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Simulation of Long Duration Shaking in the Oakland Urban Region Using the USGS San Francisco Bay Area Seismic Velocity Model

Session: Numerical Modeling of Earthquake Motion, Rupture Dynamics, Seismic Noise, Wave Propagation and Inverse Problems [Poster]

Type: Poster

Date: 4/23/2021

Presentation Time: 03:45 PM Pacific

Description: 

In this study, we test whether incorporating additional fine-scale information, e.g., Vs30 measurements and higher-resolution geologic maps, into the USGS San Francisco Bay Area 3D Seismic Velocity Model helps improve accuracy of synthetic motions in the Oakland – Alameda – Emeryville urban region in comparison with observations. In this area, long duration, high amplitude coda waves have been observed during moderate (Mw ~3.5-4.5) earthquakes but are not reproduced in simulations using the USGS velocity model (e.g., Aagaard et al., 2010; Hirakawa and Aagaard, 2021, in review). These late motions are especially prominent at stations close to the shoreline of the bay, where low-velocity bay mud and artificial fills are prominent on surface maps but are not explicitly represented in the 3D velocity model. The large amplitude coda waves likely represent seismic reverberations in the shallow, low-velocity Quaternary surface layer present in the East Bay. We use SW4, a finite-difference seismic wave propagation code, to simulate motions from 10 moderate magnitude earthquakes with hypocenters in the East Bay. Our goal is to make adjustments to the velocity model that lead to more accurate synthetic motions. We find that the current relationship that defines the velocity of the uppermost (Quaternary) rock in the velocity model overestimates measured Vs30 values and that, in general, the amplitudes of late reverberations in the synthetic records can be increased by reducing velocities in this layer.

Presenting Author: Evan T. Hirakawa

Student Presenter: No

Authors