Ground‐Motion Attenuation in the Sacramento–San Joaquin Delta, California, from 14 Bay Area Earthquakes, including the 2014 M 6.0 South Napa Earthquake

Peak ground motions (acceleration and velocity) radiated by earthquakes in the San Francisco Bay area and recorded within the Sacramento-San Joaquin Delta generally attenuate faster with distance than the Next Generation Attenuation-West2 ground-motion prediction equations (GMPEs). We evaluate the attenuation for a wide set of paths into the Delta by analyzing recorded ground motions from fourteen 4 <= M < 7 earthquakes located on major Bay area faults: the San Andreas, Calaveras, Hayward, West Napa, and Green Valley faults. We select stations within azimuthal ranges of 38 degrees-114 degrees into the Delta and calculate the residuals of the peak ground motions relative to the Boore et al. (2014) GMPEs. We then fit the natural log of these peak ground acceleration and peak ground velocity residuals for each earthquake to the function a - kr(gamma), in which a is an event term and kr(gamma) is the differential attenuation. Although there is some variation in the differential attenuation obtained for each earthquake, the peak ground motions from most of the 14 events attenuate faster than predicted by the Boore et al. (2014) GMPEs. The differential attenuation does not appear to depend on azimuth or magnitude of the earthquake; however, earthquake depth may have an effect. Our results suggest that attenuation models for the Delta can be significantly improved through regionalization, although this regionalization will increase the model complexity and the epistemic uncertainty. Supplemental Content: Figures providing additional details about the station sets analyzed for each earthquake included in this study, the peak ground acceleration (PGA) and peak ground velocity (PGV) Boore et al. (2014; hereafter, BSSA14) residuals and regression results for each individual earthquake, and the PGA and PGV residuals and regression fits for each earthquake after applying our regional correction to BSSA14, and table providing additional details about each recording analyzed in this study, including station information, peak ground motions, BSSA14 residuals, V-S30 values, and data sources.