Ridgecrest aftershock stress drops from P- and S-wave spectral decomposition

Ian Vandevert, Peter M. Shearer, & Wenyuan Fan

Submitted June 18, 2024, SCEC Contribution #13462

Seismic moment and stress drop are crucial for understanding earthquake rupture processes, but their estimates often have large uncertainties for small earthquakes. Stress drop is typically inferred from an earthquake's source spectrum based on theoretical models, but poorly constrained path corrections and other modeling assumptions limit the accuracy of stress-drop estimates. Here, we compute stress drops using both P- and S-waves for the 2019 Ridgecrest earthquake sequence, compare their estimates, and evaluate the associated uncertainties. We use spectral decomposition and apply the analysis to both types of waves for the same set of earthquakes independently. Our approach fixes the corner frequency of small calibration earthquakes to reduce scatter in the estimated source parameters of the larger earthquakes. We find that assuming a lower high-frequency falloff rate for S-waves yields more consistent absolute stress-drop estimates between P- and S-waves. Our stress-drop estimates appear to increase slightly with magnitude for earthquakes with magnitudes greater than about 3.4. Furthermore, we find that the stress-drop estimates using both types of data exhibit coherent spatial variations. Earthquakes near the Coso geothermal field tend to have lower stress drops and earthquakes near the M7.1 hypocenter have higher stress-drop estimates. This spatial pattern is consistently observed in both the P- and S-wave results. We find no strong correlation between our stress-drop estimates and the M7.1 Ridgecrest earthquake slip distribution, suggesting a heterogeneous stress environment for the Ridgecrest fault system.

Vandevert, I., Shearer, P. M., & Fan, W. (2024). Ridgecrest aftershock stress drops from P- and S-wave spectral decomposition. Bulletin of the Seismological Society of America, (submitted).