appendix 1).

The visual inspection of the HWK 6 core, together with analysis of the high resolution images, reveals the presence of rip-up silt clasts and soft sediment deformation in the source layer (Figure 3.8C-D). We are currently in the process of acquiring high resolution X-ray images to see if subtle deformation features can be imaged in this core, in order to fully evaluate the effectiveness of high resolution X-ray images for identifying liquefaction features.

3.5.3 Results: how effective is coring for identifying and characterising paleoliquefaction features?

The use of hand piston coring to explore paleoliquefaction is a useful tool but, on its own, has limitations. Hand piston coring can be very useful in a reconnaissance phase to identify areas where trenches could be opened. Extraction of cores and immediate visual analysis may be enough to select areas where cores show features of suspected liquefaction origin (similar to those discussed above). Also high resolution X-ray and CT tomography of cores can help select study sites.

The advantage of hand piston coring with respect to other coring types (e.g., vibracoring) is that the mechanism to drive the drill into the ground is less likely to make water saturated sand liquefy (although some deformation is possible), and thus the natural features are likely to be preserved intact. The disadvantage is that, depending on the sedimentary materials, it may be hard or impossible to drill into the ground, e.g., a) at the Hardwick site the first 1.5 m of sediments (above water table and with a loamy sand and silt texture) were difficult to penetrate; and b) the presence of gravel and/or hard pieces of wood may impede coring. Also, some deformation may occur during drilling (due to friction at the tube walls and core compaction) that need to be taken into account when analysing the cause of the deformation.

If trenches can be excavated to fully expose liquefaction features, cores can be very useful to explore sediment below the base of the trenches. For example, in this study we were able to access deeper levels of the sedimentary section where we could undertake analysis that helped us identify the source layers for the sand blows. Identification of the source layer is helpful to guide in situ geotechnical testing for the purpose of back-calculating magnitudes of paleoearthquakes. Coring is also useful for collecting samples to establish the maximum age of paleoliquefaction events (i.e., dating of deeper sediment).

If the water table is too high and trenches cannot be excavated, cores may help to identify the presence of liquefaction but they may not be sufficient by themselves to provide confident interpretations of liquefaction features and to fully analyse the timing of the paleoliquefaction event(s). This is for two main reasons. Firstly, a substantial number of cores would be needed to make sure that there are enough pieces of evidence to build a robust interpretation of the presence of liquefaction. Secondly, the stratigraphic relationships between sediments and liquefaction features are also difficult to interpret from cores alone and therefore to estimate the timing of liquefaction events.

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4.0 AGE OF PALEOLIQUEFACTION FEATURES AT LINCOLN, CHRISTCHURCH AND CORRELATION WITH KNOWN PALEO-EARTHQUAKES

4.1.1 Age of paleoliquefaction features

The timing of paleoIiquefaction features is based on the ages of sedimentary units hosting the paleoliquefaction and younger units not affected by the paleoliquefaction found in the trenches. The sedimentary units were dated with radiocarbon samples (see discussion of age of sediments in Appendix 1). The radiocarbon sample locations can be found on trench logs in Appendix 2 and in Figure 4.1 and Figure 4.2. The radiocarbon dates are displayed in Appendix 5.

At the Hardwick site, paleoliquefaction features formed between AD 908 and AD 1336. The paleoliquefaction event occurred after the formation of the buried soil at the top of Macrounit 2 and before deposition of the crevasse splay deposits of Macrounit 1 (after Unit 6bA and before 5Cg; e.g., HWK 6 trench; Figure 4.1). The sand blow formed when unit 6bA (HWK 6 trench) was at the ground surface. The upper portion of the sand blow was subsequently eroded. The trees rooted in the paleosol died when younger fluvial sands were deposited over the erosional surface. Using stratigraphic constraints that show the liquefaction event was older than Macrounit 1, but younger than Macrounit 2, sequence analysis in OxCal estimates the timing as AD 908–1336 (Appendix 6).

At the Marchand site, paleoliquefaction features formed between AD 1017 and AD 1840 (Figure 4.2 and Appendix 5). The mottled and weathered sand dikes intrude sediment deposited in AD 995–1141. Because the paleodikes seem to extend to the base of the topsoil we do not have a layer that could constrain the minimum age of this event. No historical earthquake is known to have induced liquefaction in this area; therefore, the weathered liquefaction features probably formed before European settlement (~1840). The poorly constrained age estimated for the Marchand paleoliquefaction features overlap the age of the Hardwick features. Given the proximity of the two sites, it is possible (although not required) that they formed during the same event. It is also possible that the paleoliquefaction features found at Marchand belong to two or more events, one of them being the same as the Hardwick one. We do not however, observe any cross-cutting relationships among the paleodikes mapped in MAR 3 trench (Figure 4.2) and thus we infer that the paleoliquefaction found at this site represents one event.

