Analysis of coseismic slip distributions and stress variations of the 2019 Mw 6.4 and 7.1 earthquakes in Ridgecrest, California

Abstract

Exploring spatiotemporal variations in the regional stress field is critical for understanding the interactions between active tectonic plates. On July 4 and 6, 2019, strike-slip earthquakes of Mw 6.4 sinistral and 7.1 dextral, respectively, occurred in Ridgecrest, California, which provided an appropriate condition for studying conjugate earthquake stress variation. In this study, we used GNSS coseismic displacement to invert the coseismic slip distributions of each earthquake. According to the spatiotemporal distribution, the focal mechanisms were divided into three time periods and five subareas. Therefore, spatiotemporal variations of the stress field were calculated. Results indicate that the two earthquakes were not purely strike-slip, but had minor dip-slip components. The maximum slips of the Mw 6.4 foreshock and the Mw 7.1 mainshock were 0.7 and 3.1 m, respectively. Areas of significant stress variations corresponded to areas of larger slip distributions. During the three time periods, the maximum horizontal stress (SHmax) direction in each subarea first experienced counterclockwise and then clockwise rotation; the SHmax direction of the foreshock epicenter region changed from 3.8° to 2.8° to 5.8°. After the mainshock, the SHmax of the foreshock epicenter region rotated clockwise by ~3.0°, the deviatoric stress was ~4.5 MPa, and the stress drop in the epicenter region of the mainshock was ~4.3 MPa, from which we can judge that the stress was almost completely released. Six months before the earthquakes, the b value began to change markedly, while it stabilized after the earthquakes. In the dextral shear zone formed by the relative motions of the Pacific and North American plates, the cumulative stress tends to be jointly regulated by NW–SE and NE–SW oriented conjugate fractures. These two Ridgecrest earthquakes are a phenomenon of stress regulation.