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Modeling Path Effects From 3D Velocity Structure in the San Francisco Bay Area

Description: 

We investigate relationships between earthquake ground-motion path effects and 3D velocity variations in the San Francisco Bay Area (SFBA) using residual analysis of ground-motion models (GMMs). GMMs are a key component in probabilistic seismic hazard analysis but can be limited in predictive power due to uncertainties in modeling earthquake source, path, and site effects. Much work has been done to constrain the source and site using known physical properties, while our understanding of effects of 3D crustal properties along unique source-to-site wave propagation paths is limited. Current GMMs use simple physical terms to describe the path (e.g., a distance metric and uniform Q-attenuation) or entirely statistical ones, which can lead to greater uncertainty and inaccurate partitioning of ground-motion effects. We seek to reduce uncertainty in GMM estimates by quantifying more complex 3D effects of crustal properties on ground motions, specifically focusing on S-velocity variations along 3D raypaths.

We test the impact of two 3D velocity models developed using different methods: a smooth model derived using tomographic inversion and a model with sharp velocity contrasts derived using geologic features. We raytrace through both models and compute velocity metrics for each path, including the integral of velocity gradients (dVI) and the derivative of dVI (ddVI). We explore relationships between the velocity metrics and non-ergodic path terms computed using mixed-effects maximum likelihood regression of GMM residuals. The non-ergodic path terms exhibit linear correlations with both dVI and ddVI. We estimate path terms using these simple relationships, and we additionally apply random forest regression to estimate path terms from multi-parameter combinations. Applying the estimated path terms reduces errors in GMM estimates, especially for the multi-parameter model. These findings have the potential to improve modeling complex path effects via a simple model parameter and reduce uncertainty in seismic hazard assessments.

Session: Recent Advances in Modeling Near-source Ground Motions for Seismic Hazard Applications [Poster]

Type: Poster

Date: 4/16/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Tara

Student Presenter: No

Invited Presentation: 

Poster Number: 54

Authors