Comparing Dynamic Rupture Behavior of the Southern San Andreas Fault over Multiple Earthquake Cycles between CFM5.3 and CFM7 with a 3D Fully Dynamic Earthquake Simulator

Hasti Bordbar, & Benchun Duan

Submitted August 30, 2026, SCEC Contribution #15463, 2026 SCEC Annual Meeting Poster #031

Understanding how three-dimensional fault geometry and tectonic loading interact to control earthquake rupture propagation is critical for seismic hazard assessment. We use EQdyna, a 3D fully dynamic earthquake simulator, to investigate multicycle dynamic rupture behavior along the southern San Andreas Fault (SSAF). We compare two geometries derived from the SCEC Community Fault Model: a near-vertical model based on CFM5.3 and a variable-dip model based on CFM 7.

We examine the effects of spatially variable loading informed by the regional strain-rate orientation reported in previous studies. In the original configuration, a uniform loading direction parallel to the Parkfield section is applied along the entire fault. Under this loading, the angle between the loading direction and the local fault strike near the Big Bend reaches 40° at 10 km depth, creating a strong barrier where most ruptures terminate. In the revised configuration, loading remains parallel to the Parkfield section in the north but is rotated by 15° from the Big Bend southward, following the regional variation in strain-rate direction. This rotation reduces the loading-to-fault angle near the Big Bend to approximately 25°, weakens its effectiveness as a rupture barrier, and allows more ruptures to propagate through sections. The resulting rupture patterns are more consistent with paleoseismic observations on rupture segmentation and spatial variations in event frequency.

The two fault geometries produce distinct rupture behavior in the Mojave section. Rupture frequency is generally lower in the variable-dip model than in the near-vertical model. In the variable-dip geometry, the Mojave section coincides with a transition from southwest- to northeast-dipping fault surfaces. The three-dimensional geometric changes generate barriers near both boundaries of the section, causing ruptures to terminate either north or south of the Mojave section and producing segmentation patterns similar to those inferred from paleoseismic observations. In contrast, rupture behavior in the near-vertical model is dominated by a single major barrier at the Big Bend. Events terminating there are commonly followed by events that rupture the previously unbroken portion of the fault. Our results demonstrate that spatially variable loading and three-dimensional fault geometry jointly control rupture extent, segmentation, and recurrence along the SSAF.

Citation
Bordbar, H., & Duan, B. (2026, 08). Comparing Dynamic Rupture Behavior of the Southern San Andreas Fault over Multiple Earthquake Cycles between CFM5.3 and CFM7 with a 3D Fully Dynamic Earthquake Simulator. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)