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Correlation of peak dynamic and static coulomb failure stress with seismicity rate change after the M7.2 El Mayor-Cucapah earthquake

Kyle B. Withers, & Kim B. Olsen

Published December 2012, SCEC Contribution #1785

We have investigated the relation between the April 4 Mw7.2 El Mayor-Cucapah earthquake and seismicity rate changes in southern California and northern Baja California in the months following the mainshock. Specifically, we use a dynamic rupture model with observational constraints for the event simulated in the SCEC 3D CVM4.0 (Roten and Olsen, 2009) to calculate the changes in the resulting static (dCFS) and dynamic Coulomb failure stress, parameterized by its largest positive amplitude (peak dCFS(t)). We employ a modified cross correlation between the seismicity rate change (for both undeclustered and declustered catalogs) and both dCFS and peak dCFS(t) in time and space (as used by Kilb et. al, 2002). We find that the correlation parameter is greater for peak dCFS(t) compared to dCFS and highest for periods after the mainshock of longer than 1 week for dCFS, and a maximum at 1 month for peak dCFS(t). We perform this analysis using both CVM-4 and CVM-H, investigating, in particular, which model better describes the increased seismicity NW of the rupture. The stress changes are rotated onto the focal mechanism of the June 15, 2010 Mw5.7 aftershock as well as onto optimum oriented planes (King, 1994). For regionally rotated stresses we find that while the dCFS values are very similar for the two CVMs, the corresponding peak dCFS(t) values are noticeably different. In particular, CVM-H generates a lobe of (directivity-induced) large peak dCFS(t) between the Elsinore and San Jacinto Faults toward the Los Angeles basin not present in the results from CVM-4. However, both CVMs produce similar peak dCFS(t) lobes near San Diego. Finally, we searched for threshold levels of dCFS and peak dCFS(t) that may be required to trigger earthquakes/aftershocks of different magnitude that might provide clues to earthquake prediction; we found a possible peak dCFS(t) threshold value of 0.7 bars for aftershocks (>4000) in regions of positive static stress. Peak dynamic stress change (in bars) calculated from CVM-4 for the April 4 Mw7.2 El Mayor- Cucapah earthquake, rotated onto a strike of 310 degrees, corresponding to the rupture of the Mw 5.72 June 15, 2010 event (epicenter depicted by the white star, mainshock by black star).

Withers, K. B., & Olsen, K. B. (2012, 12). Correlation of peak dynamic and static coulomb failure stress with seismicity rate change after the M7.2 El Mayor-Cucapah earthquake. Oral Presentation at AGU Fall Meeting 2012.