SCEC Award Number 18040 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Paleoseismic Investigation of the Lytle Creek Ridge Fault: Evidence for Linked San Jacinto-San Andreas Earthquake Ruptures
Name Organization
Michael Oskin University of California, Davis Thomas Rockwell San Diego State University
Other Participants
SCEC Priorities 1e, 2e, 5b SCEC Groups SAFS, Geology, FARM
Report Due Date 03/15/2019 Date Report Submitted 05/04/2019
Project Abstract
We conducted a paleoseismic and structural investigation of a newly discovered, active, low-angle normal fault located between the San Andreas fault and the northernmost San Jacinto fault. The Lytle Creek Ridge fault (LCRF) strikes west, dips ~30° to the north, and cuts through Lytle Creek Ridge, within the releasing step where the San Jacinto approaches within 3.5 km of the San Andreas fault. The LCRF is uniquely positioned and mechanically favored to record earthquakes that jump this releasing step and breach the Cajon Pass Earthquake Gate. We found evidence for at least two meter-scale coseismic offsets within a hand-excavated, 2.5-meter deep trench across the LCRF. Based on mechanical modeling, these large slip events are probably due to earthquake ruptures that jump from the SAF to the SJF (or visa versa). The trench contains abundant charcoal, and dating is in progress at the writing of this report. The results of this study will provide a critical paleoseismic data set for assessing the rupture behavior of the Cajon Pass Earthquake Gate.
Intellectual Merit The newly discovered Lytle Creek Ridge fault (LCRF) lies within the releasing step where the San Jacinto approaches within 3.5 km of the San Andreas fault. Slip events on this low-angle (30° dip) normal fault are very likely triggered by ruptures that jump this step. We conducted a paleoseismic investigation of the LCRF where it forms an uphill facing scarp, ponding a mixture of gravel and silty sand strata. At least two, and possibly three 2+m slip events are evident, with abundant detrital charcoal for 14C dating. Fieldwork and 14C geochronology are ongoing at the time of submission of this report. Complete results of this study will be presented at the SCEC annual meeting. The results of this research will provide critical tests of fault behavior and fault interaction through the Cajon Pass Earthquake Gate.
Broader Impacts This project constitutes a portion of the Ph.D. thesis of Alba Mar Rodriguez-Padilla, a female Ph.D. student at UC Davis. A second Ph.D. student from San Diego State University, Drake Singleton, and an undergraduate student, Michael Levison, also participated. Understanding the rupture history through the Cajon Pass provides essential input to seismic hazard models of the San Andreas and San Jacinto fault. As the lapse time on both faults now exceeds their respective recurrence intervals, a better understanding of the potential for very large earthquakes will help planners prepare for such an event prior to its occurrence.
Exemplary Figure Figure 3. Coulomb models (Lin and Stein, 2004) for stress increase and optimal slip direction on a 30° north-dipping LCRF in response to rupture on the vertical San Andreas fault (left), San Jacinto fault (middle), or both (right). Color corresponds to change in north-south dilational strain. In each case, slip is tapered to simulate constant stress drop of approximately 6 MPa, driving a comparable stress increase on LCRF. Highest stress change, with slip direction consistent with that observed in outcrop, occurs for the case where slip occurs on the San Andreas fault and San Jacinto fault.