SCEC Award Number 25291 View PDF
Proposal Category Individual Research Project (Single Investigator / Institution)
Proposal Title Calibration of the Near-Surface Seismic Structure in the San Francisco Community Velocity Model (SFCVM)
Investigator(s)
Name Organization
Kim Olsen San Diego State University
SCEC Milestones A1-1, D3-2, D3-3 SCEC Groups CEM, GM, RC
Report Due Date 03/15/2026 Date Report Submitted 05/05/2026
Project Abstract
 We have calibrated the near-surface structure of the San Francisco Community Velocity Model (SFCVM) with the aim to improve its ground motion prediction efficacy using Fourier Amplitude Spectral (FAS) bias up to 1 Hz via 3D physics-based wave propagation simulations for a series of Mw3.8–4.5 earthquakes. In areas of systematic underprediction we implement a Vs30-constrained spatially varying low-velocity taper (LVT), with the optimal station-specific thickness determined from trial-and-error simulations for different taper depths. The optimal spatially variable LVT reduces the FAS bias for areas outside the basins (defined as sites with surface Vs > 1 km/s) by up to 18% on average for the small earthquakes. In the San Francisco East Bay area, the addition of a shallow (25 - 75 m) low-velocity layer (Vs = 250 m/s) further improves the FAS bias by increasing the amplitudes and duration of the synthetics. In areas where the reference model systematically overpredicts amplitudes of the source ensemble, we calibrate the model by removing the near surface (up to 900 m) decrease in velocities, which further improves the FAS bias. The depth to the isosurface of Vs = 1.0 km/s (Z1.0) in the SFCVM is increased by up to about 100 m outside the basins due to the implementation of the optimized LVT, with implications for the depth-dependent basin amplification terms used in Ground Motion Models.
SCEC Community Models Used Community Velocity Model (CVM)
Usage Description We extracted the SFCVM from SCEC UCVM for the physics-based simulations, and used the 'explorer' tool to plot cross sections of the model.
Intellectual Merit We have shown that techniques such as a low-velocity taper and a high-velocity extension can significantly improve the accuracy of ground motion prediction in the San Francisco Bay Area CVM. Moreover, the use of physics-based ground motion simulations of small earthquakes (~M4) is useful for establishing the ground motion prediction error in community velocity models.
Broader Impacts The estimated spatially-variable low-velocity taper (LVT) and other calibrations has increased the accuracy of the ground motion prediction from SFCVM. The calibrated SFCVM establishes a basis for improved seismic hazard assessments in Northern California. The depth to the isosurface of Vs = 1.0 km/s (Z1.0) in the SFCVM is increased by up to about 100 m outside the basins due to the implementation of the optimized LVT, with implications for the depth-dependent basin amplification terms used in Ground Motion Models.
Project Participants Kim Olsen (PI)
Anupam Patel (PhD student)
Exemplary Figure Figure 2. Illustration of the LVT (left) and the effects on ground motions at station BK-BKS (right) for LVT=600m tapering depth for the 2018/01/04 Mw4.38 event. Credit: Anupam Patel
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