A Hybrid Implicit–Explicit XFEM Framework for Fully Coupled Hydro-Mechanical Dynamic Simulation of Injection-Induced Seismicity

Accurate simulation of injection-induced seismicity requires to solve for strongly coupled hydro-mechanical physics describing processes acting over widely separated spatiotemporal scales, ranging from reservoir scale fluid diffusion to fault nucleation and rapid dynamic rupture. In this study, we present a monolithic hydro-mechanical dynamic framework based on the extended finite element method (XFEM) for modeling fluid-induced fault reactivation governed by rate-and-state friction. Faults are represented as embedded displacement discontinuities within a poroviscoelastic medium, enabling a consistent treatment of fault slip, unilateral contact constraints, stress-dependent permeability evolution, and fluid exchange between the fault and the surrounding porous matrix.

To overcome the computational cost associated with fully implicit time integration, we develop a hybrid implicit–explicit (IMEX) time-integration strategy. The implicit solver is employed during the quasi-static and nucleation phase, while an explicit scheme is activated only during the coseismic stage, once a prescribed slip-velocity threshold is exceeded. This adaptive solver switching allows accurate resolution of the dynamic rupture with substantial reduction of the computational effort. The approach is combined with adaptive time stepping to efficiently capture both slow interseismic evolution and fast seismic transients within a unified framework.

Numerical simulations of fluid injection into a faulted reservoir demonstrate that, despite unconditional stability, fully implicit schemes require minimum time steps comparable to the Courant–Friedrichs–Lewy limit to accurately resolve rupture nucleation and propagation. In contrast, the proposed IMEX formulation can reproduce fault slip evolution, stress redistribution, frictional weakening, seismic moment, and event magnitude with high fidelity, while reducing computational cost by approximately 60–77% relative to fully implicit simulations. Differences between the two approaches are primarily limited to peak slip velocities and rupture speeds, whereas rupture timing, accumulated slip and event-scale seismic metrics remain consistent.

The proposed XFEM-based IMEX framework provides a robust and computationally efficient tool for simulating injection-induced seismicity, offering a practical pathway toward reservoir scale simulations of coupled fault–fluid systems relevant to geo-energy applications and seismic hazard assessment.