Spatiotemporal Variations of In Situ V_p/V_s Ratios During the 2019 Ridgecrest Earthquake Sequence Suggest Fault Zone Condition Changes

AbstractThe 2019 Mw 7.1 Ridgecrest earthquake was the largest event in California over the past 20 years. The earthquake was preceded by a sequence of foreshocks. However, the physical processes leading to the mainshock remain unclear. Here, we image the ratios of compressional (P)‐ to shear (S)‐wave velocity (Vp/Vs) in the fault zones and examine the spatial and temporal evolution of near‐source material properties during the Ridgecrest earthquake sequence. We find that the Vp/Vs ratios are spatially homogeneous in the rupture zones, indicating a lack of fault‐zone material difference along strike. We identify an anomalously low Vp/Vs ratio fault patch near the mainshock hypocenter before its occurrence, which returned to the background value after the earthquake. This low Vp/Vs ratio suggests fluid overpressure, which may have facilitated the nucleation of the 2019 Ridgecrest mainshock.