Examining the relationship between stress drop and peak ground acceleration for small-to-moderate earthquakes in the San Francisco Bay Area

Daniel T. Trugman, & Peter M. Shearer

Submitted August 13, 2017, SCEC Contribution #7538, 2017 SCEC Annual Meeting Poster #264 (PDF)

Theoretical and observational studies have suggested that inter-event variability in the median ground motions of larger (M>5) earthquakes is controlled primarily by the dynamic properties of the earthquake source, such as Brune-type stress drop. Analogous results remain equivocal for smaller events due to the lack of comprehensive and overlapping ground motion and source parameter datasets in this regime. Here we investigate the relationship between peak ground acceleration (PGA) and dynamic stress drop for a new dataset of 5297 M>1.5 events occurring in the San Francisco Bay Area from 2002 through 2016. For each event, we measure peak ground acceleration on horizontal-component channels of stations within 100km distance and estimate stress drop from P-wave spectra recorded on vertical-component channels from the same stations. We then develop a nonparametric ground motion prediction equation using a mixed-effects generalization of the Random Forest algorithm that we use to model the joint influence of magnitude, distance, and near-site effects on observed PGA. We find a strong correlation between dynamic stress drop and the residual between the observed and model-predicted PGA of each event, with the events with higher-than-expected PGA associated with higher values of stress drop. The strength of this correlation increases as a function of magnitude but remains significant even for smaller magnitude events with corner frequencies that approach the observable bandwidth of the acceleration records. Mainshock events are characterized by systematically higher stress drop and PGA than aftershocks of equivalent magnitude. Coherent local variations in the distribution of dynamic stress drop provide observational constraints to support the future development of nonergodic ground motion prediction equations that account for variations in median stress drop at different source locations.

Key Words
stress drop, ground motion, source properties, peak ground acceleration, earthquake hazard

Citation
Trugman, D. T., & Shearer, P. M. (2017, 08). Examining the relationship between stress drop and peak ground acceleration for small-to-moderate earthquakes in the San Francisco Bay Area. Poster Presentation at 2017 SCEC Annual Meeting.


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Ground Motions