THE PALEOEARTHQUAKE RECORD OF SAN GORGONIO PASS AND MAXIMUM MAGNITUDE OF SOUTHERN SAN ANDREAS FAULT RUPTURES

The San Gorgonio Pass Thrust Fault Zone is a key structural barrier along the San Andreas Fault (SAF) that has been proposed to control the extent of rupture and thus damages from large earthquakes in the greater Los Angeles region. To determine the timing and length of past ruptures on the San Gorgonio Pass Thrust Fault Zone, we undertook a series of trenching investigations to document evidence for late Holocene ground-rupturing earthquakes. A consistent history of ruptures was resolved over the last 1500 years: excavations at Millard Canyon and Lion Canyon within the San Gorgonio Pass Thrust Fault Zone both reveal two ruptures. Farther southeast, no ruptures traversed the Garnet Hill strand of the SAF in the last 600 years, but two ruptures in the last 1500 years were documented on the Banning strand of the SAF. Overlap amongst the paleoearthquake event ages from these sites (derived from radiocarbon dates) indicate the most recent rupture on the San Gorgonio Pass Thrust Fault Zone occurred ca. 1209 CE and the penultimate event ca. 622 CE. The role of the San Gorgonio Pass Thrust Fault Zone as a barrier to rupture on the SAF is illuminated when these results are compared to trench data along the Coachella and San Bernardino sections of the SAF, which hosted up to 7 and 8 ruptures in the last 1500 years, respectively. Timing of events on the neighboring SAF sections limits the most recent event to a 60-km rupture connecting just the San Gorgonio Pass Thrust Fault Zone and Banning sections, whereas the penultimate event could have spanned the entire San Bernardino section through to the Coachella section, producing a 190-km rupture impacting Cajon Pass to the Salton Sea. A primary result of these efforts is that the San Gorgonio Pass Thrust Fault Zone impedes throughgoing ruptures, inhibiting ~75% of SAF ruptures on either side. A secondary result is that the San Gorgonio Pass Thrust Fault Zone appears more prone to rupture along with SAF sections the southeast, consistent with dynamic rupture modeling through this region. Regional hazard implications are profound: USGS Scenario Shake Maps model MMI VI (Strong Shaking) is limited to the Inland Empire for ruptures terminating on or just northwest of the San Gorgonio Pass Thrust Fault Zone, much less than the strong shaking predicted for much of the greater Los Angeles area in a ShakeOut Scenario event.