Calibration of the Near-Surface Seismic Structure in the San Francisco Bay Area Velocity Model

Anupam Patel, & Kim B. Olsen

Submitted September 7, 2025, SCEC Contribution #14930, 2025 SCEC Annual Meeting Poster #TBD

The near-surface seismic structure to depths of around 1–2 km, and specifically shear wave velocity (Vs), strongly modulates the seismic response. In tomographically derived Earth models, however, properties of the shallow (< ∼2 km) subsurface are typically poorly constrained where borehole, geotechnical, or well data are absent (outside sedimentary basins). By contrast, the Vs structure below ∼2 km is generally better resolved by tomography because of broader data coverage and stronger inversion constraints. To improve the shallow structure, Hu et al. (2022a,b) and Yeh & Olsen (2024) applied a low-velocity taper (LVT) constrained by Vs30 substantially, which significantly improved the fit between simulations and recordings at sites outside southern California basins. In this study, we have used 0-1 Hz 3D physics-based wave propagation simulations and strong motion recordings for Mw4.0-4.4 events to estimate the optimal thickness distribution of LVT implemented in the San Francisco Community Velocity Model (SFBA VM-Beta version). For each event, we conducted simulations including LVTs with depths between 0 and 2100 m and a minimum Vs of 250 m/s, and computed the Fourier amplitude spectrum (FAS) for all LVT depths. The model with a constant tapering depth of 1200 m produced the minimum FAS bias of 46%, but indicated that an LVT with spatially varying tapering depths may further reduce bias in predicting ground motions. Toward that goal, we plan to run additional events and spatially interpolate the optimal tapering depths at each site. The spatially-variable LVT distribution will allow refined ground motion prediction, as well as update of Z1.0 and Z2.5 depths for use in Ground Motion Models.

Citation
Patel, A., & Olsen, K. B. (2025, 09). Calibration of the Near-Surface Seismic Structure in the San Francisco Bay Area Velocity Model. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Community Earth Models (CEM)