SCEC Award Number 13130 View PDF
Proposal Category Collaborative Proposal (Special Fault Study Area)
Proposal Title Collaborative Research: Dynamic models of potential earthquakes in the western San Gorgonio Pass Region
Name Organization
David Oglesby University of California, Riverside Michele Cooke University of Massachusetts Amherst
Other Participants Jennifer Tarnowski (UCR)
Alex Hatem (UMass)
SCEC Priorities 4, 6, 3 SCEC Groups FARM, SDOT
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
Within the San Gorgonio Pass (SGP) region in California, the San Andreas fault (SAF) system forms a restraining bend that is one of the crucial “pinch points” along the fault system. In this region, the SAF follows what appears to be a number of non-vertical and non-coplanar active segments [e.g., Yule, 2009]. Does this significant break in the surface continuity of the SAF serve as a barrier to through-going rupture in this region? Spontaneous dynamic rupture modeling is designed to help answer questions such as this. As a first step toward addressing the likelihood of through-going rupture at the SGP, we have been constructing dynamic models of earthquakes on the intersection between the San Bernardino strand of the SAF with the SGP thrust fault system on the western end of the SGP. We received our funding quite late in 2013, so we have not yet gotten our fault geometry fully implemented and models run. However, as described in our annual report on our Ventura Basin project, we are in the process of incorporating realistic fault geometry (via CUBIT meshing software) with our dynamic finite element code FaultMod [Barall, 2009]. We have also carried out (with local UCR funds) a passive-source trapped-wave seismic experiment in this region so that we may gain insight into fault connectivity, which then may be used in our dynamic models. The results will have impact on seismic hazard throughout Southern California, where ground motion depends strongly on the nature of rupture on the San Andreas Fault.
Intellectual Merit Our main progress thus far has been on two fronts. First, we have been using fault geometries from our collaborators at UMass Amherst to construct faulting models; these will be implemented in CUBIT meshing software over the next few months and will form the basis of our dynamic modeling efforts. We will experiment with different geometrical parameterizations of the intersection of the SAF and SGP systems.
Such fault parameterizations will be incorporated into the CUBIT meshing software from Sandia National Laboratory and used in our dynamic faulting models to determine the circumstances (if any) under which rupture may propagate through the geometrical knot in the SGP region.

Based on the body of research on discontinuous fault systems [e.g., Harris et al., 1991; Kase and Kuge, 1998; Duan and Oglesby, 2006; Lozos et al., 2012], a key issue in these models will be the connectivity of the fault system. If the faults come close to each other or directly connect, the likelihood of throughgoing rupture is far greater than if the faults are separated by significant distance (i.e., over 2-3 km). Surface fault mapping does not necessarily answer the question of fault connectivity at depth. Toward this goal, we have been using local UCR funds to set up a passive-source seismic array on the western SGP thrust fault to search for fault zone trapped waves. If trapped waves can be shown to propagate across the SAF-SGP fault intersection, then a strong argument can be made for the continuity of the fault system, and thus the relatively high likelihood of throughgoing rupture.

We had the instruments in the ground for approximately 3 months, and ended the deployment at the end of February 2014. We are now sorting through and analyzing the data for earthquakes and trapped waves; we hope to have this analysis completed by the beginning of the summer. The results of this experiment will help us in accurately parameterizing the continuity of the fault system in the SGP region, and will improve the accuracy of our dynamic rupture models. Furthermore, our work is part of the SCEC San Gorgonio Pass SFSA, and will directly dovetail with paleoseismic and seismological work in the area.
Broader Impacts As noted above, the results of this seismic experiment and numerical modeling will have significant impact beyond the seismic hazard of the SGP region. As seen in the TeraShake2 numerical modeling exercise [Olsen et al., 2008], through-going rupture on the SAF in SGP leads to strong seismic waves that are funneled directly into the Los Angeles Basin, leading to strong resonance and other amplification effects that greatly endanger lives and structures in the nation’s second largest city. A better characterization of the risk of such an event will be extremely valuable for engineers and emergency responders, and could affect factors from zoning and construction to insurance rates.
Exemplary Figure Figure 1. Example of proposed fault geometry for the intersection of the SAF with the SGP fault system.