We do not have a clear stratigraphic link to confirm that the paleoliquefaction at Hardwick and at Marchand represent the same shaking event. However, if we assume a link given the close proximity (2 km) between the two sites and the similar responses during the modern liquefaction events, the timing of the paleoevent would be constrained to AD 1019–1337 (see Appendix 5) by using the slightly younger constraint on the age of the unit hosting liquefaction at the Marchand site. In summary and with the data available, we can confirm that at least one liquefaction event occurred in the Lincoln area between AD 908 and AD 1336 (or AD 1019–1337 assuming the Hardwick and Marchand events are the same), although we cannot rule out the occurrence of another event in the period AD 1017–1840.

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Figure 4.1 Paleoliquefaction sand blow and dikes observed in the HWK 6 trench. Note that the sand blow age is constrained by the ages of layer 6bA (paleosol) and the base of layer 5Cg. Note that presence of an erosional unconformity between 6bA and 5Cg that provides evidence that the upper part of the sand blow has been eroded. Calibrated ages are shown in Appendix 5.

Figure 4.2 Paleoliquefaction observed in the MAR 3 trench. Note dikes appear subhorizontal in the north trench wall because the wall is subparallel to the dike strikes (Appendix 3). Modern dikes cut through paleodikes, indicating a younger relative age. We can only constrain the maximum age of the paleoliquefaction with the age the host sediment (Unit C(g)). Calibrated ages are shown in Appendix 5.

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4.1.2 Correlation of paleoliquefaction features with known paleoearthquakes

In Figure 4.3 we have plotted known historic and prehistoric earthquakes in the wider region in relation to the timing of paleoliquefaction at, and distance to, our site. We have also plotted other active fault sources that are close to the site but for which we do not have a seismic record (Forsyth et al., 2008; Stirling et al., 2012). For the AD 908–1336 (or AD 1019–1337) liquefaction event, potential strong-shaking sources are the Alpine Fault, the Hope Fault (Conway, Hope River and Hurunui segments) and the Porters Pass Fault. Alternatively, a local source, either one of the mapped faults (Figure 1.1 and Figure 4.3) or a yet unidentified fault could have also been the source.

Large earthquakes on the Alpine Fault, Porters Pass Fault or other regional faults provide possible sources responsible for the paleoliquefaction that occurred at the Hardwick site at AD 908–1336. The other fault with a paleoearthquake in this time period is the Hope Fault, but we rule this out because the Mw 7.1 1888 Amuri Earthquake on the Hope River segment of the Hope fault did not cause liquefaction in eastern Christchurch (Hutton, 1888), and magnitudes assigned to other segments of the Hope Fault (Stirling et al., 2012) are similar to the Hope River segment, as are distances to our sites. We also exclude the Greendale Fault, since its penultimate event is likely to have happened >20,000 years ago (Hornblow et al., in review; Villamor et al., 2012), and the Ashley Fault, which has not ruptured in the last 1300 years (Sisson et al., 2001).

If the paleoliquefaction found at the Marchand site occurred after the paleoliquefaction at the Hardwick site, several fault sources that ruptured during the period A.D. 1340–1840 could have been responsible for it (Figure 4.3). Our preferred candidate is the Porters Pass Fault. This is because the timing of the most recent event on the Porters Pass Fault seems to coincide with the closure of Moncks Cave in Sumner, Christchurch, by a rock avalanche (Jacomb, 2008; 2009). The location of the avalanche is only 22 km away from the Marchand site. It is possible that both sites experienced strong shaking simultaneously.

The temporal associations between paleoearthquakes and paleoliquefaction presented here are only preliminary. More paleoliquefaction sites are required to assess the likely source for the strong shaking. Furthermore, several active faults, closer to the site than those discussed above and for which there are currently no paleoearthquake records, or local unrecognised faults, could have been responsible for the paleoliquefaction described here. Given that 10 of the 11 liquefaction events documented in the Christchurch area during the 2010–2011 earthquake sequence occurred in earthquakes sourced from blind faults, there is a strong possibility that paleoliquefaction events were similarly triggered by blind fault earthquakes. For example, Bastin et al. (in review) tentatively attributed some of the pre-2010 liquefaction features in eastern Christchurch to the 1869 Mw 4.9 Christchurch earthquake. Recently identified offshore active faults (Barnes et al., 2011) provide additional seismic sources to consider.

