How does in situ stress rotate within a fault zone? Insights from explicit modeling of the frictional, fractured rock mass

We quantitatively investigate the spatial stress variations within fault zones by explicitly incorporating macroscopic fracture networks in a multilayer fault zone model. Based on elastic crack theory, we first derive a unified constitutive relationship for frictional fractures, featuring elastic and plastic shear deformation and shear-induced normal dilatancy. To honor the progressively accumulated damage across a fault zone, we establish a fractured multilayer model including randomly-oriented frictional fractures with varying densities from layer to layer. Under the specific boundary conditions of a fault zone, the global mechanical response of each layer is quantitatively related to the deformation of the interior fractures. Stress variations and effective elastic property changes are systematically studied considering the influences of fracture properties and pore pressure. We show that the major principal stress always rotates toward a limiting angle of 45 with respect to the fault slip direction and that differential stress invariantly decreases with the fracture density. However, mean stress increases for an unfavorably-oriented fault and decreases when the regional major principal stress trends at a small angle (< 45°) to the fault slip direction. Accumulated damage also results in a decrease and increase in the effective Young’s modulus and Poisson’s ratio, respectively. The influences of fracture properties, pore pressure and fracture network can be attributed to their control on the fracture deformation components and relative proportion. Our model can predict continuous variations of stresses and effective elastic properties from intact country rock, through fractured damage zone, to the plastic fault core of a mature fault