Testing_Basic_SHA_Assumption_Wyss_2015

Testing_Basic_SHA_Assumption_Wyss_2015 - Testing the Basic...

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Testing the Basic Assumption for Probabilistic Seismic-Hazard Assessment: 11 Failures by Max Wyss INTRODUCTION In the probabilistic seismic-hazard assessment ( PSHA ), one cal- culatesthepeakgroundacceleration( PGA )(alternativelyvelocity, displacement,orintensity)atagivenlocationthatisexpectednot to be exceeded by a probability chosen by expert judgment (dif- fering in different cases) and during a selected period. This is usually done by considering the influence of several seismogenic sourcesinthevicinityofasite.Herewe considerthesimple case inwhichonesource capableofalargeearthquakedominatesthe PGA calculation, a situation often encountered in practice. The first parameter one needs to know for estimating the PGA is the maximum magnitude of earthquakes M max that is possible in each seismogenic source. When one of the M max events clearly dominates the PGA at the site in question, as in this case, one speaks of the maximum credible earthquake ( MCE ) affecting the site. Estimating the PGA due to the MCE is specified as one of the requirements for assessing the seismic hazardforlargedamsasdetailedbytheInternationalCommis- sion on Large Dams ( ICOLD, 2010 ). Oncethe MCE isidentified,oneneedstoknowtheannual probabilityofthe MCE ,whichisproportionaltoitsrecurrence time. The PSHA method assumes the occurrence rate (or probability of occurrence) of the MCE can be estimated by extrapolating the occurrence rate and magnitude distribution of small earthquakes in the volume in question. In cases of large MCE s, the extrapolation is performed over one to five orders of magnitude. This basic assumption of the standard PSHA method that theoccurrenceprobabilityof MCE scanbederivedbyextrapo- lationfromtheoccurrencerateofsmallearthquakesisnotsup- ported by a physical theory. I am showing here that this hypothesisfails11outof11tests.Thetestsconsistofcompar- ing the recurrence intervals derived using this assumption for 11 fault segments known to generate major earthquakes and for which the recurrence intervals are known from direct ob- servation or estimated from paleoseismology. The recurrence intervals estimated by the PSHA method are larger than the observed ones by factors ranging from 3.3 to 178 (average 44, median 28) in sixcases from California, one from Alaska, and onefromChina.InthreecasesfromtheNorthAnatolianfault zone ( NAFZ ), the discrepancy factors range from 100 to 12,000 (average and median 10,000). In most regions, the earthquake catalog is too short for containing the MCE , let alone several of its occurrences, to es- timate the recurrence time. Therefore, the distribution of ob- served small earthquakes, described by Log N .0136 a bM .0133 1 .0134 inwhich N isthecumulativenumberand a and b areconstants tobederivedlocally, isextrapolatedto estimatetheprobability of the MCE .
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