Abstract
The 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.
Plain Language Summary
Understanding the earthquake nucleation process has direct implications for earthquake physics and seismic hazards. Specifically, identifying the geophysical processes within fault zones that precede and result in subsequent earthquakes has been of great interest to the earthquake science community. This study explores how the ratio of compressional (P) wave speed to shear (S) wave speed changes in both space and time and their correlations with the subsequent seismicity evolution, focusing on the 2019 Ridgecrest earthquake sequence in California. In most slipping areas of the earthquakes, the P‐wave to S‐wave speed ratios are relatively uniform. However, in places where faults end, cross each other, or change direction, we observe unusual values. We find high ratios near the three major earthquakes on a small scale. Additionally, the ratios change where the main earthquake (magnitude 7.1) initiated. The ratios are low between a moderate (magnitude 5.4) and the main (magnitude 7.1) earthquake and increase after the main earthquake, indicating the presence of over‐pressurized fluids near the earthquake source. The associated high pore pressure might have helped nucleate the Ridgecrest mainshock. Our findings show that these speed ratios can be highly sensitive to seismic activities and could help us better understand how earthquakes start.
Key Points
We examine the spatial and temporal evolution of near‐source material properties during the 2019 Ridgecrest earthquake sequence
The spatially homogeneous Vp/Vs ratios in the rupture zones indicate little structural resistance during the earthquake ruptures
The temporal changes in Vp/Vs ratios suggest fluid presence, which may have facilitated the nucleation of the 2019 Ridgecrest mainshock