SCEC Award Number 21020 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Using focal mechanisms within regions of off-fault deformation to constrain active fault configuration of the southern San Andreas fault
Investigator(s)
Name Organization
Michele Cooke University of Massachusetts Amherst Jack Loveless Smith College Scott Marshall Appalachian State University
Other Participants Hanna Elston, PhD candidate
SCEC Priorities 3a, 1e, 1a SCEC Groups SDOT, SAFS, CXM
Report Due Date 03/15/2022 Date Report Submitted 03/09/2022
Project Abstract
 Geodetic data that can resolve slip rates along relatively isolated faults struggle to recover geologic slip rates along close parallel strands, such as the southern San Andreas fault. Because focal mechanisms occur at depth and often closer to the slipping portions of faults within the interseismic period, the stresses derived from focal mechanisms may better resolve some details of active faulting than surface GPS velocities within a joint inversion. Here, we develop and test a new approach to invert off-fault stress information for slip distribution. We test the approach by inverting stresses produced by two forward models, one with a single planar fault and a second with the CFM based configuration of active faults in the region. We use the planar fault models to ascertain the optimal distribution of off-fault stress data for recovering the interseismic slip distribution. The joint slip rate inversions perform better than either the velocity or stressing-rate tensor alone. Because focal mechanisms do not provide complete stress tensors, we also compare inversions using the deviatoric stressing rate tensors and normalized deviatoric stressing rate tensors. The normalized deviatoric stressing rate tensors, which are similar to the data currently derived from focal mechanisms, have large misfits due to the lack of magnitude information. The validation completed in this study demonstrate the potential improved resolution of constraining interseismic slip rates from joint inversion of surface velocity vectors and deviatoric stressing rate tensors, if we are able to infer stressing rate tensors from focal mechanisms.
Intellectual Merit Because focal mechanisms occur at depth and often closer to the slipping portions of faults within the interseismic period, they may improve the resolving power of surface GPS velocities in joint slip rate inversions. Here we test a new approach to invert off-fault stress information for interseismic slip distribution. We find that either full stressing rate tensors or deviatoric stressing rate tensors improve upon the recovery of forward model slip rates than those from surface velocities alone. The validation completed in this study demonstrate the potential improved resolution of constraining interseismic slip rates from joint inversion of surface velocity vectors and deviatoric stressing rate tensors, if we are able to infer stressing rate tensors from focal mechanisms.
Broader Impacts This study is the first to demonstrate that off-fault stress information from the interseismic period could inform slip distribution. The validations completed in this study test this new method with the goal of showing how stressing rate tensor information, if it were available from focal mechanisms, could constrain interseismic slip rates at depth. This project supports both a female UMass PhD candidate, Hanna Elston, who is also a 1st generation college student, and a female PI, Cooke, with deafness.
Exemplary Figure Fig. 1 A) Optimal spacing of surface velocities (triangles) and stress tensors (circles). Open circles show points with sufficient nearby focal mechanism in the current catalog to infer stress state. B) Current focal mechanism catalog and GNSS stations.