Getting pushy with the San Gorgonio Pass: Investigating active fault geometries with crustal deformation models

Jennifer L. Beyer, Michele L. Cooke, & Scott T. Marshall

Published August 12, 2017, SCEC Contribution #7534, 2017 SCEC Annual Meeting Poster #219

Assessment of seismic hazards in southern California may be improved with more accurate characterization of the active San Andreas fault strands within the San Gorgonio Pass region. On-going debate centers on the activity level and active geometry of the Mill Creek and Mission Creek strands. We investigate five forward crustal deformation models with alternative three-dimensional active geometries of the Mill Creek and Mission Creek strands. Each model produces fault slip rates that match some, but not all, of the available geologic strike-slip rates on both the San Andreas and San Jacinto faults. The calculated misfits to the geologic strike-slip rates reveal two best-fitting models of the active fault configuration. In addition to matching many strike-slip rates on the San Andreas and San Jacinto faults, the Inactive Mill Creek model satisfies geologic evidence of no recent slip at Upper Raywood Flats, but the North-dipping Mission Creek model, with low slip rates at Upper Raywood Flats, incorporates additional active portions of the Mission Creek and Galena Peak strands interpreted by Morelan et al. [2016]. The two best-fitting models produce similar uplift patterns with significant differences in the hanging walls of the dipping faults. Additional uplift rate data in these regions and geologic slip rates along the northern fault strands are needed to inform active fault geometries. This could give preference to one of the two models if the mismatch between model and geologic slip rates deviate between the two currently preferred models.

The mismatch between the model results and geologic slip rates may owe to epistemic uncertainties in active fault structure as well as interpreted slip rates. To assess where the analysis may incorporate incorrect assumptions, we develop models that prescribe geologic strike-slip rates at investigated sites to the corresponding positions along the simulated fault meshes of the model. The faults outside of these regions slip freely in response to tectonic loading and the prescribed slip rates. Distortion maps of the model results reveal regions of incompatibility at the two branches of the southern and northern strands of the San Andreas fault. In these regions, the prescribed fault slip rates are not effectively accommodated along the simulated fault surfaces. This incompatibility may indicate that the fault configuration is inaccurate at these branches or included incorrect slip rates.

Key Words
San Gorgonio Pass, San Andreas Fault, Mill Creek, slip rates, BEM

Citation
Beyer, J. L., Cooke, M. L., & Marshall, S. T. (2017, 08). Getting pushy with the San Gorgonio Pass: Investigating active fault geometries with crustal deformation models. Poster Presentation at 2017 SCEC Annual Meeting.

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