Implications of a reverse polarity earthquake pair on fault friction and stress heterogeneity near Ridgecrest, California

Peter M. Shearer, Nader S. Senobari, & Yuri Fialko

Published September 8, 2024, SCEC Contribution #13701, 2024 SCEC Annual Meeting Poster #020

Earthquake focal mechanisms are estimated from seismic observations and provide valuable information on fault geometry and crustal stress orientation at depth. Most focal mechanisms are spatially correlated, that is, mechanisms tend to be similar to those of neighboring earthquakes. However, on rare occasions earthquake pairs are observed that appear nearly opposite in orientation, as evidenced by seismograms that are flipped in polarity. These extreme examples of focal mechanism diversity are valuable because they provide strong constraints on fault and stress properties at depth. Here we apply the Matrix Profile algorithm to 100 days of continuous data from the 2019 Ridgecrest, California, sequence and find many examples of reversely polarized waveforms. We focus our analysis on a particularly well-recorded reverse-polarity earthquake pair among the Ridgecrest aftershocks. Our analysis shows that they are at 10 km depth in the crust but only 115 m apart and that their fault planes differ in orientation by less than 20 degrees. This implies either unusually low values of fault friction, which permit faults to slip even when they are far from their optimal faulting orientation, or strong changes in stress orientation at depth, perhaps caused by residual stresses from prior earthquakes. Although definitive conclusions are lacking, we favor local stress heterogeneity as being more consistent with other observations for the Ridgecrest region.

Citation
Shearer, P. M., Senobari, N. S., & Fialko, Y. (2024, 09). Implications of a reverse polarity earthquake pair on fault friction and stress heterogeneity near Ridgecrest, California. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology