Evaluation and Updates for the USGS San Francisco Bay region 3D seismic velocity model (SF-CVM) with focus on shallow sediments in the Central Bay Block

Evan T. Hirakawa, & Brad T. Aagaard

Published September 10, 2023, SCEC Contribution #13145, 2023 SCEC Annual Meeting Poster #200

We evaluate and update the shallow (<500 m) velocity structure in the USGS San Francisco Bay region 3D seismic velocity model (SF-CVM) in the Central Bay Block (CBB) by comparing recorded and simulated motions using the SF-CVM from ~1000 ground-motion records for 14 Mw 3.5-4.5 earthquakes near the Hayward and Calaveras faults. The CBB lies between the San Andreas and Hayward faults and spans much of the San Francisco Bay urban region. The current version of the SF-CVM (v21.1) contains a widespread Quaternary zone with ubiquitously low, likely unrealistic velocities (80 m/s) at the surface spread throughout much of the CBB. This shallow velocity structure leads to synthetic ground velocities with peak values that are much higher than observations, especially in areas where low velocity sediments are not likely to be found (e.g., in the South Bay foothills). We combine two datasets to refine the properties of the Quaternary zone in the SF-CVM: (1) maps of Quaternary surface geologic units (Knudsen et al., 2000; Witter et al., 2006) and (2) 1D velocity profiles from downhole logs (Boore, 2003). We use the maps to divide the SF-CVM Quaternary zone into “coarse-grained” and “fine-grained” units and derive separate velocity-depth curves for them by regression of the downhole data. The resulting model has lower velocity sediments around the SF Bay, particularly in SF Bay Mud, and increased velocities in areas of older alluvium and dune sands. We show a significant improvement in simulated PGV across most ground-motion records when using the modified model: in most cases, unrealistic ground motions produced by SF-CVM v21.1 are reduced in the new model as a result of increased surface velocities. The model still does not reproduce long-duration reverberations commonly observed at sites in the East Bay Plain, which may be the aim of future updates.

Key Words
velocity model, San Francisco Bay Area, simulations

Citation
Hirakawa, E. T., & Aagaard, B. T. (2023, 09). Evaluation and Updates for the USGS San Francisco Bay region 3D seismic velocity model (SF-CVM) with focus on shallow sediments in the Central Bay Block. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)