Updates to the USGS San Francisco Bay region 3D seismic velocity model (SF-CVM) with a focus on near surface (<1 km) properties

Evan T. Hirakawa, & Brad T. Aagaard

Submitted September 7, 2025, SCEC Contribution #14656, 2025 SCEC Annual Meeting Poster #TBD

We present updates to the USGS San Francisco Bay region 3D seismic velocity model (SF-CVM; most recent version v21.1) which are focused on shallow (< 1 km) properties west of the Rodgers Creek-Hayward-Calaveras fault system. The geology-based SF-CVM uses seismic wavespeed–depth rules to populate different rock units within a geologic framework model (GFM) with seismic properties. We identify a few issues that arise in the shallow Quaternary–Tertiary units from the current configuration of rules and the GFM: (1) The current Vs–depth rule for the Quaternary zone imposes a very low wavespeed of Vs = 80 m/s at the surface. This may be valid in some places, such as around the shores of the SF Bay, where “Bay Mud” is present; however, the Quaternary zone spreads over a vast part of the SF Bay region where such a low wavespeed is not realistic. (2) The current Vs–depth rule contains a very sharp jump from 85 m/s at 25 m depth to 418 m/s at 50 m, which does not seem realistic. (3) The Quaternary wavespeed rule intersects the Tertiary rule at 150 m depth, which leads to a wavespeed reversal in many places where the Quaternary zone overlies the Tertiary; we consider this to be unrealistic as well. We show that the first two problems lead to simulated ground motions with amplitudes much larger than observations.
We address these issues by developing new, smoothly varying wavespeed–depth rules by fitting a power-law function to existing downhole wavespeeds and applying these new rules to a partitioned Quaternary zone, which was obtained from overlying a Quaternary surface geologic map that distinguishes fine-grained (e.g., mud/clay) from coarse-grained (e.g., gravel) sediments. We also use a new rule for Tertiary rock, constructed so that the Quaternary rule tapers into the Tertiary, avoiding any possible velocity reversals at depth. We evaluate the current and updated seismic velocity model by simulating ground motions from 16 Mw 3.5–4.5 earthquakes in the region and comparing synthetic motions with observations. We find that the updated model significantly improves amplitude overestimates resulting from v21.1, mostly due to the changing of the Quaternary wavespeed rule. We show that ground motions are still underestimated in some places, particularly around the East Bay Plain. We provide some possible avenues for future updates, including modifications to the GFM.

Key Words
Community Velocity Model, San Francisco Bay Area, Northern California

Citation
Hirakawa, E. T., & Aagaard, B. T. (2025, 09). Updates to the USGS San Francisco Bay region 3D seismic velocity model (SF-CVM) with a focus on near surface (<1 km) properties. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Community Earth Models (CEM)