Shear Wave Splitting in the Vicinity of the 2019 M7.1 Ridgecrest, California Earthquake

The M7.1 earthquake sequence of Ridgecrest, California, has been the focus of numerous geoscientific studies since it occurred in early July 2019. To systematically investigate the spatial distribution and possible temporal variations of upper crustal anisotropy in the aftershock zone of the earthquake, local events recorded by three temporary stations from July to December 2019 were used for shear wave splitting analysis. The technique of minimizing the lesser of the two eigenvalues of the covariance matrix of the seismograms was utilized to search for the splitting parameters (including the fast orientation and splitting time). To ensure reliability of the results, we manually screened all the automatically determined measurements, and adjustments were made to the time window and frequency, if necessary, to filter out noises and exclude non-S signals. In most areas away from the fault zones, the resulting fast orientations were mostly consistent with the approximately N-S orientation of the maximum horizontal compressive stress (SHmax), while fast orientations near the fault zones were mostly parallel to the strike of the faults. Such a parallelism was most clearly observed in the intersection region of orthogonal faults. An intriguing preliminary observation is that some aftershocks with ray paths sampling the main fault zone show NW-SE (i.e., fault parallel) fast orientations, which were gradually replaced by N-S (i.e., SHmax parallel) fast orientations. If this feature can be confirmed by additional studies, it may indicate progressive healing of the fault zone damaged by the M7.1 earthquake.