Inverting for interseismic deep slip on closely spaced faults using surface velocities and stressing rate tensors

Hanna M. Elston, Michele L. Cooke, Jack P. Loveless, & Scott T. Marshall

Published September 11, 2022, SCEC Contribution #12148, 2022 SCEC Annual Meeting Poster #187

Debate persists about the partitioning of active strike-slip between the southern and northern pathways of the southern San Andreas fault (SSAF) through the San Gorgonio Pass. Inversions of interseismic surface velocities that are often used to inform fault activity can struggle to uniquely resolve the 3D slip-rate distribution along closely spaced faults such as the two pathways of the SSAF. Because earthquake focal mechanisms contain information on stress at depth, closer to interseismic deep slip than GNSS surface velocities, local stress states inferred from microseismic focal mechanisms may provide additional constraints on patterns of deep slip. Here, we present an inverse approach that utilizes both surface velocities and subsurface stressing rates to constrain the interseismic deep slip rates and the consequent relative activity of faults.

We assess the inverse approach by inverting forward model-generated stressing rates and surface velocities to recover fault slip distribution for two models: a single planar strike-slip fault and a complex model simulating the SSAF. Forward models simulate interseismic loading in a two-step back-slip like approach and produce both regularly spaced stressing rates and surface velocities as well as stressing rates at points of recorded microseismicity and surface velocities at GNSS station locations. Because focal mechanisms only provide normalized deviatoric stress tensors, we invert the full, deviatoric and normalized deviatoric forward-model generated stressing rate tensors to assess the impact of removing stress magnitude from the constraining data.

The inversions including stress magnitude information recover the slip rate distribution well. The inversions recover the locking depth with a broader transition zone than prescribed in the forward model due to the smoothing-based regularization imposed in the inversion. Stressing rate inversions recover the forward model slip rates better than the surface velocity inversions. Inverting regularly spaced stress and velocity information jointly optimizes recovering the forward model slip distribution. Joint inversions of both surface velocities and local stress states may improve constraints on the interseismic deep slip rates in regions of complex faulting such as the SSAF. Successful inversions will require methods to adjust the focal mechanism-derived normalized deviatoric stress tensors so that they approximate stressing rate magnitudes within the crust.

Key Words
Inverse model, Forward model, stress state, surface velocity

Citation
Elston, H. M., Cooke, M. L., Loveless, J. P., & Marshall, S. T. (2022, 09). Inverting for interseismic deep slip on closely spaced faults using surface velocities and stressing rate tensors. Poster Presentation at 2022 SCEC Annual Meeting.

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