Effect of Porosity and Permeability Evolution on Injection-Induced Aseismic Slip

It is widely recognized that fluid injection can trigger aseismic fault slip. However, the processes by which the fluid-rock interactions facilitate or inhibit slip are poorly understood and some are oversimplified in most models of injection-induced slip. In this study, we perform a 2D anti-plane shear investigation of aseismic slip that occurs in response to fluid injection into a permeable fault governed by rate-and-state friction. We account for porosity and permeability changes that accompany slip, including dilatancy, and quantify how these processes affect pore pressure diffusion, which couples to aseismic slip. Fault response to injection has two phases. In the first phase, slip is negligible and pore pressure closely follows the standard linear diffusion model. Pressurization eventually triggers aseismic slip close to the injection site. In the second phase, aseismic slip front expands outward and dilatancy causes pore pressure to depart from the linear diffusion model. We quantify how prestress, injection rate, permeability and other fluid transport properties affect the slip front migration rate, finding rates ranging from 10 to 1,000 m/day for typical parameters. The migration rate is strongly influenced by the fault's closeness to failure and injection rate. The total slip on the fault, on the other hand, is primarily determined by the injected volume, with minimal sensitivity to injection rate. Additionally, we show that when dilatancy is neglected, slip front migration rate and total slip can be several times higher. Our modeling demonstrates that porosity and permeability evolution, especially dilatancy, fundamentally alters how faults respond to fluid injection.