Dynamic Rupture Modeling of Coseismic Interactions on Orthogonal Strike-Slip Faults

Julian C. Lozos

Accepted April 19, 2021, SCEC Contribution #10971

The San Andreas Fault System is dominated by right-lateral strike-slip faulting. However, a large number of smaller orthogonal 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 34 hours later 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 strike-slip faults as earthquake gates: what conditions might make them rupture together versus separately, how likely is a rupture on one fault to activate a large cross-fault, and is this persistent behavior versus something changes over multiple earthquake cycles? Here, I use the 3D finite element method to simulate dynamic ruptures on orthogonal strike-slip fault systems with several geometrical configurations. In models in a homogeneous elastic half space with uniform initial stresses on both faults, which isolate the effects of fault geometry, I find that a stopping phase from rupture hitting the end of one fault is necessary to initiate rupture on the other. A rupture on one fault that crosses the other and continues on does not activate the second fault. Thus, the rupture pattern and earthquake size in an orthogonal fault system may be heavily dependent on the nucleation location, reinforcing the idea that such systems do function as earthquake gates. I compare these uniform stress models to ones in which the fault system is in a regional stress field, which produces different initial stresses on each fault, as well as to ones in which the faults are in a viscoplastic medium rather than an elastic one.

Lozos, J. C. (2021, 04). Dynamic Rupture Modeling of Coseismic Interactions on Orthogonal Strike-Slip Faults. Poster Presentation at 2021 Seismological Society of America Annual Meeting.