Leveraging Local Depth Phases for Improved Hypocenter Analysis and Discovery of a Thick Seismogenic Zone in Ridgecrest, California

Abstract Accurate earthquake location is of fundamental importance for understanding seismogenic processes, revealing the Earth's interior structure, and mitigating seismic hazard. However, precisely determining the depth of an earthquake is often challenging due to the severe trade‐off between focal depth and origin time, especially in the absence of nearby seismic stations. To address this challenge, we have developed an integrative procedure for reliably and efficiently identifying the sP depth phase in local and regional seismic records. After picking the traveltimes of first‐arriving P, S waves and sP depth phases, we sequentially refine earthquake hypocenter (longitude, latitude and depth) and origin time within a Bayesian inversion framework. The efficacy of the proposed depth phase identification procedure and earthquake location method is validated through the analysis of small‐to‐moderate aftershocks that occurred within 2 months of the 2019 M w 7.1 Ridgecrest earthquake. Our study shows that including depth phases can significantly reduce location uncertainty in depth by a factor of five. Moreover, the results achieved by jointly using first arrivals and depth phases are less dependent on the background velocity model, enabling more accurate location estimates for 86.6% of the examined earthquakes. In regions northwest of the mainshock nucleation area, the base of the seismogenic zone is located generally below 10 km, likely sandwiching a much shallower brittle‐to‐ductile transition zone (<4 km) beneath the Coso geothermal site. This locally abrupt change in rock rheology may modulate the rupture propagation of large earthquakes.