SCEC Award Number 20204 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Dynamic Rupture Modeling of Coseismic Interactions on Orthogonal Strike-Slip Faults
Investigator(s)
Name Organization
Julian Lozos California State University, Northridge
Other Participants
SCEC Priorities 1d, 1e, 2e SCEC Groups FARM, CS, SAFS
Report Due Date 03/15/2021 Date Report Submitted 03/14/2024
Project Abstract
 Intersecting orthogonal strike-slip faults with opposite senses of slip pose the question of what allows rupture to propagate through the junction and through both faults versus confining rupture to a single fault. I conduct dynamic rupture simulations on simplified orthogonal strike-slip fault systems, to determine which conditions produce rupture on both component faults. In models with uniform initial tractions on both faults, slip on the first fault must reduce normal stress on the second fault for it to rupture. If the first fault ends at the cross fault, a stopping phase causes the cross fault to rupture. In models where I resolve a uniform regional stress field on the faults, only a narrow range of stress orientations allow multifault ruptures. These results will be helpful for evaluating hazard near orthogonal strike-slip faults.
Intellectual Merit The question of rupture through orthogonal strike-slip fault systems is fundamentally an earthquake gate question. A 90-degree branch or bend is a sharp geometrical discontinuity, made more complex by the different senses of slip on the two faults. Rupture can approach the fault junction from three or four directions (depending on whether the fault system is T-shaped or +-shaped), and there are many possible rupture paths. The question of which rupture path occurs from which nucleation point under which stress conditions epitomizes the earthquake gate problem.

Although the model setup here is very simple compared to real-world faults, the results of this parameter study will hopefully be useful as a baseline for understanding rupture hazard on orthogonal strike-slip faults, as well as a starting point to motivate and set up site-specific studies on real faults.
Broader Impacts This study will help understand the basic rules of multi-fault rupture on orthogonal strike-slip faults. Investigating these basic behaviors can provide groundwork for better understanding past earthquakes or sequences on orthogonal or conjugate strike-slip faults, as well as for forward modeling of possible rupture behaviors on other such fault pairs.

This work will also help with hazard assessment. In a most broad sense, it will help refine how to estimate rupture hazard at mapped orthogonal fault junctions. More specifically, this work may be able to impact how the Uniform California Earthquake Rupture Forecast (UCERF) handles the probability of multi-fault rupture on orthogonal faults. The current iteration of UCERF handles all fault junctions based on the angle between the faults, and it does not allow multi-fault ruptures through 90° junctions (Field et al., 2014). This work can contribute new physics-based rules to how UCERF handles orthogonal faults, rather than a simple angle constraint.
Exemplary Figure Figure 2 (which is figure 5 in the published GRL manuscript) illustrates the dynamic processes that control rupture path through an orthogonal strike-slip fault system.

Caption: Stress effects of initial rupture directions. Top: the cross fault is compressed first, which prevents multi-fault rupture. Center: the cross fault is extended first, which allows rupture to propagate through both faults. Bottom: a high-shear-stress stopping phase allows bilateral rupture on the long fault.