Paleoseismic investigation and mechanical modeling of rupture behavior through Cajon Pass

Alba M. Rodriguez Padilla, Michael E. Oskin, Thomas K. Rockwell, Irina Delusina, & Drake M. Singleton

Published August 13, 2019, SCEC Contribution #9514, 2019 SCEC Annual Meeting Poster #142

Assessment of the probability of multi-fault ruptures requires a better understanding of the mechanics and paleoseismic history at fault junctions. We investigate the junction of the San Jacinto fault with the San Andreas fault at Cajon Pass. Here the San Jacinto Fault (SJF) terminates adjacent to the San Andreas Fault (SAF) in a 3.5km wide releasing step-over where their mapped surface traces do not intersect. The potential for dynamic rupture transfer through this zone has been demonstrated (Lozos, 2016), yet physical evidence of linkage is yet to be documented. The recently discovered Lytle Creek Ridge Fault (LCRF) is a low-angle normal fault that accommodates extension within the step-over between the SJF and the SAF, and roots into the San Andreas fault at a shallow depth (~1km). By itself, this fault is too shallow and too short to be seismogenic, or to act as a transfer structure. However, the LCRF is uniquely positioned and mechanically favored to record ruptures that jump the Cajon Pass Earthquake Gate, providing the first opportunity to assess the rupture behavior of this junction from field observations. We present a paleoseismic dataset that shows that at least four earthquakes have bridged the gap. We integrate a Bayesian analysis of radiocarbon ages and statistical analysis of pollen recovered from the stratigraphic sequence to characterize the timing of events recorded in the LCRF trench site. Our trench records the 1812 earthquake, a ~1600AD event, and two earlier events. Using a 3D finite element mechanical model based on the slip distributions in Lozos (2016) and the offsets in our trench site, we constrain potential slip models of the San Jacinto fault as ruptures approach its termination. Our preliminary results show that to activate the LCRF, slip on the San Jacinto fault must taper steeply (~10^3) and halt abruptly about 2-5km northwest of the LCRF. The combination of our paleoseismic dataset, pollen analysis, and numerical model characterize the timing of joint rupture of the SJF and the SAF, as well as the mechanics of this behavior.

Key Words
multi-fault rupture, earthquake gate, paleoseismology, mechanical modeling, San Andreas Fault System

Citation
Rodriguez Padilla, A., Oskin, M. E., Rockwell, T. K., Delusina, I., & Singleton, D. M. (2019, 08). Paleoseismic investigation and mechanical modeling of rupture behavior through Cajon Pass . Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)