UCLA

Initial results of an ongoing project to develop a site response model specific to Northern California are presented, focusing on the San Francisco Bay Area (SFBA). Although current NGA-West2 ground motion models were developed for application across broad regions (such as California), they have the potential to be biased when applied to particular sub-regions. This study investigates the impacts of the unique geology in the SFBA on site response features. A model conditioned on time-averaged shear-wave velocity in the upper 30 m (VS30) and broad geomorphic categories (future iterations of the model will also consider depths to shear-wave velocity isosurfaces, z1.0) is used in this study. A critical element in the modeling process is to reflect the impact of different geologic conditions through the selected geomorphic categories, including (from rock to soil) Franciscan, Great Valley Sequence, and Tertiary rock, shelf deposits, valleys, and basins. When all sites are grouped, site responses inferred at 748 ground motion stations across the region exhibit a trend with VS30 that is generally consistent with statewide trends. However, mean residuals within four geomorphic categories exhibit strong differences, highlighting important effects of the sub-regional geology.