Poster #120, Stress and Deformation Over Time (SDOT)

An innovative technique combining forward and inverse approaches to resolve interseismic deep slip and locking depths on closely spaced faults

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Poster Presentation

2020 SCEC Annual Meeting, Poster #120, SCEC Contribution #10677
In the San Gorgonio Pass region, forward Boundary Element Method (BEM) models of complex 3D faulting show that multiple viable fault configurations can match some but not all geologic slip rates. Inversions of interseismic surface velocities often struggle to uniquely resolve slip rates on faults in closely spaced systems, such as the southern San Andreas fault (SAf). Local stress states inferred from microseismic focal mechanisms during interseismic periods may provide additional insight on fault slip rate distribution. We present an inverse approach that uses either interseismic stressing rate tensors or surface velocities to estimate the distribution of slip rates along 3D fault surfaces the San Gorgonio Pass region. Additionally, we demonstrate that this innovative technique can provide information to constrain interseismic deep slip, active subsurface fault geometry and locking depths.

To test the performance of the inverse approach, we utilize two forward 3D BEM models, a single strike-slip fault model and a southern SAf model that includes nearby faults. The forward BEM models simulate interseismic loading in a two-step approach to produce surface velocities and stressing rate tensors at points that we use in inversions. The single fault model inverts a regular grid of model results, while the southern SAf model inverts model results at regional GPS station coordinates and the centers of clusters of focal mechanisms from the catalog. Both inverse approaches estimate slip distribution on faults, which we compare to the slip prescribed within the forward models.

Both inversions of the single fault recover the locking depth prescribed within the forward model, but the inversions predict a broader transition zone from locked to slipping behavior due to the smoothing-based regularization within inversions. Because the stressing rate tensors already include far-field loading, points farther than 10 km from the faults do not add information to the inversion. In this way, inversions of slip rate from stressing rate tensors differ from surface velocity inversions where far-field points are needed to capture far-field deformation. Consequently, stressing rate tensor inversions may be better able to delineate slip rates among closely spaced faults. Comparisons of inverse and forward model results demonstrate the utility of using stressing rate tensors from interseismic focal mechanisms to estimate slip, especially deep slip, during interseismic periods.