Accounting for site effects in probabilistic seismic hazard analyses of southern California: Overview of the SCEC phase III report

This article presents an overview of the Southern California Earthquake Cen ter (SCEC) Phase-III effort to determine the extent to which probabilistic seismic hazard analysis (PSHA) can be improved by accounting for site effec ts. The contributions made in this endeavor are represented in the various articles that compose this special issue of BSSA, Given the somewhat arbitrary nature of the site-effect distinction, it must be carefully defined in any given context. With respect to PSHA, we define the site effect as the response, relative to an attenuation relationship, averaged over all damaging earthquakes in the region. A diligent effort has been made to identify any attributes that predispose a site to greater or lower levels of shaking, The most detailed maps of Quaternary geology are n ot found to be helpful; either they are overly derailed in terms of disting uishing different amplification factors or present southern California stro ng-motion observations are inadequate to reveal their superiority. A map ba sed on the average shear-wave velocity in the upper 30 m, however, is found to delineate significantly different amplification factors. A correlation of amplification with basin depth is also found to be significant, implying up to a factor of two difference between the shallowest and deepest parts of the Los Angeles basin. In fact, for peak acceleration the basin-depth co rrection is more influential than the 30-m shear-wave velocity, Questions r emain, however, as to whether basin depth is a proxy for some other site at tribute. In spite of these significant and important site effects, the standard devi ation of an attenuation relationship (the prediction error) is not signific antly reduced by making such corrections, That is, given the influence of b asin-edge-induced waves, subsurface focusing, and scattering in general, an y model that attempts to predict ground motion with only a few parameters w ill have a substantial intrinsic variability. Our best hope for reducing su ch uncertainties is via waveform modeling based on first principals of phys ics. Finally, questions remain with respect to the overall reliability of attenu ation relationships at large magnitudes and short distances, Current discre pancies between viable models produce up to a factor of 3 difference among predicted 10% in 50-yr exceedance levels, part of which results from the un certain influence of sediment nonlinearity.