GEOMORPHIC EVIDENCE FOR LATE QUATERNARY SLIP ON THE NORTHERNMOST SAN JACINTO FAULT ZONE, AND IMPLICATIONS FOR SLIP TRANSFER TO SAN ANDREAS FAULT

Paleoseismic and geologic data on the southern San Andreas fault zone (SAFZ) show that there is a significant difference in earthquake recurrence and slip rate on the fault north and south of Cajon Pass. Transfer of slip from the SAFZ to the San Jacinto fault zone (SJFZ) is the likely cause of the decrease in slip rate, and paleoseismic data suggest some ruptures on the SAFZ have diverted down the SJFZ. A better understanding of the fault structure and measurements of slip rate on the northern SJFZ are needed to test these hypotheses. We are working to understand where slip is transferred between the SAFZ and the SJFZ in the Cajon Pass area by mapping and dating offset geomorphic features along the three major strands of the SJFZ from where it exits the San Bernardino basin to its northwest termination in the San Gabriel Mountains. Field investigations were complimented by the use of LiDAR imagery to investigate the fault structure and look for potential slip rate or paleoseismic sites. Two potential slip rates sites as well as several sites that have the potential to bracket the timing of faulting have been identified along the northern SJFZ. Preliminary results show a relatively steady decrease in geomorphic evidence of slip to the northwest. Shallow trenching on the SJFZ where it is closest to the SAFZ revealed no evidence for surface rupture in the past 2000 years. This result was unexpected and may indicate that slip is transferring between the SAFZ and SJFZ further to the northwest, or to the southeast in the San Bernardino basin.