Combining forward and inverse approaches to resolve interseismic deep slip and locking depths on closely spaced faults

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

Published August 16, 2021, SCEC Contribution #11593, 2021 SCEC Annual Meeting Poster #121 (PDF)

Inversions of interseismic surface velocities alone often struggle to uniquely resolve the fault slip rate distribution in systems with branching or closely spaced faults, such as the southern San Andreas fault (SAf). Local stress states inferred from microseismic focal mechanisms may provide additional constraints on interseismic deep slip because they contain information about stress state at depth and closer to the interseismic deep slip than GPS surface velocities. Here, we invert surface velocities and subsurface stressing rates, both individually and jointly, to estimate the distribution of slip rates on the southern SAf and nearby faults. The inverse approach we present can constrain both the interseismic deep slip rates that reveal the relative activity of faults and the fault locking depths. Debate persists on the activity of the northern pathway for earthquake rupture through the San Gorgonio Pass. Our inversions may be able to inform the activity of the northern pathway.

Forward models that include the 3D complex geometry of faults within the San Gorgonio Pass region simulate interseismic loading of the southern SAf in a two-step back-slip like approach. The models produce both stressing rate tensors at points of recorded microseismicity and surface velocities at GPS station locations. We invert the forward model-generated stressing rates and surface velocities to recover the fault slip distribution and assess the inverse model performance. The inversions of forward model-generated stressing rate tensors and surface velocities recover a broader locking depth transition zone than prescribed due to the smoothing-based regularization applied in the inversion. Stressing rate tensor inversions resolve slip rates better than the surface velocity inversions. Inverting stress and surface velocity information simultaneously improves the fit to the forward-model slip distribution. Joint inversions of both surface velocities and local stress states may improve constraints on the interseismic deep slip rates and locking depths in regions of complex faulting.

We would like to estimate the slip rate distribution on the southern SAf, by inverting both stress states derived from microseismic focal mechanisms and GPS surface velocities. Prior to this inversion, we need ways to adjust the focal mechanism deviatoric stress tensors so that they approximate stressing rate within the crust.

Key Words
inverse model, forward model, interseismic, slip rate

Citation
Elston, H. M., Cooke, M. L., Loveless, J. P., & Marshall, S. T. (2021, 08). Combining forward and inverse approaches to resolve interseismic deep slip and locking depths on closely spaced faults. Poster Presentation at 2021 SCEC Annual Meeting.

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