The Effects of Bulk Friction and Cohesion in 2D Dynamic Models of the 1971 San Fernando Earthquake

Guadalupe Bravo, David D. Oglesby, Elyse Gaudreau, Gareth J. Funning, Edwin Nissen, & James Hollingsworth

Submitted September 7, 2025, SCEC Contribution #14474, 2025 SCEC Annual Meeting Poster #TBD

Deformation in an earthquake is often partitioned between slip on the fault and off-fault processes. Significant off-fault deformation was measured for the 1971 Mw 6.6 San Fernando Earthquake, where the steeply dipping Sylmar fault segment, which cuts through both soft and stiff sedimentary layers, has a higher percentage of vertical off-fault deformation than the shallower-dipping Tujunga Fault segment, which cuts through stiffer sedimentary layers (Gaudreau et al., 2023). Our aim is to understand what may have contributed to the differences in off-fault deformation between the two fault segments. Using 2D dynamic finite element models (Barall, 2009), we explore the effects on slip and surface deformation of different fault geometries and material properties, including the implementation of Drucker-Prager plasticity. Specifically we undertake a parameter study of how plasticity, primarily focusing on bulk friction and cohesion, can affect slip and off-fault deformation. As we decrease the bulk friction and cohesion parameters, our models diverge from our elastic models for the two corresponding fault segments. In particular, these parameters affect the partitioning of slip and off-fault deformation near the surface, where our models with smaller bulk friction and cohesion values have less slip at the surface than those with larger bulk friction and cohesion. For low values of bulk friction, the cohesion takes on increased importance near the surface. We find that our models need to be rather finely tuned in both plasticity parameters to produce deformation patterns qualitatively similar to that observed in the 1971 event. The need for such fine-tuning may highlight a limitation of the 2D models or other aspects of our problem formulation.

Citation
Bravo, G., Oglesby, D. D., Gaudreau, E., Funning, G. J., Nissen, E., & Hollingsworth, J. (2025, 09). The Effects of Bulk Friction and Cohesion in 2D Dynamic Models of the 1971 San Fernando Earthquake. Poster Presentation at 2025 SCEC Annual Meeting.

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