Sensitivity of deformation to activity along the Mill Creek strand of the San Andreas fault within the San Gorgonio Pass

Jennifer L. Beyer, & Michele L. Cooke

Published August 15, 2016, SCEC Contribution #6865, 2016 SCEC Annual Meeting Poster #060

A significant portion of the deformation budget across irregular fault networks can be accommodated by permanent off-fault deformation that accumulates between earthquakes. This deformation may correspond to a reduction in the slip rates along the faults within these complex networks, which in turn may contribute to uncertainty when evaluating seismic hazards within southern California. We use well-validated 3D Boundary Element Method (BEM) models to investigate the irregular fault geometry through the San Gorgonio Pass. Within this region, on-going debate centers on the activity level and geometry of the Mill Creek strand of the San Andreas fault. Here we compare BEM models with and without an active Mill Creek strand, while investigating varying geometries of an active Mill Creek strand. One alternate fault configuration consists of an active segment of the Mission Creek fault transferring slip to the Galena Peak fault. All models are run with a range of boundary conditions to account for epistemic uncertainty of tectonic loading, providing a range of slip rates and surface deformation. The presence of an active Mill Creek strand decreases the dextral slip rates on other faults within the San Gorgonio Pass; however, each model results in fault slip rates that match several of the available geologic slip rates. An active Mill Creek strand also produces much more localized uplift, while the model without a Mill Creek strand produces a more distributed uplift pattern in the San Bernardino Mountains. For each fault geometry, we also compare two end member models representing the interseismic loading of the crust and deformation across multiple earthquake cycles. While the interseismic models simulate the overall loading of faults between earthquakes, the multi-cycle models simulate crustal stresses that generate permanent off-fault deformation such as evidenced by microseismicity. These end member models produce distinct stressing rate patterns. The multi-cycle model reveals that in regions of high fault complexity, the sum of the slip on faults does not equal the plate velocity and a significant portion of the strain is accommodated as distributed off-fault deformation. Consequently, assessment of seismic hazards in the southern California may be compromised due to insufficient characterization of the complex fault geometry.

Key Words
San Gorgonio Pass, Mill Creek, southern California, BEM

Citation
Beyer, J. L., & Cooke, M. L. (2016, 08). Sensitivity of deformation to activity along the Mill Creek strand of the San Andreas fault within the San Gorgonio Pass. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)