SCEC Award Number 20055 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
SCEC Priorities 3a, 1e, 1a SCEC Groups SDOT, SAFS, CXM
Report Due Date 03/15/2021 Date Report Submitted 03/09/2021
Project Abstract
 The San Gorgonio Pass region hosts multiple nearby active strands for which both subsurface active configuration and slip rate are under-constrained. Geodetic data that can resolve slip rates or fault geometry along relatively isolated faults struggle to recover geologic slip rates along close parallel strands of the San Andreas fault in this region. Because focal mechanisms occur at depth and closer to the slipping portions of faults within the interseismic period, they can, in some instances, better resolve details of active faulting than interseismic surface GPS velocities. Here we develop and test a new approach to invert off-fault stress information from the interseismic period for slip distribution. We test the approach by inverting stresses produced by two forward toy models, one with a single planar fault and a second with the CFM based configuration of active faults in the region. We varied the location and spacing of off-fault stress data in the planar model to pin-point the ideal configuration for recovering the interseismic slip distribution. We compare slip rate inversions of both the forward model produced surface velocities at the permanent GPS sites and the stressing rate tensor at 10-15 km spaced points throughout the seismogenic crust, where focal mechanisms might occur. The stressing rate tensor inversion recovers the prescribed slip rates better than the surface velocity inversion (misfit 1.1 and 2.5 mm/y respectively). The validation completed in this study demonstrate the utility of new method for constraining interseismic slip rates from stress tensors inverted from the focal mechanisms catalog.
Intellectual Merit Because focal mechanisms occur at depth and closer to the slipping portions of faults within the interseismic period, they can, in some instances, better resolve details of active faulting than interseismic surface GPS velocities. Here we test a new approach to invert off-fault stress information from the interseismic period for slip distribution. We find that the stressing rate tensors may recover the forward model slip rates better than the surface velocities. The validations completed in this study demonstrate the utility of new method for constraining interseismic slip rates from stress tensors inverted from the focal mechanisms catalog. This approach may be particularly useful in regions with closely space parallel fault strands where inversions of interseismic GPS velocities struggle to resolve slip rates.
Broader Impacts This study is the first to demonstrate that off-fault stress information from the interseismic period can be inverted to reliably inform slip distribution. The validations completed in this study test this new method with the goal of using stress tensors inverted from focal mechanisms to constrain interseismic slip rates at depth. This project supports a female UMass PhD candidate, Hanna Elston, who is also a 1st generation college student, and a female PI with deafness.
Exemplary Figure Fig. 3 A) Forward model interseismic strike-slip rate distribution with a 20 km locking depth. B, C) Correlation of prescribed slip rate and inversions from surface velocity (B) or stressing rate tensor (C) inversion predicted average strike-slip rates for elements below 20 km.