Stick Slip Instabilities and Strain Localization Dynamics in a fluid-infiltrated fault gouge zone model

Xiao Ma, & Ahmed E. Elbanna

Published August 15, 2017, SCEC Contribution #7780, 2017 SCEC Annual Meeting Poster #180

Stick-slip instabilities and shear localization have been long observed in sheared fault gouge, yet their underlying microscopic mechanisms are still not fully understood. Although numerous studies of the stick slip behavior within fault gouge have been conducted over the past two decades, studies on the role of fluids in promoting or regulating these instabilities are still limited on both the experimental and theoretical fronts. Given that most fault zones in active tectonic settings are fluid infiltrated, it is imperative to develop gouge specific models that incorporates fluids effects and enable prediction of strength evolution and localization bands for a wide range of pressure and strain rates.

Here, we use a non-equilibrium thermodynamics model, the Shear Transformation Zone (STZ) theory, to resolve gouge scale inelastic processes while integrating the role of fluids using Darcy’s flow approximation and Terzaghi’s effective stress concepts. In the STZ theory, the internal gouge state is quantified by a state variable called compactivity which is closely related to gouge porosity and evolves according to the roles of thermodynamics. Heterogeneities in compactivity are seeds for spontaneous localization of strain in the gouge layer even in the absence of pore fluids or thermal effects. The existence of fluids further enriches the model and modify the stability boundaries. In this study we consider frictional and frictionless grains as well as presence or absence of acoustic vibrations. We conducted linear stability analysis on the system of governing equations with dry and wet condition as well as with and without strain localization. Furthermore, we carry out numerical simulations to study the fully nonlinear response in the different cases.

Our preliminary results suggest that transition from dilation to compaction may occur during rapid stress drop and that the existence of pore fluids alters the stick slip dynamics changing the recurrence times and the boundary between stable and unstable slip. Hydraulic properties are also shown to control the shear band evolution including its peak strain rate and maximum width. We discuss the implications of our model for athermal strength evolution in fault zones and possible extensions to incorporate shear heating and thermal pressurization effects.

Key Words
Stick Slip, Strain Localization, Pore fluids

Citation
Ma, X., & Elbanna, A. E. (2017, 08). Stick Slip Instabilities and Strain Localization Dynamics in a fluid-infiltrated fault gouge zone model . Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)