Group A, Poster #103, Fault and Rupture Mechanics (FARM)

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

David D. Oglesby, Elyse Gaudreau, Gareth J. Funning, Edwin Nissen, & James Hollingsworth
Poster Image: 

Poster Presentation

2023 SCEC Annual Meeting, Poster #103, SCEC Contribution #13263 VIEW PDF
The 1971 San Fernando earthquake produced a rich pattern of ground motion and deformation that continues to be explored over 50 years later. As one of the only large, surface-rupturing, urban historical earthquakes in Southern California, it is of great importance in understanding the effects of potential future earthquakes in the region. The fault geometry has two main thrust segments (Sylmar and Tujunga) offset from each other by ~1300 m. The Sylmar segment appears to have a steep surficial dip of ~55°, which cuts through young, soft sediments near the surface, while the Tujunga segment has a shallower near-surface dip of ~25°, and cuts through somewhat older and stiffer near-surface mater...ials. Both segments appear to converge at depth to a single surface with a dip of ~40°.

A key observation in this event is the large amount of off-fault deformation measured in recent analyses by Gaudreau et al. (2023). Vertical off-fault deformation was found to be larger as a percentage of total offset on the Sylmar segment, while strike-parallel off-fault deformation was larger on the Tujunga segment. In the current work, we use 2D dynamic finite element models to investigate how fault geometry and Drucker-Prager plasticity might have affected vertical off-fault deformation near the Sylmar and Tujunga segments.

We find that plastic models with homogeneous material properties produce significant amounts of off-fault deformation in both Sylmar and Tujunga models, but that variations in near-surface dip alone are not sufficient to explain the observed differences between the segments, implying that spatially heterogeneous material properties may be crucial in understanding the physical source of these effects. The results have implications for understanding the near-surface behavior of faults that can be crucial for predicting ground motion and damage near fault traces.