Does pore-pressure induced permeability enhancement aid fluid-induced aseismic ruptures outpace pore-fluid migration?

Abstract:

The migration of seismicity in response to subsurface fluid injection is classically connected to diffusive pore-fluid movement. However, a growing body of observations provide evidence for distant induced earthquakes at very short times since injection, which are difficult to explain with the plausible range of reported crustal diffusivities. Recent studies suggest that such anomalously fast seismicity migration might, instead, be explained by elastic-stress perturbations provided by fluid-induced, aseismic ruptures which have outpaced pore-pressure diffusion. However, on linear slip-weakening faults with constant damage-zone permeability, a 1D shear rupture can aseismically outpace the pore-pressure front only under high overpressures and when background stress is very close to the residual strength(τ_b≈τ_r). This parameter regime seems too narrow to be operating across the diverse geotectonic settings where such fast seismicity migration has been observed. Here, we examine if pore-pressure induced permeability enhancement could provide an additional mechanism that allows such ruptures to outpace pore-pressure diffusion under less restrictive conditions.

We model a 1D shear rupture, pressurised by a constant overpressure (Δp) source, with damage zone permeability varying exponentially with effective normal stress.When friction is quasi-constant, we find that the shear rupture grows self-similarly with the non-linear pore-pressure diffusion. Outpacing solutions emerge at systematically smaller pre-stress to strength ratios as permeability enhancement or over-pressure to normal stress (σ) ratios increase. For more pronounced slip-weakening, permeability enhancement permits aseismic outpacing at much lower Δp/σ values than for constant permeability fault zones. Aided by permeability enhancement, these ruptures outpace pore-pressure diffusion even when τ_b is substantially smaller than τ_r . Importantly, we find that the smallest value of τ_b/τ_r for outpacing to occur, and the extent of outpacing at a given τ_b/τ_r, are both controlled by the level of permeability enhancement and Δp/σ. Our results show that permeability enhancement and linear slip-weakening together can explain the occurrence of faster-than-diffusion ruptures over a wide range of hydromechanical conditions .