Plasticity and Off-Fault Deformation in the 1971 San Fernando Earthquake

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

Published September 8, 2024, SCEC Contribution #13708, 2024 SCEC Annual Meeting Poster #125 (PDF)

Slip and off-fault deformation both occur during an earthquake, with the former measured on the fault and the latter away from it. Significant off-fault deformation was measured for the 1971 San Fernando Earthquake, in which the steeply-dipping Sylmar segment, which passes through soft near-surface sedimentary layers, experienced a higher percentage of vertical off-fault deformation compared to the shallower-dipping Tujunga fault segment, which passes through firmer sedimentary rocks (Gaudreau et. al 2023). Our aim is to learn about what may have caused this off-fault deformation and its heterogeneity by performing dynamic rupture modeling and analyzing the effects of the different fault geometries and material properties. We compute 2D dynamic finite element models using the FaultMod code (Barall 2009), varying parameters such as stress, material properties, friction, cohesion, and fault geometry to understand how they affect slip and ground motion. We find that bulk friction and cohesion and their interaction with nonplanar, dipping fault geometry can significantly impact the generation of off-fault failure and deformation, in some cases producing higher slip in models with plasticity than in otherwise equivalent elastic models. In addition, we observe qualitatively more vertical off-fault deformation within 1 km of the fault trace in our models for the Sylmar Segment than in our models for the Tujunga Segment, in agreement with observations.

Citation
Bravo, G., Oglesby, D. D., Gaudreau, E., Funning, G. J., Nissen, E., & Hollingsworth, J. (2024, 09). Plasticity and Off-Fault Deformation in the 1971 San Fernando Earthquake. Poster Presentation at 2024 SCEC Annual Meeting.

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