The role of poroelasticity and dilatancy in governing the transition from aseismic to seismic slip during fluid injection

Most faults at seismogenic depths can be described as fractures or discontinuities in a fluid-saturated porous medium and thus, the theory of poroelasticity offers a practical mechanical description of the natural fault environment. However, poroelasticity is rarely considered in simulations of fault slip. Poroelasticity incorporates the two-way coupling of solid and fluid phases where pore-pressure change,  e.g., due to slip, strains the rock matrix and volumetric strain causes changes in pore pressure. During earthquake nucleation, inelastic dilatancy may also induce pore pressure changes. A complex interplay of pore pressure in the bulk and shear zone emerges when we consider the multiple processes coupled to slip on a fault governed by rate-and-state friction. Here, we present an efficient spectral boundary integral code that allows for 2D quasi-dynamic rate-and-state simulations of slow and fast slip with fully coupled and simultaneous state-dependent dilatancy, fluid injection, and two-way coupled diffusive poroelastic bulk response. The method allows for anisotropic shear-zone permeability, while the bulk is considered to be isotropic and homogenous. We can thus simulate three diffusion time scales at once: along the shear zone, across the shear zone, and due to wavelength-dependent bulk diffusion. We apply the code to understand nucleation and repeated fault ruptures with a realistic pore-pressure injection history from a field experiment. We compare different cases with and without dilatancy, larger or smaller differences in drained and undrained poroelastic properties, and varying bulk diffusivity. By systematically increasing the dilatancy coefficient, we observe a transition from highly unstable seismic slip to a migrating slow slip front to quasi-static slip localized to highly pressurized areas. More surprisingly, we find that differences in drained and undrained poroelastic properties and bulk diffusivity strongly influence fault slip stability. A larger difference between drained and undrained Poisson’s ratio or higher bulk diffusivity results in more stable slip during injection, fewer ruptures, and delayed nucleation. These effects appear to be of comparable importance to dilatancy. We conclude that the poroelastic properties of the bulk, which are typically ignored, play a critical role in the stability and determining if slip is seismic or aseismic.