What Can the Brune-type Stress Drops of Ridgecrest Aftershocks Tell Us About the Mainshock Rupture?

Abstract:

With its high seismicity rate and good station coverage, the July 2019 Ridgecrest, California sequence provides an worthy opportunity to explore spatial and temporal patterns in earthquake stress drop. Here we use a spectral decomposition approach to calculate Brune-type stress drops for more than 11,000 precisely located and well-recorded earthquakes that occurred in the Ridgecrest study region from year 2000 through the end of 2019. Ridgecrest aftershocks have higher stress drops than background seismicity, possibly due to their deeper hypocenters. While we observe a slight increase in stress drop with magnitude, this finding strongly depends on the assumed high-frequency falloff rate inherent to the Brune model. Most intriguing, coherent spatial patterns of stress drop in the aftershock sequence correlate with the slip distribution of the M 7.1 mainshock, whose northwest rupture tip terminated in a long-lived zone of enervated stress drop. These preliminary observations point to a potential link between mainshock rupture dynamics and the Brune-type stress drops of smaller earthquakes.