ce 5603 seismic hazard assessment …courses.ce.metu.edu.tr/.../02/lecture_1_introduction.pdfce 5603...

CE 5603 SEISMIC HAZARD ASSESSMENT

LECTURE 1: INTRODUCTION

By : Prof. Dr. K. Önder Çetin

Middle East Technical University

Civil Engineering Department

http://www.metu.edu.tr/

http://www.metu.edu.tr/

Course Outline

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Course Outline

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Why Assess Seismic Hazard Levels ?

4

"Every once in a while, something bad happens as a result of an earthquake, and

probabilistic seismic hazard analysis is the basis on which one reckons how often

bad happens at some place of interest" (Cornell)

Seismic Hazard vs. Seismic Risk

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"In 1910, the Seismological Society of America identified the three parts of the

earthqake problem that merit study: the event itself (when, where and how

earthquakes occur), the associated ground motions, and the effects on structures.

These are still the fundamental elements in evaluating earthquake risk." (McGuire,

2004)

"Seismic hazard analysis is the foundation of seismic risk and/or seismically

induced failure analysis of facilities; therefore seismic hazard analysis can be used

to determine risk-based seismic design and construction practice. Seismic hazard

analysis can also be developed and used as an end in itself, generally in the form

of seismic hazard maps." (Cornell)


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More specifically, "risk" is identified as the product of "hazard" and "vulnerability".

Along with more formal definitions of "hazard" and "risk", a clear distinction

between the frequently confused words in terminology can be made using specific

examples:


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If an engineer is utilizing a building code in his/her design, the seismic coefficients

and the design spectrum to be used are the results of a "hazard" analysis, i.e. it

categorizes the zones in a specific area prone to earthquake effects. A hazard map

doesn't define whether your structure to be built is safe or not against failure, nor it

tells the order of expected damage.


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Imagine a residence, built in accordance with seismic regulations, at a distance of

5 km from the active fault. It'll potentially be prone to higher "hazard" than a similar

structure located at 50 km away from the fault. However, this does not necessarily

mean that the residence located far away is under less "risk". Given the conditions

that the latter is very poorly constructed, it will probably be under higher "risk",

although located in a zone of lighter "hazard". The same conditions apply vice

versa.


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Imagine two identically constructed (same structural system, same construction

quality) structures at adjacent lots, having the same in-situ conditions. Building A

inhabits residents, however Building B is the headquarters of an international

software company. Two buildings are under the same "hazard" level, and expected

to undergo similar light damage, however the calculated annual income loss due to

temporary repair and operational loss at Building B makes it susceptible to higher

"risk". Similar examples are valid for other types of structures such as tunnels,

dams and transportation infrastructure.

Probabilistic vs. Deterministic

Seismic Hazard Assessment

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Supporters of probabilistic and deterministic philosophies in seismic hazard

analyses have been almost continuously debating, sometimes ironically, on the

proper selection of hazard levels and calculation of ground motions, since the

probabilistic methodology first sprung back in late 1960's. Here is a pinch from two

opposite views:



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Probabilistic View Supported

"The deterministic approach to seismic hazard analysis DSHA, seems to be very

different animal from PSHA. DSHA deals with fascinating !!! things like MCE

(Maximum Credible Earthquake), or SSE (Safe Shutdown Earthquake), or OBE

(Operating Basis Earthquake) terminology plied in the large dams nuclear reactor

trades.

In the heyday of DSHA, back in 1960's, and 1970's when there were still

acknowledged wise men in the Earth and Engineering Science related to

earthquakes, one or may be a few of these people would decide the

MCE/SSE/OBE's and where they would be likely to occur. A little cookbook ground

motion estimation would then ensure and –bingo!- seismic design criteria. This

doesn't sound like much serious things like nuclear reactors, and people have

been on the lookout for something better ever since. This, of course is PSHA,

essentially the only other game in town."



