Structural control on the 2019 Ridgecrest earthquake from local seismic tomography

We apply local seismic tomography to 159,754 P-wave and 101,751 S-wave arrival times of 5287 local earthquakes to determine 3-D images of P- and S-wave velocity (Vp, Vs), Poisson's ratio (ν) and Vp azimuthal anisotropy in the source zone of the 2019 Ridgecrest earthquake in Southern California. Its big foreshock (M6.4) and mainshock (M7.1) took place in a high-velocity (high-V) and low-velocity (low-V) transition belt. The source zone exhibits significant low-Vs and high-ν anomalies in the lower crust and uppermost mantle, probably reflecting crustal fluids associated with upwelling mantle flow. A high consistency between the seismogenic fault of the 2019 Ridgecrest earthquake and a low-Vs and high-ν anomaly suggests that the fluids in the fault zone affected the rupture process. Fast-velocity directions (FVDs) of Vp azimuthal anisotropy in the upper crust reflect effects of faults and cracks, whereas mineral alignments affect the FVDs in the low-V parts of the lower crust. Dominant NW–SE and E–W FVDs are revealed in the study area, indicating that the Pacific-North American plate relative motion causes the crustal deformation of the study area.