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Figure 4.3 Timing of major historic earthquakes, prehistoric fault ruptures and large landslides in the wider region. Timing information from: Alpine Fault, Berryman et al. (2012); Hope Fault – Conway segment, Langridge et al. (2003); Hope Fault – Hope River segment, Cowan and McGlone (1991); Hope Fault – Hurunui segment, Langridge et al. (2013) and Khajavi et al. (in review); Poulter Fault, Berryman and Villamor (2004); Acheron Rock Avalanche, Smith et al. (2012); Porters Pass Fault, Howard et al. (2005); Ashley Fault, Sisson et al. (2001); Moncks Cave, Jacomb (2008; 2009). Location information for all faults from Litchfield et al. (2014) and references therein. Note that vertical black dotted lines associated with individual faults indicate that there are no paleoearthquake data. Vertical red dotted lines indicate that paleoearthquake studies suggest that no fault ruptures occurred during that period.

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5.0 CONCLUSIONS • Comparison of liquefaction and landform maps suggest liquefaction in the 2010–2011

Canterbury Earthquake Sequence occurred predominantly in the alluvial sediments underlying the Yaldhurst 1 surface and coastal sand dunes. That is, liquefaction mainly occurred in association with alluvial and sedimentary environments that are younger than ~2500 years in the Canterbury area.

• Liquefaction occurred in association with point bar deposits at the Marchand study site near Lincoln. This spatial correlation was also observed by other researchers working in similar environments in the Canterbury region and is widely observed in international literature.

• At the Hardwick study site, also near Lincoln, liquefaction ejecta appears to be associated with crevasse splay deposits, but the liquefied sand is sourced from deeper sediments. However, how the crevasse splay influences the occurrence of liquefaction could not be determined within the scope of this study.

• The sizes of the 2010–2011 liquefaction features are similar in the Hardwick and Marchand sites: sand blow sizes range from 10 to 40 cm in height and 200 to 400 cm in width, and dike widths range from 1 to 3 cm. The sand blow sizes represent the composite sand blow dimensions after accumulation of ejecta from the 4 September 2010, 22 February 2011 and 13 June 2011 liquefaction events. Similar sand blow sizes are described from the M 5.9 1988 Saguenay, Quebec earthquake. In contrast, sand blows found in the New Madrid Seismic Zone (earthquakes typically of Mw 6.8–8) are commonly 1–2 m thick, 10s of metres wide, 10s–100s of metres long, and associated with sand dikes 10s–100s of centimetres wide.

• Apart from the characteristics of ground shaking (PGA, frequency content, and shaking duration), other factors that appear to control the size of liquefaction features are the liquefaction susceptibility of the sediments and the size of liquefiable sediment bodies. With regard to the latter we observe that the size of point bars in the meander-belt of the Mississippi River floodplain in the vicinity of the New Madrid Seismic Zone can be tens of kilometres long and hundreds of metres wide, whereas the point bars in the Waimakariri system of the Canterbury Plains are usually less than 1 km long and only a few tens of metres wide. The correlation between the size of fluvial deposits and liquefaction features is likely to be related to the volume of liquefiable source sand.

• At the Hardwick and Marchand sites we found paleoliquefaction features dated at AD 908 to AD 1336 and AD 1017 to AD 1840, respectively. If both events are the same (perhaps likely given the close proximity between the two sites and their similar responses during the 2010–2011 liquefaction events), the timing of the paleoliquefaction event is constrained to between AD 1019 and AD 1337.

• Large earthquakes on the Alpine Fault and Porters Pass Fault occurred during the same time span as the paloeliquefaction event observed at our study sites. Other nearby known and unknown local and regional faults could also be earthquake sources responsible for the paleoliquefaction that occurred at the Hardwick and Marchand sitea at AD 1019 and AD 1337. Causative faults that we can rule out include the following: the Hope Fault (the Mw 7.1 1888 Amuri Earthquake did not cause liquefaction in eastern Christchurch); the Greendale Fault (penultimate event is likely to have happened >20,000 years ago) and the Ashley Fault (no rupture in the last 1300 years).

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With respect to advantages and disadvantages of each technique tested during this project we conclude:

• Geomorphic mapping, aided by DEMs developed from LiDAR data, is useful to select areas, such as point bar environments, that may be prone to liquefaction. High resolution DEMs and aerial photographs are required to identify these subtle geomorphic features in areas of very low relief.

