Slip perturbation during fault reactivation by a fluid injection

Slip orientation inferred from fault striae or focal mechanism datasets is commonly used in stress inversion methods based on the Wallace-Bott hypothesis. The hypothesis postulates that slip on a fault plane is collinear with the orientation of the resolved shear stress. It is valid for a single planar fault subjected to a homogeneous far-field stress. However, the experimental displacement data from an induced fault reactivation experiment, conducted in the Mont Terri rock laboratory, Switzerland, indicated multiple triggered slip orientations, thereby
preventing application of the above inversion method. We present numerical and analytical results of slip on a reactivated fracture with a non-uniform fluid pressure distribution. Using these models, we evaluate the reasons for the inconsistency of our observations and the traditional Wallace-Bott hypothesis and test the physical effects of various parameters on fault slip. In the fully coupled hydromechanical numerical model (three-dimensional distinct element method), fluid pressure at a point on the fault surface is increased stepwise (assumed planar and singular) until shear reactivation of the fault is induced. We studied two different models with high and low fault plane stiffness to represent hard and soft rock masses, respectively. The model shows that high fault stiffness preserves the planarity of the fault plane, while low fault stiffness permits dilation and morphological changes of the fracture related to fluid pressure diffusion. The highest slip perturbation was observed in the low stiffness model due to the change of the fracture shape, controlled by the non-uniform pressure distribution. The Eshelby analytical solution confirmed that the more the fracture is dilated, the more the corresponding resolved shear
stress is perturbed. Additionally, when compared to dilation and fault aperture, the friction angle has the most influence on the angular difference between geomechanical slip vectors and resolved shear stress.