Assessing kinematic compatibility of fault geometry and slip rates along the southern San Andreas fault system in the San Gorgonio Pass region

Jennifer L. Hatch, & Michele L. Cooke

Published August 15, 2018, SCEC Contribution #8758, 2018 SCEC Annual Meeting Poster #165

Assessment of seismic hazards in southern California may be improved with more accurate characterization of geometry and slip rates along 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. Crustal deformation models with alternative three-dimensional active geometries of the Mill Creek and Mission Creek strands produce fault slip rates that match some, but not all, of the available geologic strike-slip rates at sites along both the San Andreas and San Jacinto faults. Sites with disagreement between the model and geologic slip rates indicate where the model geometry is incompatible with the interpreted geologic slip rate. We test the kinematic compatibility of slip rates and fault geometry by investigating mechanical models that prescribe geologically determined strike-slip rates within the model at positions corresponding to the sites of the geologic investigations. The faults outside of these regions slip freely in response to tectonic loading, fault interaction and the influence of the sites of prescribed slip. We assess the kinematic incompatibility of the models by calculating the off-fault distortion, here defined as the sum of the curl and the divergence of the surface velocity field.

Here, we investigate the kinematic incompatibility of the San Gorgonio Pass region using a suite of models. Because most geologic investigations provide ranges for slip rates, one model prescribes the mean geologic slip rates at investigated sites and a second one prescribes the slip rate limit that minimizes the misfit to the freely slipping model. Off-fault distortion maps of the model results reveal regions of kinematic incompatibility at the branch of the San Andreas fault near Indio Hills, as well as along the San Bernardino strand near Cajon Pass. These same regions have freely slipping model slip rates that over- or under-estimated the geologic slip rates, indicating that the prescribed fault slip rates are not effectively accommodated along the simulated fault surfaces. These incompatibilities at Indio Hills and Cajon Pass suggest that we have incorporated inaccurate fault configuration at these branches or included incorrect slip rates.

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

Citation
Hatch, J. L., & Cooke, M. L. (2018, 08). Assessing kinematic compatibility of fault geometry and slip rates along the southern San Andreas fault system in the San Gorgonio Pass region. Poster Presentation at 2018 SCEC Annual Meeting.

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