Coupled Boundary Element and Finite Volume Methods for Modeling Fluid-Induced Seismicity in Fault Networks within Low-Permeability Rocks

Abstract

To better understand the mechanics of injection-induced seismicity, we developed a two-dimensional numerical code to simulate both seismic and aseismic slip on non-planar faults and fault networks driven by fluid diffusion along permeable faults. Our approach integrates a boundary element method to model fault slip governed by rate-and-state friction with a finite volume method for simulating fluid diffusion along fault networks. We demonstrate the method's capabilities with two illustrative examples: (1) fluid injection inducing slow slip on a primary rough, rate-strengthening fault, which subsequently triggers microseismicity on secondary, smaller faults, and (2) fluid injection on a single fault in a network of intersecting faults, leading to fluid diffusion and reactivation of slip throughout the network. In both cases, the simulated slow slip migrates more rapidly than the fluid pressure diffusion front. The observed migration patterns of microseismicity in the first example and slow slip in the second example resemble diffusion processes but involve diffusivity values that differ significantly from the fault hydraulic diffusivity. These results support the conclusion that the microseismicity front is not a direct proxy for the fluid diffusion front and cannot be used to directly infer hydraulic diffusivity, consistently with some decametric scale in-situ experiments of fault activation under controlled conditions. This work highlights the importance of distinguishing between mechanical and hydrological processes in the analysis of induced seismicity, providing a powerful tool for improving our understanding of fault behavior in response to fluid injection, in particular when a network of faults is involved.