Stick-slip dynamics in dry and fluid saturated granular fault gouge investigated by numerical simulations

Abstract

Mature faults in the earth comprise a granular fault gouge created due to communition and fragmentation of host rock at the core of the damage zone. It is well understood that granular interactions in this fault gouge govern the dynamics of the fault. The stick-slip dynamics in a sheared granular layer in experiments and simulations is understood to simulate similar physical mechanisms that are involved in earthquakes. The dynamics of a sheared granular fault gouge can be strongly affected by its particle properties, shear driving velocity, confining stress, temperature and pore fluid. Fluids are found to play a significant role in fault mechanics, where they can not only alter the physicochemical properties of gouge materials, but also affect the stick-slip dynamics of the fault. Laboratory experiments have been widely used to investigate the underlying physics of granular interactions in fault mechanics, showing the important role of fluids in slip nucleation and failure process. However, experiments lack detailed information at grain scale to understand the role of fluids in stick-slip dynamics. Furthermore, in experiments the different effects of fluids, such as physicochemical and hydro-mechanical are not distinguishable. Numerical simulations at grain scale however allow unraveling such questions. Here, a 3D coupled Computational Fluid Dynamics-Discrete Element Method is used to model stick-slip dynamics in a granular fault system with fluids. First, a dry granular fault gouge is modeled and its dynamics for different particle properties and under different loading configurations is studied. Next, the effect of capillary cohesion at low saturation degrees, where the capillary bridges are in pendular regime, is studied. The results show an increase of recurrence time between slips and an increase in stress drop due to a rearrangement of the particles owing to the presence of rather low cohesive forces between the wet granular particles. This particle rearrangement results in more stable configurations of the granular layer as indicated by an increase in particle coordination number and shear stiffness of the layer during the stick phase. The results for a fluid saturated fault gouge show that slip events are characterized by a higher drop in friction coefficient, in potential energy and thickness of the gouge layer compared to the dry conditions. The drained fluid saturated granular fault gouge shows higher values for pre-seismic potential energy, describing that the fluid leads to more stable configurations of the granular layer illuminated by a higher particle coordination number. The higher potential energy drop is accompanied by a higher release of kinetic energy in fluid saturated granular fault gouge. The spatial correlation of regions with high fluid velocity, particle-fluid interaction and particle kinetic energy during slip show that the mechanisms of particle rearrangement, increase of fluid pressure and particle-fluid interaction forces are strongly coupled phenomena explaining the macroscopic observations. The high dynamic fluid pressure is due to the fast fluid flow during slip caused by particles rearrangements. The high fluid flow in turn introduces high drag forces on particles leading to a high particle kinetic energy. The coupled fluid-particle simulations provide grain-scale information allowing an in-depth understanding of slip instabilities. The observations in this research emphasize the important role that fluid flow and fluid-particle interactions may play in tectonic fault zones and show in particular how DEM models can help understand the hydro-mechanical processes that dictate fault slip.