Loading Rate-Dependent Elastoviscoplasticity in San Andreas Fault Observatory at Depth (SAFOD) Fault Gouge: Implications for Repeating Earthquakes and Fault Zone-Guided Waves

Abstract:

Deformation experiments on phyllosilicate-rich fault gouges reveal velocity-strengthening behavior and monotonic strength evolution in response to perturbations in slip velocity below ~10^-4 ms^-1. Fault gouge from the actively creeping zones at the San Andreas Fault Observatory at Depth (SAFOD) exhibits similar monotonic strength evolution and has been described in terms of rate-state friction-velocity dependence and ageing behavior. While these parameters provide phenomenological descriptions of gouge rheology on relatively short timescales, they are commonly applied in numerical simulations of repeating earthquakes within the SAF creeping section, often being adjusted arbitrarily in order to match seismological observations. With first assuming a deformation constitutive law, we performed comprehensive microstructural and mechanical characterization of fault gouge from the SAFOD Central Deforming Zone (CDZ). An in-situ displacement sensor was developed to provide direct measurements of gouge deformation under various loading conditions, including constant and variable strain rate and constant and variable shear stress. Constant and variable strain-rate tests confirm previous observations of low shear strength and reveal viscoplastic deformation below the frictional yield strength. Variable loading rate tests demonstrate an apparent yield stress for viscoplastic behavior at low loading rates, and a transition to elastic behavior with increasing loading rate up to 0.02 MPas^-1. The elastic response of the gouge constrains the static shear modulus ~500 MPa, providing a lower bound of ~450 ms^-1 for the shear velocity of the SAFOD fault core. Our microstructural and mechanical characterization of the gouge is consistent with the physical interpretation of an elastically perfect elastoviscoplastic solid. Parameterizing this model with our experimental data demonstrates general agreement with the observed loading rate-dependence of the gouge and provides a physical description of the deformation mechanisms. By relating the model parameters to micromechanical processes, gouge rheology may be employed as a constraint in models of the SAF creeping section and provides a physical framework for understanding lithological changes on the fault with depth.