Detailed traveltime tomography and seismicity around the 2019 M7.1 Ridgecrest, CA, earthquake using dense rapid-response seismic data

Abstract:

We derive detailed 3D models of seismic P- and S-wave speeds (V_P and V_S, respectively) and a local earthquake catalog for the region around the 2019 M_w7.1 Ridgecrest earthquake in the Eastern California Shear Zone. The study uses traveltimes from aftershocks of the M_w7.1 Ridgecrest earthquake recorded by over 250 seismic sensors deployed rapidly in several array configurations [Catchings et al. 2020] and stations of the regional network. The raw waveform data are first processed using automated procedures to detect and locate over 94,000 earthquakes—more than two and a half times the number of events reported by the published catalog of the Southern California Seismic Network (36,343 events). Over 2.47M P- and 1.52M S-wave arrival times are used in a new traveltime tomography method based on stochastic Voronoi-cell subspace projections of the model space [Fang et al. 2020] and a robust 3D ray tracer that uses the Fast Marching Method for solving the eikonal equation [White et al. 2020]. The derived high-resolution V_P, V_S, and V_P/ V_S models correlate closely with surface geology and spatial seismicity patterns. Salient features include (a) high V_P (3.7% faster than surrounding material on average) and low V_S (4.3% slower) volumes coinciding with the Coso geothermal field, a conspicuous gap of seismicity, and the NW terminus of the Eastern Little Lake Fault (ELLF) as well as (b) low V_P (10.8% slower), low V_S (7.1% slower), and low V_P/V_S (2.3%) volumes coinciding with the Garlock fault and the SE terminus of the ELLF, and (c) strong cross-fault velocity contrasts coinciding with the NE terminus of aftershocks associated with the Southern Little Lake Fault, and an isolated swarm of seismicity on the Garlock fault to the SW of the ELLF.