SCEC Award Number 18200 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Assessing kinematic compatibility of fault geometry and slip rates along the southern San Andreas fault system
Investigator(s)
Name Organization
Michele Cooke University of Massachusetts Amherst
Other Participants Jenn Beyer, PhD student
SCEC Priorities 3a, 1a, 1c SCEC Groups SDOT, CXM, SAFS
Report Due Date 03/15/2019 Date Report Submitted 05/02/2019
Project Abstract
 Crustal deformation models with alternative 3D fault geometry within the San Gorgonio Pass region produce fault slip rates that match some, but not all, of the available geologic strike-slip rates (Beyer et al. 2018). Disagreement between the model and geologic slip rates highlight locations where the interpreted fault geometry is incompatible with the geologic slip rate. Here, we investigate for the first time the kinematic compatibility of slip rates and fault geometry with mechanical models that limit strike-slip rates at the locations of the geologic investigations to within the range of interpreted slip rates at those sites. The faults outside of these regions slip freely in response to tectonic loading, fault interaction and the influence of prescribed slip. We assess the kinematic incompatibility by mapping the off-fault distortional strain. Distortion maps reveal regions of kinematic incompatibility at the branch of the San Andreas fault near Indio Hills, as well as within the hanging wall of the San Gorgonio Pass thrust and near the juncture of the San Jacinto and San Andreas faults at Cajon Pass. The local off-fault distortion indicates that the inferred geologic slip rates at these sides are not effectively accommodated along the interpreted fault surfaces. The incompatibilities highlight regions that would benefit from better constraints on active fault configuration and slip rates. Additionally, we compare slip rates within and without hard linkage of the San Andreas and San Jacinto faults at Cajon Pass and show that hard- or soft-linkage, are plausible and yield similar long-term slip rates.
Intellectual Merit The project is the first to examine kinematic compatibility of geologic slip rates and interpreted fault geometry and the findings highlights regions that require further study. Off-fault distortion maps of the model results reveal regions of kinematic incompatibility at the branch of the San Andreas fault near Indio Hills, and to a lesser degree at Cajon Pass and within the hanging wall of the San Gorgonio Pass thrust. These incompatibilities suggest that we have either incorporated inaccurate fault configuration in these areas or included incorrect slip rates. Further geological and geophysical investigations should address the fault geometry and slip rate uncertainties at Indio Hills.
The comparison of slip rates within and without hard linkage of the San Andreas and San Jacinto fault at Cajon Pass show that without additional independent data, both variations of fault connection, hard linkage or soft linkage, are plausible. The small difference in slip rates suggests that for models of long-term slip rates (e.g. multi-cycle models), it doesn’t matter significantly whether the two faults are hard linked or not.
Broader Impacts The project is the first to examine kinematic compatibility of slip rates and interpreted fault geometry. This project supported a female PhD student at UMass - Amherst and a hearing-impaired fe-male PI.
Exemplary Figure Figure 4: Change in off fault distortional strain map for model where slip rate is limited to within the range of geo-logic rates at sites of geologic investigations. Change in distortion is calculated form the reference model. The col-or of the circle indicates the differences between the prescribed slip rate and the slip rate in the reference model of Fig. 2. High distortion at Indio Hills and small distortion at Cajon pass and in the hanging wall of the San Gorgonio Pass thrust indicate regions where geologic slip rate are kinematic incompatible with fault geometry. Further in-vestigations should work to resolve these incompatibilities.