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Can deformation rates across the Carrizo Plain segment of the San Andreas Fault be explained by vertical migration of the locked to-creeping transition?

Lucile Bruhat, & Paul Segall

Published August 9, 2017, SCEC Contribution #7456, 2017 SCEC Annual Meeting Poster #087

Most geodetic inversions of surface deformation rates consider the depth distribution of interseismic fault slip-rate to be time invariant. However, some numerical simulations show down-dip penetration of dynamic rupture into regions with velocity-strengthening friction, with subsequent up-dip propagation of the locked to-creeping transition. These models are particularly attractive to investigate the discrepancy between geodetically- and seismically-derived locking depths.

Recently, Bruhat & Segall (GJI, 2017) developed a new method to characterize interseismic slip rates, that allows slip to penetrate up dip into the locked region. This simple model considers deep interseismic slip as a crack loaded at constant slip rate at the down-dip end. It provides analytical expressions for stress drop within the crack, slip, and slip rate along the fault. These expressions make use of an expansion of the slip distribution in Chebyshev polynomials, with a constraint that the crack-tip stress be non-singular. The simplicity of the method enables Monte Carlo inversions for physical characteristics of the fault interface, establishing a first step to bridge purely kinematic inversions to physics-based numerical simulations of earthquake cycles.

This study extends this new class of solution to strike-slip fault environment. We focus our work on the Carrizo Plain section of the San Andreas Fault. We first investigated earthquake cycle models that include viscoelastic flow and deep interseismic creep to explain observed horizontal deformation rates. We show that with reasonable estimates for the elastic thickness and the maximum coseismic displacement, it is difficult to explain the data without appealing to deep interseismic creep and deep extent of coseismic rupture. Best fitting models suggest that the maximum coseismic rupture extends to 15 km, then tapers down to the elastic/viscoelastic transition at 20 km. Between 15-20 km, deep interseismic slip occurs, migrating vertically at 10 m/year, slowly unlocking the deepest region of the elastic crust. This model constitutes a first step at explaining the discrepancy between geodetically-derived locking depths and microseismicity along the San Andreas fault.

Key Words
Creep and deformation, Fault mechanics, Transient deformation, Carrizo Plain, Seismic cycle, Tectonic geodesy

Bruhat, L., & Segall, P. (2017, 08). Can deformation rates across the Carrizo Plain segment of the San Andreas Fault be explained by vertical migration of the locked to-creeping transition? . Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy