SCEC Award Number 23189 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title 3D Dynamic models of near-surface faulting and off-fault deformation in the 1971 San Fernando Earthquake
Investigator(s)
Name Organization
David Oglesby University of California, Riverside Elyse Gaudreau University of Victoria (Canada) Gareth Funning University of California, Riverside
Other Participants James Hollingsworth, ISTerre, Université Grenoble-Alpes, France
Edwin Nissen, School of Earth and Ocean Sciences, University of Victoria, Canada
SCEC Priorities 2d, 2e, 3g SCEC Groups FARM
Report Due Date 03/15/2024 Date Report Submitted 11/12/2024
Project Abstract
 We are aiming to explain the distribution of vertical off-fault deformation (OFD) measured in the 1971 San Fernando Earthquake, which displayed more vertical OFD on the hanging wall of the steeply dipping Sylmar segment than on the more shallow-dipping Tujunga segment. To investigate the physics of this event, we performed 2D dynamic spontaneous rupture simulations of two 2D fault segments, in preparation for a move to 3D models in the near future. Both models converge to a single segment with intermediate dip below 2 km. Our models incorporate Drucker-Prager plasticity to help model the OFD. We find that for models with high frictional coefficients and large deviatoric stresses, slip can be larger for the plastic model than for the otherwise equivalent elastic model. For models with low friction and lower deviatoric stresses, the plastic slip is smaller than the elastic slip. In the case of the shallow dipping Tujunga segment, we find that the vertical OFD is spread out over a large area on the hanging wall, making it less likely to be detected by the methods used to map OFD near the fault trace. In contrast, for the steeply dipping Sylmar segment, the vertical OFD is concentrated near the fault trace, and would likely be much easier to detect. This result agrees with observations, and points toward the parameterizations that we will use in our subsequent 3D modeling.
Intellectual Merit This project helps extend our understanding of the interacting effects of fault geometry and plasticity in determining fault slip and surface deformation. In particular, it shows that the dip angle near the surface has a strong impact on the distribution of vertical off-fault deformation, with steeper dip angles concentrating the OFD close to the fault trace. Our results set the stage for 3D modeling to come subsequently.
Broader Impacts The 1971 San Fernando earthquake is one of the only examples of a surface-rupturing thrust event in an urban setting. As such, this research helps to explain not only the physical processes that may have taken place in that earthquake, but it also helps in anticipating the effects of future earthquakes on similar faults in the Los Angeles basin and beyond. Our results have implications for seismic hazard, engineering, zoning, and disaster preparedness and response.
Exemplary Figure Figure 3: Comparison between Tujunga and Sylmar segment models for both elastic and plastic materials. a) Tujunga slip, b) Sylmar slip, c) Tujunga vertical displacement (hanging wall to the right), d) Sylmar vertical displacement (hanging wall to the right). In agreement with observations, the plastic Sylmar model experiences more vertical off-fault deformation (note peak in blue curve on hanging wall) than the plastic Tujunga model.
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