Path Effects and Crustal Properties in the San Francisco Bay Area

Tara Nye, Grace A. Parker, Annemarie S. Baltay, Evan T. Hirakawa, Kyle B. Withers, & Morgan P. Moschetti

Published September 8, 2024, SCEC Contribution #13634, 2024 SCEC Annual Meeting Poster #170

We investigate relationships between earthquake ground motion path effects and crustal properties for 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 large uncertainty and variability. GMMs rely on parameters describing the earthquake source, path, and site conditions, and much work has been done on source and site using known physical properties (e.g., magnitude; Vs30, time-averaged Vs in the upper 30 m; and Z1.0, depth to the 1 km/s Vs isosurface). Conversely, understanding of how crustal properties of unique source-to-site propagation paths influence ground motion is limited; current GMMs use simple physical terms to describe the path (e.g., distance) or entirely statistical ones, which can lead to greater uncertainty and inaccurate partitioning of ground-motion effects. To improve GMMs and reduce their uncertainty, we explore relationships between empirical path effects and physical parameters encompassing various crustal properties in the SFBA.

Using ground motion recordings from 1129 M3–6 earthquakes at 834 stations (~72,000 two-component horizontal records), we compute residuals between observed ground motion and NGA-West2 GMM estimates for peak ground acceleration, peak ground velocity, and 5% damped pseudospectral acceleration. We partition residuals into event and site terms using mixed-effects maximum likelihood regression and assume the remaining residual represents path effects. We consider correlations between these path effects and a variety of parameters, including well-established descriptors (e.g, seismic velocity and attenuation) and more novel crustal properties (e.g., fault density and orientation). We also explore relationships with traditionally site-prescribed parameters (e.g., high frequency attenuation, kappa; and Z1.0) as the delineation between the path and site components of a raypath is ambiguous. By evaluating a wide range of parameters, we aim to establish how the heterogeneous conditions of the SFBA influence earthquake ground motion. Additionally, we aim to identify novel relationships to improve GMM accuracy and decrease aleatory variability with the goal of better characterizing seismic hazard.

Key Words
San Francisco Bay Area, path effects

Citation
Nye, T., Parker, G. A., Baltay, A. S., Hirakawa, E. T., Withers, K. B., & Moschetti, M. P. (2024, 09). Path Effects and Crustal Properties in the San Francisco Bay Area. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)