ACCOUNTING FOR SITE EFFECTS IN PROBABILISTIC SEISMIC HAZARD ANALYSIS: OVERVIEW OF THE SCEC PHASE III REPORT

SUMMARY Probabilistic seismic hazard analysis (psha) depends on empirical attenuation relations that quantify the amplitude of ground motion as a function of distance, magnitude, and sometimes other parameters. Given the variety of factors complicating earthquake shaking, attenuation relationships have a large degree of uncertainty (i.e., Observations are highly scattered about predicted values). An important question is the degree to which this scatter can be reduced by accounting for site effects. In this context we define a site effect as the tendency for mean ground motion, given all potentially damaging earthquakes throughout the region, to be significantly different from the predicted value. The so-called phase iii effort of the southern california earthquake center has concentrated on identifying and accounting for such site effects in psha. In approximately 10 papers that will be published together in the near future, we have carried out both empirical and theoretical investigations of how attenuation relations can be evaluated and modified to account for site effects. Our findings show that certain site attributes, such as detailed geological classification and/or depth to basement at sediment sites, are correlated with ground motion amplitudes. We are presently quantifying how accounting for these systematic differences influences seismic hazard analysis.