Mechanical Models of Fault Slip Rates in the Transverse and Peninsular Ranges, CA

Savannah C. Devine, Hugh Harper, & Scott T. Marshall

Published September 10, 2023, SCEC Contribution #13133, 2023 SCEC Annual Meeting Poster #146

The Transverse and Peninsular Ranges of Southern California host a geometrically complex network of seismically active faults that pose challenges to deformation modelers. Many of these faults have finite lengths, are disconnected, and intersect other faults making accurate geometric representation difficult. Additionally, the Transverse Ranges hosts mostly contractional deformation while the Peninsular Ranges exhibits mostly strike-slip faulting making the selection of appropriate boundary conditions challenging. Here, we present three-dimensional mechanical models of this region that are driven only by the total Pacific-North American plate motion and slip on the San Andreas, San Jacinto, and Garlock faults. The advantage of this method is that the boundary conditions are appropriate for the entire Pacific-North American plate boundary in southern California and could in the future be used to simulate the mechanics of the entire plate boundary. Using these boundary conditions and a 3D fault mesh based on the SCEC CFM v5.3, we use a Boundary Element Method implementation to solve for the full three-dimensional distribution of slip rates on 83 faults. To evaluate model performance, we compare model slip rate predictions to UCERF3 average slip rate estimates along with individual point-based estimates made by individual investigators. 46/68 (68%) model-predicted average fault slip rates fall within the UCERF3 slip rate ranges with a root mean squared error (RMSE) of 1.03 mm/yr. The largest slip rate discrepancies occur on the San Gabriel, Pine Mountain, and Big Pine faults, which may be presently inactive. We find that rendering these three faults inactive results in an improvement in all goodness-of-fit metrics where 47/63 (75%) UCERF3 slip rates fit with an RMSE of 0.40 mm/yr. We also compare this same model to existing point-based long-term slip rate estimates and fit 39/69 (60%) with an RMSE of 0.79 mm/yr. The model fit to independent slip rate data implies that total plate motion in conjunction with strike-slip along the “Big Bend” of the San Andreas fault, the Garlock, and San Jacinto faults is sufficient to reproduce both reverse-slip rates throughout the Transverse Ranges and strike-slip rates in the Peninsular Ranges regions without additional driving forces. Overall, the models provide slip rates and distributions for 83 faults, many of which currently do not have geologic slip rate estimates and/or may not produce interseismic deformation.

Key Words
Deformation Modeling, Earthquakes, Mechanical Modeling, Faults

Citation
Devine, S. C., Harper, H., & Marshall, S. T. (2023, 09). Mechanical Models of Fault Slip Rates in the Transverse and Peninsular Ranges, CA. Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)