Evaluation of Uncertainties of the Northern California Velocity Model Adopted for the CyberShake Study 24.8 Using Simulations of Small Earthquakes

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ID: 320219
2026
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Abstract
Summary This study evaluates the performance of the velocity model adopted for the CyberShake 24.8 (CS24.8) study when used to constrain wave propagation in three-dimensional regional-scale physics-based simulations for seismic hazard estimates. The CS24.8 study was developed to estimate seismic hazard in a subdomain surrounding the San Francisco Bay Area (SFBA) in California, adopting a physics-based finite-difference scheme for frequencies up to 1 Hz; above that, a stochastic scheme with site-specific adjustments is used. The velocity model adopted in the CS.24.8 study was a modified version of the USGS regional velocity model developed for the SFBA. The evaluation of the velocity model is based on comparisons between simulated and recorded ground motions for 18 small-to-moderate local earthquakes. Our analysis focuses on two frequency ranges: 0-1 Hz for estimating wave-propagation uncertainties for users of the CS24.8 study, and 1-5 Hz to provide insight into the performance of the velocity model for future Cybershake studies in this region. Metrics based on Fourier amplitude spectra (FAS) and waveforms’ duration are used for quantitative evaluation of the velocity model. Two aspects of the simulated ground motions are analyzed: (i) the median and variability of the ground motion in the region and (ii) wave propagation effects for specific source-site pairs. For (i), the velocity model leads to an underprediction of the FAS ranging from 0.1 LN-units at 0.3 Hz to 0.5 LN-units at 1 Hz for the horizontal component, and an underprediction of the duration by a factor of 2. The underprediction can be explained by the 400 m/s minimum shear-wave velocity adopted in the CS24.8 velocity model, which is larger than the actual values in the soft marine quaternary sediments of the SFBA, where most stations are located. The spatial variability of the FAS from the simulations over the region is lower than that from the observations over the frequency range 0.3 to 1 Hz, suggesting that the 3-D velocity structure is too smooth. When extending the analysis up to 5 Hz, the underprediction pattern increases up to 0.7 LN-units, and the spatial variability of the ground motions increases, reconciling the gap observed at lower frequencies. For (ii), the evaluation shows that the 3-D simulations improve the accuracy of wave propagation effects for the FAS compared to the ergodic ground-motion models (GMMs) for frequencies less than 0.7 Hz and have similar accuracy up to 1 Hz, being the maximum frequency solved in the physics-based scheme of the CS24.8 study. When extending the analysis above 1 Hz, misrepresentations in the 3-D velocity model introduce noise into the simulated ground motions, leading to a less accurate estimate of the FAS at these frequencies compared to GMMs. Our results inform users of the CS24.8 study that the physics-based simulation (up to 1 Hz) offers performance comparable to or better than standard GMMs, while accounting for wave-propagation uncertainties. These findings can guide future refinements of the velocity model.
Reference Key
openalex_W7167813013 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors Camilo Pinilla-Ramos, Yehuda Ben-Zion, Norman Abrahamson, Maxime Lacour, Xiaofeng Meng
Journal geophysical journal international
Year 2026
DOI
10.1093/gji/ggag245
URL
https://doi.org/10.1093/gji/ggag245
Keywords Keywords not found

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