Abstract
It is widely recognized that fluid injection can trigger fault slip.
However, the processes by which the fluid-rock interactions facilitate
or inhibit slip are poorly understood and some are neglected or
oversimplified in most models of injection-induced slip. In this study,
we perform a 2D antiplane 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 pore dilatancy and permeability
changes that accompany slip, and quantify how these processes affect
pore pressure diffusion, which couples to aseismic slip. The 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 of the fault eventually triggers
aseismic slip in the immediate vicinity of the injection site. In the
second phase, the aseismic slip front expands outward and dilatancy
causes pore pressure to depart from the linear diffusion model. Aseismic
slip front overtakes pore pressure contours, with both subsequently
advancing at constant rate along fault. We quantify how prestress,
initial state variable, injection rate, and frictional properties affect
the migration rate of the aseismic slip front, finding values ranging
from less than 50 to 1000 m/day for typical parameters. Additionally, we
compare to the case when porosity and permeability evolution are
neglected. In this case, the aseismic slip front migration rate and
total slip are much higher. Our modeling demonstrates that porosity and
permeability evolution, especially dilatancy, fundamentally alters how
faults respond to fluid injection.