Implications for San Andreas Fault Ruptures Based on New Evidence from the Cabazon, CA Paleoseismic Site, San Gorgonio Pass Fault Zone

The 10-km section of the San Gorgonio Pass fault zone (SGPFZ) between the Banning and San Bernardino strands of the San Andreas fault is composed of roughly orthogonal, alternating strike slip and thrust fault sections. In order to investigate whether the complexity of the fault system is a barrier to thoroughgoing ruptures and consequently a limit on earthquake magnitude, we excavated a mega-trench across a compressional section of the SGPFZ approximately 2 km northeast of Cabazon, CA. The mega-trench was located at the base of the San Bernardino Mountains and exposed Pleistocene Cabazon Formation in the hanging wall block, thrust over Holocene alluvial units. Sediments in the 9-m-deep trench consisted of boulder to cobble debris flow deposits and silty sand fluvial sequences, which buried and were interbedded with incipient soil horizons. We present evidence for five earthquakes in the mega-trench, determined by several generations of fault scarps buried by progressively younger sediments. Vertical separation for each event was small, on the order of 30 to 60 cm at the paleo-ground surfaces. Preliminary radiocarbon dating of charcoal collected from the deformed sediments constrains the five observed events to the past ~6000 years and the most recent event to ca. 500-700 years ago; additional dates are pending that will enable us to determine the age of each earthquake. Based on the existing age control, the average earthquake recurrence interval for the SGPFZ is ~1400 years, seven times longer than the average interval for neighboring sections on the southern San Andreas fault. If the earthquakes recorded at the Cabazon site record great San Andreas fault ruptures like those modeled in the 2008 ShakeOut Scenario, much of the vertical slip must occur within the bedrock in the hanging wall or in blind faults in the alluvium below the site. Alternatively, these earthquakes may represent more local, M6-M7 events that rupture the San Gorgonio Pass and possibly portions of the San Andreas fault. Either result requires that other strands of the SGPFZ must be active to maintain slip across this complex system.