IN-SITU EVIDENCE FOR FLUID-INDUCED ASEISMIC SLIP EVENTS ALONG FAULT ZONES

Stress measurements performed with the Hydraulic Test on Preexisting F ractures (HTPF) method, prior to and after conducting some large scale fluid injections in a granite rock mass, provide evidence for the occ urrence of important stress heterogeneities associated with a fault zo ne. Modelling this fault zone as a weaker moduli inclusion does not ex plain the observed stress perturbations. The best model that can easil y account for the observations is shear stress release along the fault zone induced during the injections. The absence of a seismic event, c ompatible with the size and the location of the slip area, implies tha t this slip motion occurred quasi-statically. A similar aseismic shear stress relief mechanism helps also to explain the extreme inconsisten cy (up to 165-degrees) between slip directions associated with some of the injection induced microseismic events and the shear stress direct ions expected from the regional stress field as determined from the HT PF data. This implies that the regional stress field cannot be determi ned only from the focal mechanisms of fluid-induced microseismic event s, since these can be influenced very strongly by local stress heterog eneities.