USING PALEOSEISMOLOGY TO UNDERSTAND RUPTURES AT FAULT BRANCHES: NEW INSIGHTS FROM A PENINSULAR SAN ANDREAS FAULT PALEOSEISMIC RECORD

Increasingly, multi-fault ruptures have been recognized and hazard models have evolved to consider fault network behavior rather than that of individual faults. The San Andreas Fault (SAF) has major fault branches with the San Jacinto (SJF) and San Gregorio (SGF) faults. The SAF-SJF step-over has experienced large historical ruptures in 1812 (Mw 7.5) and 1857 (Mw 7.8). The SAF-SGF intersection experienced the 1906 (Mw 7.9) San Francisco earthquake rupture. The 2002 Denali Fault rupture (DFR, Mw7.9), with similar geometric and geologic characteristics, provides an analog. The DFR propagated onto the Totschunda Fault (TF). We found that the higher accumulated stress path, as defined by the elapsed time and slip rate on each fault branch, was favored. We find that applying this reasoning to the San Andreas fault explains major surface rupture behavior at fault branches.

We recently completed a paleoseismic investigation with a preliminary chronology of a new Peninsular San Andreas fault paleoseismic site; Monte Bello, located 76 km and 378 km from the southern and northern ends of the 1906 rupture, respectively. The preliminary event age estimates correlate with onshore and offshore paleoseismic records along the North Coast section of the fault.

A refined paleoseismic chronology from the Monte Bello site will be used to test the notion that the Peninsular section of the SAF has failed exclusively in 1906-type earthquakes during the past 800 years across a major fault branch, or independently but close in time with possible SA/SGF ruptures. This has implications for future fault behavior because what may most strongly control ruptures at fault branches is stress resulting from prior earthquakes.