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

Description: 

Seismic community velocity models (CVMs) are foundational for many basic and applied topics ranging from derivations of earthquake source properties to simulations of ground motions. The CVM quality directly impacts the accuracy and epistemic uncertainty of simulated wave fields generated by scenario earthquakes or 3D physics-based simulations. We present an innovative methodology to evaluate the effect of epistemic uncertainty of the CVM on key ground motion intensity measures related to seismic hazard analysis. This methodology involves using point sources to simulate wave fields from small earthquakes located in different parts of the crustal volume of interest and comparing the results to observations. Using small events and the point source approximation facilitates a focus on properties of the CVM rather than finite source ruptures. Evaluating differences between recorded and simulated data in spectral amplitude, phase matching, and signal duration allows us to estimate epistemic uncertainties and identify features and subregions of the CVM to improve. Here, we apply this framework to evaluate the performance of the Northern California CVM developed for the CS24.8 CyberShake study. The evaluation employs three-dimensional ground motion simulations of sets of local small-to-moderate earthquakes with magnitudes ranging from 3.5 to 4.5. Because of the relatively low ground-motion amplitude induced by these earthquakes, the near-surface materials are expected to be well-represented by a linear-elastic stress-strain relationship at all recording sites. Therefore, the misfit between recorded and simulated waveforms can be mainly attributed to structural complexities not fully represented in the used CVM and, to a lesser extent, the source parameterization (location, depth, focal mechanism, source time function). We will present the spatial distribution of residuals in the spectral domain, identify areas where the CVM performs best, describe the impacts of the results on the CyberShake simulations, and provide important guidance for future refinements of CVMs.

Session: Challenges and Opportunities in Constraining Ground-motion Models from Physics-based Ground-motion Simulations - II

Type: Oral

Date: 4/17/2025

Presentation Time: 10:45 AM (local time)

Presenting Author: Camilo Ignacio

Student Presenter: No

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