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
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 03/23/2021
Project Abstract
California’s San Andreas Fault System is dominated by right-lateral strike-slip faulting. However, many smaller orthogonal or conjugate left-lateral structures also exist. Some, such as the Garlock Fault or Pinto Mountain Fault, are large enough to be mapped without having had a historic earthquake. However, the existence of other smaller orthogonal structures is often highlighted only when they rupture in conjunction or sequence with a larger mapped fault. The 2019 Ridgecrest sequence, which included a M6.4 rupture on a left-lateral fault followed by a M7.1 earthquake on an orthogonal right-lateral fault, exemplifies this. The Ridgecrest example raises questions as to what conditions led to the source faults rupturing in two closely-spaced earthquakes as opposed to one single larger event. That extends to broader questions about general behaviors of orthogonal or conjugate strike-slip faults: what conditions might make them rupture together versus separately, and how likely is a rupture on one fault to activate a large cross-fault? At a basic level, there are three possible rupture behaviors at an orthogonal fault junction: rupture occurs on one fault and the second remains uninvolved, a rupture begins on one fault and propagates through the junction onto the second fault, or a rupture occurs on one fault and then a second occurs later on the other fault. Here, I use the 3D finite element method to conduct dynamic rupture simulations of earthquakes on orthogonal fault systems, under a variety of initial stress conditions, to determine which conditions cause or prevent orthogonal multi-fault ruptures.
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 This is still a work in progress, so the figures in this interim report are still preliminary. That said, Figure 1 is the most representative one at the moment (March 2021).