Spatial and temporal variations of seismic azimuthal anisotropy following the 2019 ridgecrest earthquake sequence in southern california

Abstract

To investigate the spatial and temporal variations of seismic azimuthal anisotropy in the vicinity of the conjugate fault network activated by the 4 July 2019 (UTC) M6.4 foreshock and the 6 July 2019 M7.1 mainshock in Ridgecrest, California, a total of 1,470 shear wave splitting measurements were obtained from local events recorded by five seismic stations over the period of July 2019 to January 2020. The results suggest a strong asymmetry in anisotropy forming mechanisms across the NW-SE striking Eastern Little Lake Fault, which is the main fault of the area. In the area located to the northeast of the main fault, the observed fast orientations are dominantly N-S, which is parallel to the maximum horizontal compressive stress. In the area surrounding the main fault, the fast orientations are primarily parallel to the strikes of multiple fault zones including the main fault and the crossing faults, rather than solely consistent with that of the main fault, which may indicate along-strike variations in fault strength of the main fault. To the southwest of the main fault, the observed anisotropy is jointly controlled by faults or regional stress. The observed NE-SW-oriented anisotropy in the vicinity of two previously proposed blind faults confirms the existence of faults. The splitting times of anisotropic areas with different inducements are independent of the focal depths, suggesting that either the stress- or structure-induced anisotropy is mostly located in a shallow layer in the top several kilometers. A nearly 90-degree switch in the fast orientations and greatly reduced splitting times from a group of nearby earthquakes may indicate fault zone healing.