In Situ Evidence for fluid-induced aseismic slip events along fault zones

Abstract

Stress measurements performed with the Hydraulic Test on Preexisting Fractures (HTPF) method, prior to and after conducting some large scale fluid injections in a granite rock mass, provide evidence for the occurrence of important stress heterogeneities associated with a fault zone. Modelling this fault zone as a weaker moduli inclusion does not explain the observed stress perturbations. The best model that can easily account for the observations is shear stress release along the fault zone induced during the injections. The absence of a seismic event, compatible with the size and the location of the slip area, implies that this slip motion occured quasi-statically. A similar aseismic shear stress relief mechanism helps also to explain the extreme inconsistency (up to 165°) between slip directions associated with some of the injection induced microseismic events and the shear stress directions expected from the regional stress field as determined from the HTPF data. This implies that the regional stress field cannot be determined only from the focal mechanisms of fluid-induced microseismic events, since these can be influenced very strongly by local stress heterogeneities.