Evaluation and Updates for the USGS San Francisco Bay Region 3D Seismic Velocity Model in the East and North Bay Portions

ABSTRACT We update the eastern and the northern portions of the detailed domain of the U.S. Geological Survey San Francisco Bay region 3D seismic velocity model (SFVM) based on comparisons of recorded and synthetic ground motions from 20 moderate (Mw 3.7–4.6) earthquakes. We modify the current SFVM (v.08.3.0) by assigning alternate property-versus-depth relations to the existing 3D geologic model. In some places, changes correspond to reassigning correct relations in which geologic units appear to be mislabeled, and in other places we subdivide geologic units where mapped geologic boundaries are missing from the 3D models so that we can implement a velocity contrast across a boundary. We also make ad hoc adjustments to velocity rules near the surface in some areas to better fit arrival times (specifically, in the Livermore basin). The updates reduce misfits in waveform correlation, travel time, cumulative absolute displacement, and peak ground velocity and are included in v.21.1 of the model. The selected earthquakes are small enough so that we neglect finite-source effects and model them as point sources. This allows us to assume that observed waveform characteristics are the result of path effects, and discrepancies between synthetic and recorded motions arise from misrepresentation of the elastic properties. Our analysis suggests refining the 3D geologic model, and adjusting the rules assigning properties to the geologic units will further improve the accuracy of the SFVM for simulating earthquake ground motions.