COULD A KEYSTONE FAULT BLOCK EXPLAIN THE OVERDUE TIMING OF A M7.5+ EARTHQUAKE ALONG THE SOUTHERN SAN ANDREAS FAULT?

The Coachella segment of the San Andreas Fault (SAF), southern California has not experienced an earthquake since ~1725, and may be 150+ years overdue. We mapped its southern terminus near Bombay Beach, CA, where the main trace of the San Andreas is less defined southward, and becomes a progressively more dispersed zone east of the Durmid Ladder Structure (DLS) of the SAF. The DLS of the SAF changes into the Brawley Seismic Zone (BSZ) along the West Pope Fault Zone (WPFZ). Our field studies, image analysis, and geophysical evidence show the WPFZ to be a complex fault zone that fits the keystone model. Fletcher et al., 2016 defined keystone faults as "system-spanning, cross-cutting faults that regulate slip events on other faults". The WPFZ separate two disparate fault blocks that may also inhibit rupture through the right-lateral boundary between the SAF and BSZ. The WPFZ is a NNW-striking right-stepping right-normal fault zone, with tips near master faults of the Hidden Spring and Extra Fault Zones on the North American and Pacific side of the SAF. The WPFZ a) is at a high angle to the SAF and BSZ, b) is a major structure that is highly deformed, c) is locked and does not exhibit much evidence of creep, unlike other faults nearby, d) lack of creep is confirmed by InSAR and UAVSAR interferograms, and e) has no recorded seismic activity. A seismic quiet zone coincides with all the faults that converge at the WPFZ, including the Hidden Spring Fault Zone, the Extra Fault Array, the southernmost SAFZ, and the West Brawley Fault. The four small recent earthquakes (April May 2019) that occurred near or within the West Brawley Fault may reflect stress release in a region that is critically stressed and close to rupture. This adds urgency to understand the geodynamics and structures of this region.

Reference:

Fletcher, J.M., Oskin, M.E., and Teran, O.J., 2016, The role of a keystone fault in triggering the complex El Mayor-Cucapah earthquake rupture: Nature Geoscience, v. 9, p. 303–307, doi:10.1038/ngeo2660.