Source, Site and Path Characterization of the July 2019 M7.1 and M6.4 Ridgecrest Earthquake Sequence

Grace A. Parker, Annemarie S. Baltay, Eric M. Thompson, John Rekoske, & Dan McNamara

Submitted August 12, 2019, SCEC Contribution #9458, 2019 SCEC Annual Meeting Poster #TBD

The 2019 July 5 M7.1 Ridgecrest, California earthquake generated peak ground motions in excess of 50% g and 60 cm/s near the fault and was recorded on over 700 local and regional seismic stations across California and eastern Nevada. In addition, the July 4 M6.4 earthquake was also well-recorded as were numerous aftershocks, including six M5+ and over 50 M4+ events. Given the large number of nearly co-located events, this dataset allows us to rigorously model the spatial and temporal variability in ground motion. We compare the recorded horizontal ground motions, processed for peak-ground acceleration, velocity and psuedo-spectral acceleration, to several NGA-West2 ground-motion models (GMMs) to understand the spatial distribution of the ground motions. We take a non-ergodic approach and use a mixed effects regression method to partition the ground motion residuals into the separate contributions of source, site and path, and to evaluate the uncertainty of each component. To account for the coupling of the site and path terms at large source-to-site distances caused by the spatial clustering of the events, we utilize additional NGA-West2 recordings in computing site-specific terms. At long periods, the repeatable site terms clearly highlight strong amplification in basins across Southern California and deamplification in the mountainous regions. At short periods, there is less spatial coherence. We also explore the ability of VS30, the time-averaged shear wave velocity in the upper 30m, to capture observed site response, as well as the effect of measured versus proxy-based VS30 estimates. The high-frequency source terms are reflective of earthquake stress drop and show both along-fault and on/off-fault dependence, as well as temporal variability. We consider a per-event differential attenuation term to investigate the variation in both anelastic attenuation (Q) and geometrical spreading across Southern California. Understanding the dependence of the source, site, and path terms on geology, velocity structure and fault proximity highlights physical mechanisms that generate and control ground motion and allows us to add more predictive physics into fully non-ergodic models to improve the accuracy and precision of GMMs.

Citation
Parker, G. A., Baltay, A. S., Thompson, E. M., Rekoske, J., & McNamara, D. (2019, 08). Source, Site and Path Characterization of the July 2019 M7.1 and M6.4 Ridgecrest Earthquake Sequence. Poster Presentation at 2019 SCEC Annual Meeting.


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