PLASTICITY AND OFF-FAULT DEFORMATION IN THE 1971 SAN FERNANDO EARTHQUAKE

Slip and off-fault deformation occur during an earthquake, with slip being measurable deformation on the fault, and off-fault being deformation that is measured away from the fault. Significant off-fault deformation was measured for the 1971 San Fernando Earthquake, finding that the Sylmar segment of the fault experienced more vertical off-fault deformation, while there was more horizontal off-fault deformation on the Tujunga segment (Gaudreau et al. 2023). Our aim is to learn about what may have caused this off-fault deformation and its heterogeneity by performing dynamic models of the faulting process on this fault system and analyzing the effects of the different fault geometries and materials.

Our current studies of the Sylmar and Tujunga fault segments have utilized 2D dynamic finite element models using FaultMod (Barall 2009). In our models, we set parameters such as stress, material properties, friction, cohesion, and fault geometry to understand how they affect slip and ground motion. We have been modeling both plastic and elastic models for each of the fault segments. As we work on tuning our 2D models, we have found that both bulk friction and cohesion make a difference in how plastic or elastic our models behave. In addition, we are seeing qualitatively more vertical off-fault deformation in our models for the Sylmar Segment than in the Tujunga Segment.