Dynamics of Fluid-Driven Slip on a 3D Heterogeneous Fault With Rate-and-State Friction

A three-dimensional quasi-dynamic finite element approach is presented for the simulation of fluid-driven seismicity on a heterogeneous fault with rate-and-state friction. The coupled nonlinear hydro-mechanical equations of the fault and surrounding matrix are solved monolithically, using the Imperial College Geomechanics Toolkit, to obtain the fluid pressure and displacement fields. This study solves for friction on the fault surfaces, wherein the augmented Lagrangian method is used to apply the contact constraints in a finite element framework. A stick-predictor slip-corrector algorithm is developed for the rate-and-state friction law to obtain better convergence. It is found that the spatial mesh size and temporal time step must meet specific criteria in order to guarantee the convergence of an iterative solver. The fault is represented as a surface in the model using zero-thickness interface elements. The heterogeneous fault has velocity-weakening asperities, surrounded by creeping velocity-strengthening barriers. Changes in pore pressure trigger an aseismic slip front propagating along the creeping barriers, leading to the failure of asperities. The characteristics of fluid-induced aseismic slip are described, along with its interaction with seismogenic asperities. Numerical results show that the seismicity of a heterogeneous fault can be determined using a geometry-material diagram based on two measurable fault parameters: the ratio $a/b$ of the frictional parameters of the creeping region, and the areal density of the velocity-weakening asperities. It is shown that these two parameters can control the post-seismic distribution of slippage around each asperity, which can be a trigger for secondary seismic events on neighboring asperities.