Spatio-temporal variations of stress and strain parameters in the San Jacinto fault zone

Niloufar Abolfathian, P Martínez-Garzón, & Yehuda Ben-Zion

Under Review 2018, SCEC Contribution #8106

We analyze the background stress field around the Jacinto fault zone (SJFZ) in southern California with a refined inversion methodology using declustered focal mechanisms of background seismicity. Stress inversions applied to the entire fault zone, and 3 focus areas with high level of seismicity, provide 3D distributions of the maximum horizontal compression direction (SHmax), principal stress plunges and stress ratio . The results are compared with coseismic strain parameters derived from direct summation of earthquake potencies and b-values of frequency-size event statistics. The main stress regime of the SJFZ is strike-slip, although the northwest portion near Crafton Hills displays significant transtension. The SHmax orientation rotates clockwise with increasing depth, with the largest rotation (23 degrees) observed near Crafton Hills. The principal stress plunges have large rotations below ~9 km, near the depth section with highest seismicity rates and inferred brittle-ductile transition zone. The rotations produce significant deviations from Andersonian theory for strike-slip faulting, likely generating observed increased dip-slip faulting of relatively deep small events. The stress ratio parameters and b-value results are consistent with increasing number of dip-slip faulting below ~9 km. The derived coseismic strain parameters are in good agreement with the stress inversion results. No large-scale stress rotations are observed across the time of the 2010 Mw 7.2 El Mayor-Cucapah earthquake. The stress ratio near the Trifurcation area of the SJFZ changes after the El Mayor-Cucapah earthquake toward transpression, but this may have been produced by a local M > 5.4 event.

Citation
Abolfathian, N., Martínez-Garzón , P., & Ben-Zion, Y. (2018). Spatio-temporal variations of stress and strain parameters in the San Jacinto fault zone. Pure and Applied Geophysics, (under review).


Related Projects & Working Groups
SDOT, FARM, Seismology,Tectonic Geodesy