SCEC Award Number 16231 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Modeling Rupture through the Brawley Seismic Zone Stepover: Can Ruptures Propagate between the San Andreas and Imperial Faults?
Investigator(s)
Name Organization
David Oglesby University of California, Riverside Thomas Rockwell San Diego State University
Other Participants Christos Kyriakopoulos, postdoctoral researcher
Aron Meltzner, postdoctoral researcher
SCEC Priorities 4e, 4a, 3e SCEC Groups CS, SoSAFE, FARM
Report Due Date 03/15/2017 Date Report Submitted 03/14/2017
Project Abstract
 We modeled the configuration of the southern San Andreas and Imperial faults (SAF, IF), and the Brawley Seismic Zone (BSZ), and ran several dynamic rupture scenarios to test whether the SAF and IF could rupture together in a large earthquake. The motivation for this study was the observation that at least two past southern SAF events are very similar in timing to IF events, suggesting that either they ruptured simultaneously or in sequence. Our modeling suggests that large earthquakes may rupture through this large step-over, depending on rupture directivity and the presence of connecting faults or cross-faults, supporting the paleoseismic observations. Nucleation on the SAF favors rupture of both the SAF and IF in the step-over region, whereas nucleation on the IF ruptures primarily the western (IF) strand of the step-over. We interpret this asymmetry to reflect the geometrical complexity of the system, which leads to dynamic clamping and unclamping of the system at different locations and at different times. We also modeled the cross-faults that are evident as seismicity lineaments to test their affect on ruptures through the BSZ, and found that they do little to impede through-going ruptures. We are currently testing whether the cross-faults may act as triggers for large earthquakes, as the BSZ commonly has earthquakes up to M5 that result in perceived elevated earthquake likelihood.
Intellectual Merit We tested the possibility of a through-going rupture in the Salton Trough region. Our results indicate that the ability of rupture to propagate through is strongly influenced by the complex, nonplanar, overlapping fault geometry in the region. If the fault geometry is fixed, the likelihood of through-going rupture depends on two major factors: 1) Nucleation location, and 2) pre-stress level.
Broader Impacts A great earthquake on the southern San Andreas fault, with surface rupture from Parkfield to the Salton Sea, will cut most major lifelines into southern California, including highways and major water conduits. One major highway that has been considered safe in such an event is the I-8 corridor, as it runs south of the southern terminus of the San Andreas fault. In this study, we consider multiple models that test whether it may be possible that a large earthquake can rupture through the large releasing step that comprises the Brawley Seismic Zone and produce rupture on both the San Andreas and Imperial faults. Part of the impetus for this study is the recognition that some large earthquakes on the Imperial and southern San Andreas fault are indistinguishable in timing and could represent large earthquakes that ruptured both fault segments. If so, then all major highways and byways into southern California could be cut by such an event.
The finite element mesh and dynamic rupture models were prepared and executed by Postdoc Researcher Christos Kyriakopoulos.
Exemplary Figure Provided as exemplary figure in attached report, which is the same as figure 4.
Figure Caption. Final slip distribution for simulations with Cross Faults. (Left column from Top to Bottom) Simulations with nucleation in the south on the Imperial fault with S=2.0, 0.45; (Right column from Top to Bottom) Simulations with nucleation in the north on the southern San Andreas fault with S=2.0, 0.45; Note: models are plotted in a local coordinate system (km) with the y-axis roughly parallel to the main fault strike. Northeast is to the left.