Earthquake ruptures in poroelastic media involve a suite of complex phenomena arising from stick‐slip frictional instabilities and thermo‐hydromechanical couplings. In this study we propose a fully implicit, time‐adaptive, and monolithically coupled finite element model to simulate dynamic earthquake sequences in poroviscoelastic media. We consider a Kelvin‐Voigt viscoelastic material and characterize the impact of inertial effects on injection‐induced earthquakes. We present, for the first time, dynamic simulations of ruptures in rate‐and‐state faults in poroelastic media. Our simulations resolve the full earthquake cycle, including the interseismic, spontaneous earthquake nucleation, and dynamic rupture phases. We compare dynamic simulations with quasi‐dynamic ones, in which inertial effects are neglected and the slip singularity is resolved through a radiation damping approximation. Viscous dissipation models the physical process of seismic wave attenuation: As viscous damping increases, the patch size and the maximum fault slip become smaller, hence decreasing the expected earthquake magnitude. From a computational perspective, viscoelasticity helps avoid spurious high‐frequency oscillations during wave propagation. By including inertial effects, the dynamic model accounts for transient fluctuations of pressures and solid stresses during rupture, which are neglected in the quasi‐dynamic approach. Understanding these transient perturbations may shed light on the role of pore pressure in the mechanism of dynamic earthquake triggering. The poroviscoelastic dynamic approach is a good compromise between the inviscid, fully dynamic model, and the quasi‐dynamic one. A small amount of viscous damping allows us more efficient calculations, while preserving the most relevant features of dynamic ruptures, in particular slip velocities, accumulated slip, and seismic moment released.

Understanding the risk associated with anthropogenic earthquakes is essential in the development and management of engineering processes and hydraulic infrastructure that may alter pore pressures and stresses at depth. The possibility of earthquakes triggered by reservoir impoundment, ocean tides, and hydrological events at the Earth surface (hydro-seismicity) has been extensively debated. The link between induced seismicity and hydrological events is currently based on statistical correlations rather than on physical mechanisms. Here, we explore the geomechanical conditions that could allow for small pore pressure changes due to reservoir management and sea level changes to propagate to depths that are compatible with earthquake triggering at critically-stressed faults (several kilometers). We consider a damaged fault zone that is embedded in a poroelastic rock matrix, and conduct fully coupled hydromechanical simulations of pressure diffusion and rock deformation. We characterize the hydraulic and geomechanical properties of fault zones that could allow for small pressure and loading changes at the ground surface (in the order of tens or hundreds of kPa) to propagate with relatively small attenuation to seismogenic depths (up to 10 km). We find that pressure diffusion to such depths is only possible for highly permeable fault zones and/or strong poroelastic coupling.

The intensity and damage potential of earthquakes are linked to the speed at which rupture propagates along sliding crustal faults. Most earthquakes are sub-Rayleigh, with ruptures that are slower than the surface Rayleigh waves. In supershear earthquakes, ruptures are faster than the shear waves, leading to sharp pressure concentrations and larger intensities compared with the more common sub-Rayleigh ones. Despite significant theoretical and experimental advances over the past two decades, the geological and geomechanical controls on rupture speed transitions remain poorly understood. Here we propose that pore fluids play an important role in explaining earthquake rupture speed: the pore pressure may increase sharply at the compressional front during rupture propagation, promoting shear failure ahead of the rupture front and accelerating its propagation into the supershear range. We characterize the transition from sub-Rayleigh to supershear rupture in fluid-saturated rock, and show that the proposed poroelastic weakening mechanism may be a controlling factor for intersonic earthquake ruptures.