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Probabilistic View Supported



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Almost Pure Determinisim: Krinitzsky's Position

Ellis Krinitzsky, a core supporter of deterministic methods in seismic hazard

analysis defends his opinions with the following main items listed:

"Earthquakes do not occur randomly in space and time"

"Using Poissons models is wrong (lack of memory)"

"The magnitude – recurrence relation is so uncertain that it becomes meaningless"

"Probabilitic seismic assessment is a bad science"

"Only deterministic approaches (maximum earthquake) are reasonable"

Krinitzsky's and other supporters' deterministic point of view in depth can be

accessed from numerous publications, where a few are listed here as suggested

references:

Krinitzsky (1993a), Krinitzsky (1993b), Krinitzsky (1993c), Krinitzsky (2002a),

Krinitzsky (2002b), Mualchin and Krinitzsky (2003), Hatheway (2003), Castaños

and Lomnitz (2002).



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Almost Pure Determinisim: Krinitzsky's Position



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Keeping a Balance Between

Criticism on probabilistic seismic hazard analysis is primarily focused on points

such as misuse of probabilistic tools, lack of adequate data for proper handling of

uncertainties and ambiguities in applying expert opinions. However, as Bommer

(2003) contributes, the debate regarding the relative merits and shortcomings of

deterministic and probabilistic approaches are likely to continue for many years. A

big step ahead taken towards a healthier debate will probably be an agreement on

the clear definitions of hazard assessment concepts and terms.



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Keeping a Balance Between

More fair criticism towards probabilistic seismic hazard assessment can be made

upon identifying the true source of problematic issues. Probabilistic methodology

provides the neccessary tools for handling the uncertainties associated with the

earthquake and seismic hazard assessment phenomenon. However, only proper

use of these available tools will lead to an acceptable and sound analysis

workflow.

Specific examples for most of the criticism towards probabilistic methods can be

given as; misinterpretation of magnitude – recurrence relations of earthquakes

based on instrumental data obtained from records aged slightly over a century,

misuse of logic trees for combining results from analyses having different distance,

magnitude definitions, and several others. While some of these issues are

currently under research to come up with more representative analysis methods,

an inevitable amount that can not be overlooked has to do with building up a

common terminology that bridges probabilistic and deterministic methodologies.

McGuire (2001), also shows very good demonstrations of complementary roles of

deterministic and probabilistic methods for decision – making purposes.



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References

McGuire, R. K., (2004). Seismic Hazard and Risk Analysis, EERI Monograph

Series, No. 10, Earthquake Engineering Research Institute.

Cornell's monograph supply full reference details (also not sure about the

reference of paragraphs in quotes)

Krinitzsky, E. L., (1993a). Earthquake probability in engineering – Part I: The use

and misuse of expert opinion, Engineering Geology, 33, 257-288.

Krinitzsky, E. L., (1993b). Earthquake probability in engineering – Part II:

Earthquake recurrence and limitations of Gutenberg-Richter b-values for the

engineering of critical structures, Engineering Geology, 36, 1-52.

Krinitzsky, E. L., (1993c). The Hazard in Using Probabilistic Seismic Hazard

Analysis, Civil Engineering, November, 60-61.

Krinitzsky, E. L., (2002a). How to obtain earthquake ground motions for

engineering design, Engineering Geology, 65, 1-16.

Krinitzsky, E. L., (2002b). Epistemic and aleatory uncertainty: a new shtick for

probabilistic seismic hazard analysis, Engineering Geology, 66, 157-159.

Engineering, 21, 377-384.



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References

Mualchin, L., Krinitizsky, E. L., (2003). A new and defective regulation in California

for protecting critical buildings from earthquakes, Engineering Geology, 69, 415-

419.

Hatheway, A. W., (2003). How to obtain earthquake ground motions for

engineering design, 67, 403-404.

Castaños, H., Lomnitz, C. (2002). PSHA: is it science?, Engineering Geology, 66,

315-317.

Bommer, J. J., (2003). Uncertainty about the uncertainty in seismic hazard

analysis, Engineering Geology, 70, 165-168.

McGuire, R. K., (2001). Deterministic vs. probabilistic earthquake hazards and

risks, Soil Dynamics and Earthquake Engineering, 21, 377-384.

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