Joint rupture scenarios of the Southern San Andreas, Imperial and intersecting Cross Faults south of Bombay Beach

Christos Kyriakopoulos, UC Riverside

Wednesday, July 26, 2017 at 10:30 AM

Building 3, Rambo Auditorium
Kenneth Ryan

The possibility that a large portion of the Southern San Andreas Fault (SSAF) could be activated in a M>7.5 earthquake remains one of the most important issues for the correct evaluation of the seismic hazard in Southern California. The commonly accepted southern termination of the SSAF is halfway along the eastern margin of the Salton Sea, near Bombay Beach. The area between the southern end of the SSAF and the northern end of the Imperial Fault (IF) is known as the Brawley Seismic Zone (BSZ). The BSZ is characterized by small- to medium-sized earthquakes, which often manifest as seismic swarms. Recent cases of such swarms in this area include the 2012 Brawley sequence (Hauksson et al., 2013), as well as a more recent case in the fall of 2016 (Hauksson et al., 2017) that occurred immediately adjacent to the southern terminus of the SSAF. In spite of this seismic activity, to date there is no known trace of the SSAF extending south of Bombay Beach, or a trace of the IF extending north to link the two large faults. For that reason, the SSAF and IF are often considered to have little to no seismic interaction. However, paleoseismological evidence of coincident slip at the Dogwood site on the IF (Rockwell et al., 2011) and Coachella Valley (Philibosian et al., 2011) on the SSAF suggest that a through-going rupture might be possible. A major event involving the activation of both SSAF and IF could reach a magnitude of up to 8. Such an event could disrupt Interstate 8, which has long been considered to be a “safe” path in the worst-case scenario of the much anticipated “big one” in southern California. The main topics of this talk are: 1) the possibility of joint rupture of the Southern San Andreas and Imperial Faults, including the role of intersecting Cross faults; 2) the possibility of a major SSAF event being triggered by a small swarm-type earthquake in the BSZ. We are targeting the above questions by performing spontaneous dynamic rupture models that investigate a large spectrum of geometric and stress configurations. Our preliminary results show that, under certain prestress conditions, a through-going rupture is possible via a joint SSAF and IF activation. The implementation of the cross faults in the model reveals a wide variety of interaction possibilities that might affect rupture propagation. Finally, we are exploring a rupture scenario for which a moderate event will initiate on a cross fault (e.g., as part of a local seismic swarm) and propagate on the adjacent main fault, and thus cascade into a large SSAF earthquake.

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