Updating the USGS San Francisco Bay Area 3D Seismic Velocity Model to Improve Ground Motion Estimates

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

Numerical simulations of earthquake ground motions are important for seismic hazard evaluation, as they provide more insight into complex attributes of seismic motions than empirical regressions. We focus on improving the accuracy of ground motion estimates for a region east of the Hayward fault, which previous studies have identified as needing adjustment. We update the empirical rules used to assign elastic properties to the Great Valley Sequence and Cenozoic sedimentary rocks in the USGS San Francisco Bay Area 3D geologic model using ground motion simulations of local earthquakes. The current work is part of an ongoing effort to improve this model by more closely reproducing observed ground motions.

We simulate motions from five Mw 3.0-4.4 earthquakes on the Hayward Fault. The data we use to compare with our synthetics are obtained from broadband stations in local networks within ~25 km of the earthquake epicenters. Some stations lie to the west of the fault zone above Franciscan Complex rocks, while others like to the east of the fault zone above more compliant Great Valley Sequence and Cenozoic sedimentary rocks. In addition to the high seismic hazard associated with the Hayward Fault, this bimaterial contrast makes the study area particularly interesting. The current empirical rules assign seismic properties to geologic units as functions of depth; however, this methodology neglects other factors such as complex deformation or uplift, which is a prevalent feature of the local geology. In this study, we will adjust the rules to more carefully consider the geologic history of rock units. The goal is to show improvements to synthetic waveforms at frequencies up to ~1 Hz based on comparisons with observed motions. We address adjustment uniqueness by considering the best-fitting model over all earthquake simulations.

Presenting Author: Evan T. Hirakawa

Authors