Source complexity of small and moderate earthquakes in the 2019 Ridgecrest earthquake sequence

Qimin Wu, & Xiaowei Chen

Published August 16, 2021, SCEC Contribution #11635, 2021 SCEC Annual Meeting Poster #242 (PDF)

It is well known that many larger earthquakes have highly complex rupture processes with significant variation in slip and stress drop over the rupture plane. As the quantity and quality of data increase, similar source complexity is being observed for well-recorded smaller earthquakes. The 2019 Ridgecrest earthquake sequence occurred within zones of complex and distributed fault system within the eastern California shear zone (ECSZ), and included thousands of small to moderate earthquakes (M ~2-5). The foreshocks and aftershocks are well recorded by the southern California seismic network, making it an ideal case study to better understand earthquake source complexity. In this study, using the dataset organized for the community stress drop validation project, we combine both frequency and time domain analyses based on the empirical Green’s function (EGF) technique to investigate the source processes for selected M2-5 earthquakes. In the frequency domain, we apply the multiple spectral ratio method on all potential target events with M ≥ 2 in the Ridgecrest sequence to measure corner frequencies of the target events. We quantify the complexity of the stacked spectral ratios by examining their deviation from the simple omega-square source model. We also apply an improved stacking method to estimate corner frequencies and compute stress drops. In the time domain, we apply an iterative deconvolution method to retrieve relative source time functions (RSTFs), from which we quantify possible directivity effects and the existence of multiple subevents when applicable. Combining the STF and spectral ratio results, we will classify the analyzed events into “simple” and “complex” categories. Stress drop estimates from the two types of spectral analysis will be compared and then be examined jointly and separately for the two classified categories to investigate the effects of source complexity on the variability of stress drop estimates. Our results will contribute to a better understanding of fundamental earthquake rupture processes, and the limitations of Brune-type simple source model in estimating source parameters such as stress drop and source dimension and the influence on ground motion predictions.

Citation
Wu, Q., & Chen, X. (2021, 08). Source complexity of small and moderate earthquakes in the 2019 Ridgecrest earthquake sequence. Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology