How Does In Situ Stress Rotate Within a Fault Zone? Insights From Explicit Modeling of the Frictional, Fractured Rock Mass

Abstract

We quantitatively investigate the spatial stress variations within the fault zone rock mass by explicitly incorporating macroscopic fractures into a 2D multilayer model. Based on elastic crack theory, we first derive a unified constitutive law for frictional fractures, featuring elastic and plastic shear deformation and shear-induced dilatancy. To honor the varying degrees of damage across fault zones, the multilayer model is composed of varying densities of randomly oriented frictional fractures from layer to layer. Under the boundary conditions specific to fault zones, the global mechanical response of each layer is quantitatively related to the local fracture deformation. We show that the major principal stress always rotates toward a limiting angle of 45° with respect to the fault plane and that the differential stress invariantly decreases with increasing fracture density. Approaching the fault core, the mean stress can either increase or decrease, depending on whether the fault strikes at a high (>45°) or low (<45°) angle to the regional major principal stress. Accumulated damage also results in the decrease and increase of the effective Young's modulus and Poisson's ratio of the fractured rock mass, respectively. Both the fracture properties and pore pressure affect the stress variations by modulating the fracture-associated deformation and the relative proportion of the elastic and plastic components. Our model illuminates the systematic variations of in situ stresses and effective elastic properties within the damage zone of a mature fault.