By: J.H. Steidl

Abstract: This study determines site response factors that can be applied as corrections to a rock attenuation relationship for use in probabilistic seismic hazard analysis. The site response factors are amplitude and site class dependent. These amplification factors are determined by averaging ratios between observed and predicted ground motions for peak ground acceleration (PGA) and for 5% damped response spectral acceleration at T= 0.3, 1.0, and 3.0 second oscillator periods. The observations come from the strong-motion database of the Southern California Earthquake Center, and the predictions are based on the Sadigh (1993) rock attenuation relation. When separated and averaged according to surface geology, significantly different site response factors are found for Quaternary and Mesozoic units, but a sub-classification of Quaternary is generally not justified by the data. The low input-motion amplification factors are consistent with those obtained from independent aftershock studies at the PGA and T=0.3 second period. An observed trend of decreasing Quaternary site amplification with higher input motion is consistent with nonlinear soil behavior, however, the trend exists for Mesozoic sites as well implying that this may be an artifact of the Sadigh relationship. There is a correlation between larger site response factors and lower average shear-wave velocity in the upper 30 meters for low predicted PGA input motions, with an increase in the correlation with increasing period. The 0.3-second site response factors for Quaternary data in southern California determined in this study are consistent with 0.3-second NEHRP site response correction factors, however, at 1.0-second period there remain some inconsistencies. A trend is also seen with respect to sediment basin depth, where deeper sites have higher average site response factors. These results constitute a customized attenuation relationship for southern California. The implication of these customized attenuation models with respect to probabilistic hazard analysis is examined in Field et al. (this issue).

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