Revising the USGS Bay Area Velocity Model in the East Bay

Evan Hirakawa

USGS Earthquake Science Center

Date & Time
Building 3, Rambo Auditorium

The current version of the USGS Bay Area Velocity Model is a 3D representation of the seismic velocity structure in the San Francisco Bay region and has been shown to successfully reproduce many aspects of ground motion and wave propagation from local earthquakes. Still, some users have noted that there are areas in the model that could be improved and should be investigated. Here, we present an exercise to evaluate and improve the model in a subregion centered around the East Bay. Special attention is paid to the area immediately east of the Hayward Fault in the Berkeley Hills. We use a finite-difference code (SW4) to simulate wave propagation from eight Mw3.5-4.2 earthquakes with epicenters in the East Bay. The synthetic seismograms obtained are compared with observed motions from each respective event. Similarity between synthetic and observed motions is judged by measures of arrival time difference and energy mismatch. First, some questionable features in the current version of the model are addressed and modified. Then, several variations of the model are generated by adjusting velocities assigned to particular geologic units. The evaluation procedure indicates which variation best represents the true velocity structure by the chosen metrics. The final adjusted model significantly improves certain aspects of the synthetic waveforms in comparison with the current version of the velocity model.

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