• In the Canterbury region, analysis of 1940s and 1960s aerial photos was useful in identifying areas with sandy soils (well drained, hence lighter surfaces) that may be indicative of paleoliquefaction features or sand blows.

• Paleoseismic trenching is the best technique for identifying, measuring, and dating paleoliquefaction features, but can be limited by high water tables (e.g., water tables are often within 1.5 m of the surface in the Christchurch region) which may cause trench collapse. The depth to which trenches can be safely excavated limits the stratigraphic record exposed in the trench.

• GPR profiling is a useful technique for non-invasive reconnaissance of sites, but currently is not able to resolve sand dikes ~<10 cm wide and sand blows ~<10 cm thick (depending on ground and surface conditions). The GPR signal is also limited by the depth of water table.

• Hand piston coring is a useful technique for identifying some liquefaction features, such as high-angle sand dikes containing clasts and possible liquefaction source beds exhibiting soft-sediment deformation structures. However, within core samples it is difficult to distinguish liquefaction-related sand blows and sills from sand layers deposited by fluvial processes. Coring is proved useful to extend the sedimentary record into the past and to search for the source layers of the liquefied sands.

• X-ray analysis of core samples is a promising technique for identifying flow structures within sand dikes and soft-sediment deformation. However, the X-ray scanning resolution used in this study was not of high enough resolution to detect features that could suggest liquefaction.

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6.0 RECOMMENDATIONS: FUTURE RESEARCH

For future research, we recommend:

• Taking advantage of other 2010–2011 liquefaction sites to search for paleoliquefaction features, given that in our study paleoliquefaction features have been found in the same locations that liquefied during the Canterbury Earthquake Sequence. Additional paleoliquefaction studies will help to further evaluate the timing, source area, and magnitude of the paleoearthquake that induced liquefaction between AD 1019 and AD 1337, and to develop the paleoearthquake record in the Christchurch region. Under an aligned PhD study being currently undertaken, studies will be conducted at two more sites affected by the 2010–2011 liquefaction. Similar studies in the Christchurch area have been undertaken by some authors of this report (S. Bastin et al.) and will be published soon.

• Using a combination of techniques to study paleoliquefaction depending on site conditions. Some of the techniques we tested in this study, such as aerial photo analysis, high resolution X-ray, Computed Tomographhy (CT) Scanning, and other physical properties of cores, and grain magnetisation orientation, could be further explored, and will be used at the new study sites in the PhD stsudy. In the exploratory stage of a study, Cone Penetration Tests (CPTs) could be used to assess potential presence of liquefiable sand in the subsurface of areas with no historic liquefaction. In our on-going study (at sites of 2010–2011 liquefaction), we plan to use CPT logs as a correlation tool in combination with coring and trenching to understand the sedimentary architecture of the sites (see below). Shallow electrical resistivity is another technique that has proved useful in other overseas studies of paleoliquefaction but we did not test it in our study.

• Taking advantage of the imagery resources acquired after each event of the Canterbury Earthquake Sequence, as well as mapping of the liquefaction ejecta undertaken immediately after the events, to explore sedimentary environments that are prone to liquefaction. For example, point bar deposits are widely known for being highly susceptible to liquefaction. It is possible that a thorough study of the 2010–2011 liquefaction of the Christchurch area may identify other elements of the fluvial or coastal sedimentary environments with high liquefaction susceptibility. On-going research by a PhD student involves investigating the role of the crevasse splay in the occurrence of liquefaction at the Hardwick site, and will also investigate liquefaction in the coastal dune environment.

• Finally, we recommend that a national database of liquefaction features (historic and paleo-features) be created to help understand ground shaking levels and recurrence, and liquefaction recurrence in areas of New Zealand with liquefaction susceptible soils.

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7.0 ACKNOWLEDGEMENTS

We thank the Hardwick and Marchand families for land access. Caroline Hardwick provided invaluable information, including an extensive photographic record of the liquefaction events. Numerous colleagues from University of Canterbury and University of Madrid assisted with fieldwork and discussions. Bella Ansell assisted with GIS work. This study is funded by the New Zealand Earthquake Commission, the New Zealand National Hazards Research Platform and GNS Strategic Development Fund. Dougal Townsend and Nicola Litchfield provided internal GNS Science reviews.

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exploring methods to assess paleoliquefaction in the canterbury area

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