Earthquakes generated from fluid injections sometimes appear to occur much faster than would be expected from the speed of fluid migration. Bhattacharya and Viesca used observations from a controlled fluid injection to develop a model to understand the behavior of fluids and aseismic slip. They found that the aseismic slip outpaced the fluid migration, allowing for earthquake triggering to happen much faster and farther away from the initial fluid injection.
Earthquake swarms attributed to subsurface fluid injection are usually assumed to occur on faults destabilized by increased pore-fluid pressures. However, fluid injection could also activate aseismic slip, which might outpace pore-fluid migration and transmit earthquake-triggering stress changes beyond the fluid-pressurized region. We tested this theoretical prediction against data derived from fluid-injection experiments that activated and measured slow, aseismic slip on preexisting, shallow faults. We found that the pore pressure and slip history imply a fault whose strength is the product of a slip-weakening friction coefficient and the local effective normal stress. Using a coupled shear-rupture model, we derived constraints on the hydromechanical parameters of the actively deforming fault. The inferred aseismic rupture front propagates faster and to larger distances than the diffusion of pressurized pore fluid.