Poster #107, Tectonic Geodesy

Mechanical Modeling Faulting in the Transverse and Peninsular Ranges of Southern California

Savannah C. Devine, & Scott T. Marshall
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Poster Presentation

2021 SCEC Annual Meeting, Poster #107, SCEC Contribution #11616 VIEW PDF
The greater Los Angeles, California region lies within the Transverse and Peninsular Ranges of southern California and hosts numerous seismically active faults. Here, we present results from three-dimensional Boundary Element Method (BEM) models that are based on the SCEC Community Fault Model version 5.2 and simulate long-term deformation throughout the region. Previous BEM models of the region were driven by a uniform shortening rate; however, geodetic data indicate spatially variable shortening rates across the Transverse Ranges while the Peninsular ranges show no shortening. In models presented here, we drive regional deformation using a combination of the total Pacific-North American pl...ate boundary motion and geodetically determined slip rates along the San Andreas Fault. This setup tests whether tectonic plate and San Andreas fault motion can reproduce both reverse and strike-slip rates throughout the Transverse and Peninsular Ranges. In order to gauge model feasibility, we compare model predictions to existing geologic slip rate estimates. We find that the CFM5.2 model performs well, fitting 45/60 UCERF3 slip rate ranges with an average misfit of 1.2 mm/yr. Reverse Slip rates on 38/43 faults in the Transverse Ranges generally fit the geologic estimates (average misfit of 0.9 mm/yr) despite the model being driven solely by strike-slip motion. This indicates that motion on the San Andreas fault combined with tectonic plate motion, is sufficient to reproduce current-day reverse faulting rates. The largest slip rate discrepancies occurred on the Pine Mountain and San Gabriel faults where the model predicted 4.2 and 4.5 mm/yr of slip, respectively, compared to geologic estimates of 0.01 and 0.39 mm/yr respectively. Both of these faults have uncertain recent geologic activity, so we created a model with the Pine Mountain and San Gabriel faults removed and found little impact on the other predicted slip rates and an improved average misfit of 0.9 mm/yr. This result could indicate that the San Gabriel and Pine Mountain faults may be currently inactive. The model also provides slip rate estimates along the many faults that do not currently have geologic estimates. Given the preliminary success, it is possible that a similar methodology could be used to create a mechanical model of the entire Pacific-North American plate boundary.