T004-0004
Detailed nucleation process and mechanism of the July 2019 Mw 6.4 Ridgecrest, California earthquake

Monday, 7 December 2020
Poster
Min Liu, China University of Geosciences (Beijing), Beijing, China; Dalhousie University, Department of Earth and Environmental Sciences, Halifax, Canada, Miao Zhang, Dalhousie University, Department of Earth and Environmental Sciences, Halifax, NS, Canada and Hongyi Li, China University of Geosciences (Beijing), School of Geophysics and Information Technology, Beijing, China
Abstract:
The July 2019 Ridgecrest earthquake sequence, consisting of two principle earthquakes (Mw 6.4&7.1) and associated foreshocks and aftershocks, broke a nearly 20-year absence of strong earthquakes in California. It is noteworthy that a significant foreshock sequence preceded the Mw 6.4 mainshock in ~2 hours. However, the detailed nucleation process and the underlying nucleation mechanism of the Mw 6.4 mainshock and its foreshocks are still not clear. In this study, we comprehensively analyzed seismograms obtained from nine nearby stations before the Mw 6.4 mainshock using state-of-the-art methods. We utilize the Match&Locate method to build a highly complete foreshock catalog, investigate the detailed spatial and temporal evolution of the foreshocks, and discuss the potential nucleation mechanism of the Mw 6.4 mainshock and its foreshocks. The Mw 6.4 mainshock was preceded by 40 foreshocks with a local magnitude range of -0.39–4.0 (from 15:35:29 to 17:32:52 on July 4, 2017, UTC), which occurred on three individual fracture zones with different orientations. The largest foreshock (ML 4.0) likely separates the foreshock activity into two stages with different nucleation mechanisms. Earthquakes following the ML 4.0 earthquake including the Mw 6.4 mainshock can be confidently explained by the cascade triggering model (i.e., stress transfer), because they occurred within regions of increasing Coulomb stress and outside of the rupture area of the ML 4.0 event. A warm of small earthquakes initiated the seismic sequence, showing close hypocenters and high waveform similarity. By assuming an empirical stress drop of 3 Ma and a simple circular rupture model, their rupture patches are most likely overlapped. Thus, their nucleation mechanism may be explained by aseismic slip (i.e., preslip model). Our observations demonstrate preslip model and cascade model may jointly govern the nucleation process of the Mw 6.4 mainshock, but at different stages.
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