SCEC Award Number 18209 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Dynamic rupture modeling of earthquakes at the intersection of the San Andreas, San Jacinto, and Cucamonga faults, Cajon Pass, southern California – testing the effects of intersection geometry
Investigator(s)
Name Organization
Julian Lozos California State University, Northridge James Dolan University of Southern California David Oglesby University of California, Riverside
Other Participants
SCEC Priorities 1e, 2e, 1d SCEC Groups SAFS, FARM, Seismology
Report Due Date 04/30/2020 Date Report Submitted 03/15/2024
Project Abstract
 Strike-slip faults are nonplanar structures. Most large strike-slip faults have mapped complexities and discontinuities along strike, but seismological and geodetic inversions, as well as field geophysical studies, suggest dip can also vary substantially along the length of these faults. The southern San Andreas Fault, which twists from a northwest dip in the Carrizo Plain to vertical in the Mojave Desert to a southeast dip from Cajon Pass to San Gorgonio Pass, is a notable example. In this geometrical parameter study, I conduct 3D dynamic models of rupture propagation on strike-slip faults with a planar strike but a change in dip midway along the fault, to see how far rupture is able to propagate beyond the dip inflection point. I test a variety of compressional and extensional dip angles, and I vary the abruptness of the dip change. I also test several initial stress amplitudes and orientations. I find that larger and more abrupt changes in dip are more likely to stop rupture before it reaches the end of the fault. I also find that compression vs. extension has little effect on rupture extent compared to dip angle and abruptness when the fault is favorably aligned within a regional stress field, but for unfavorable alignments, compressional systems are more likely to stop rupture than extensional ones regardless of dip angle. This study may help assess possible rupture segmentation and endpoints for strike-slip faults with variable dip, as well as providing potential explanations for rupture endpoints along apparently planar fault segments.
Intellectual Merit Even though this project and its resulting paper ended up rather different from the original proposal, it is still ultimately inspired by the geometry of the San Andreas Fault in Cajon Pass. While the question of a change in dip on a strike-slip fault is somewhat removed from the multi-fault rupture issue of this earthquake gate, it still very much addresses possible causes of a San Andreas rupture stopping in Cajon Pass, and raises the question of whether a dip change on this planar-strike section of the San Andreas might be a mechanism for causing a rupture to choose the San Jacinto instead of continuing on the San Andreas. The results we have here will put us in a much better place to conduct the work we originally proposed, now that we see the role that changes in dip can play.

Because this did become a parameter study, it also applies beyond Cajon Pass, in both specific and broad ways. In one specific case, the San Gorgonio Pass Special Fault Study Area also involves dramatic changes in dip on strike-slip faults; this work can perhaps help analyze possible rupture paths through that knot. In a broader sense, this work will provide some physics-based rules for rupture negotiating dip changes, which could be useful to implement in UCERF (which currently only has rules for along-strike geometry influencing rupture endpoints). Additionally, the aspect of this paper which focuses on how surface displacement is partitioned between vertical and horizontal components as dip changes could be useful for developing surface displacement prediction equations, as well as a way to potentially infer a change in dip from measured surface displacement.
Broader Impacts Because we ended up using these funds for a broader Cajon Pass-inspired parameter study about the impacts of changes in dip on strike-slip rupture endpoints, this work will be useful not just for understanding why ruptures may or may not stop in Cajon Pass, but also in other strike-slip settings with known dip changes (such as San Gorgonio Pass further south along the San Andreas Fault, or the Alpine Fault in New Zealand). The rupture endpoints aspect of this project could be useful for future iterations of UCERF, and the surface displacement aspect could be useful for displacement prediction equations, and in turn for guiding how to engineer and zone for those expected displacements.
Exemplary Figure Figure 2 is the most exemplary figure, since it summarizes rupture behavior across our geometrical and stress parameter space.
Caption: Grids showing how rupture behavior changes across geometrical parameter space with the fault orientation rotated within the regional stress field. When the fault is rotated to a more compressional angle, there is little effect on rupture behavior for extensional geometries, but nearly all compressional geometries produce stopped ruptures. Similarly, when the fault is rotated to a more extensional angle, compressional ruptures are not strongly affected, but rupture stops on nearly all extensional geometries.
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