Coseismic reorientation of the crustal stress field at the base of the 2019 Ridgecrest earthquake fault zone

Jared T. Bryan, William B. Frank, & Pascal Audet

Published September 8, 2024, SCEC Contribution #13920, 2024 SCEC Annual Meeting Poster #047

The 2019 Ridgecrest earthquake sequence marks the largest seismic event in Southern California in over 20 years. Information on stress and damage evolution throughout the fault zone is necessary for understanding the dynamics of the event. Noise-based methods for measuring seismic velocity changes can sensitively probe changes in stress and damage, particularly in the shallow subsurface, but we need complementary methods to monitor deep crustal processes and to detect localized velocity changes. Receiver functions (RFs) provide a localized measurement of the crustal column and are uniformly sensitive to velocity changes throughout the crust. With over 1500 magnitude 5.0+ magnitude 5.0+ teleseismic earthquakes every year, RFs permit quasi-continuous monitoring of the crustal velocity structure.

We build a catalog of ~8,000 P-phase receiver functions for 31 stations near Ridgecrest using teleseismic events from 2015-2023. We use tools from optimal transport to disentangle independent modes of waveform variation in the RFs, isolating the signature of seismic velocity changes from changes in the backazimuth/slowness of the incident teleseismic waves. Using 1D velocity model inversions and perturbations to the resulting synthetic RFs, we localize the velocity perturbations at depth. We identify perturbations to a phase in the RF corresponding to a velocity change at ~15 km depth that changes coseismically, decays over ~1 year, and persists with a static offset to the present-day, in contrast to the shallow velocity change which fully recovers within 1 year. To constrain the mechanism of this deep velocity change, we then estimate time-dependent changes in seismic anisotropy and observe a spatially-coherent coseismic rotation of the fast-axis of seismic anisotropy. We suggest that these observations capture a coseismic reorientation of the crustal stress field which evolves over a longer timescale than shallow damage often observed in seismic velocity changes.

Citation
Bryan, J. T., Frank, W. B., & Audet, P. (2024, 09). Coseismic reorientation of the crustal stress field at the base of the 2019 Ridgecrest earthquake fault zone. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology