Enhancing Characterization of Near-Surface Velocity Structure(s) in the San Francisco Bay Area: A Stationary and Spatially Varying Approach

This study presents the development of two new sedimentary velocity models for the San Francisco Bay Area (SFBA) to improve the near-surface characterization of shear-wave velocity ($V_S$), with the ultimate goal of enhancing the Bay Area community velocity model. A stationary model, based solely on $V_{S30}$, and a spatially varying model incorporating location-specific adjustments were developed using a dataset of 200 measured $V_S$ profiles. Both models were formulated within a hierarchical Bayesian framework, using a parameterization that ensures robust scaling. The spatially varying model includes a slope adjustment term modeled as a Gaussian process to capture site-specific effects based on location. Residual analysis shows that both models are unbiased up to $V_S$ values of 1000 m/sec. Along-depth variability models were also developed using within-profile residuals. Applying the proposed models in the SFBA results in an increase in $V_S$ in the San Jose area and east of the Hayward Fault, supporting simulations that suggest over-amplification in these regions compared to observations. Goodness-of-fit (GOF) comparisons using linear site-response analyses demonstrate that the proposed models outperform the USGS model in capturing near-surface amplification across a broad frequency range. Incorporating along-depth variability further improves GOF scores by reducing over-amplification at high frequencies. These results underscore the importance of integrating community velocity models with detailed sedimentary velocity models to enhance regional seismic hazard assessments.