Potential for improving ground-motion relations in southern California by incorporating various site parameters

This article evaluates the possibility of improving a particular ground-mot ion relationship for predicting peak acceleration (PGA) and absolute respon se spectral accelerations at the periods of 0.3, 1.0, and 3.0 sec in southe rn California. We use the attenuation model of Abrahamson and Silva (1997), which Lee et al. (2000) found satisfactory for this region. We examine dif ferences between observed and predicted values (residuals) as a function of several site attributes to determine whether corrections can be made to im prove the predictions. This study differs from that of Steidl (2000) in tha t we use an attenuation model that accounts for sediment amplification and nonlinearity (Steidl used only a rock-site relationship). Residuals are sig nificantly correlated with basin depth. Depending on the specific frequenci es considered, ground motions at the deepest part of the basin average 30% to 80% higher than at the edge of the basin. Residuals rue also significant ly correlated with the estimates of the average amplification of peak veloc ity due to the basin velocity structure (Olsen, 2000). Since the basin dept h is correlated with the average basin amplification, there is no need to c orrect the ground-motion model for both effects. Detailed geology is genera lly found unhelpful in improving ground-motion predictions. Overall, correc ting the ground-motion relation reduces the standard error of ground-motion predictions by about 5%. Whether the ground-motion relation modifications suggested here are significant in terms of the implied seismic hazard is ev aluated in Field and Petersen (2000). The weak correlation of residuals with respect to site parameters motivated us to apply the test proposed by Lee et nl. (1998). This involves plotting residuals versus residuals for stations that have recorded more than one e arthquake. To the extent that systematic site effects cause the misfit betw een observations and the ground-motion model, such a plot will show correla tion among the residuals. Correlation coefficients are, surprisingly, very low, ranging from 0.16 for PGA residuals to 0.26 for 3-sec response spectra . Thus it seems that it will be very difficult to refine ground-motion pred iction equations beyond the very general categories now in use, improved ph ysical understanding of the site, source, and path contributions must play a major role in any future efforts to reduce the uncertainty in the ground- motion predictions.