FIRST RESULTS OF ASEISMIC FAULT SLIP AND LEAKAGE PRECEDING AN EARTHQUAKE INDUCED DURING AN IN SITU FAULT REACTIVATION EXPERIMENT IN SHALES (MONT TERRI FS EXPERIMENT, SWITZERLAND)

Understanding fault reactivation is critical in geologic wastewater disposal, hydraulic fracturing, and CO₂ sequestration because it may result in enhanced fault permeability, potentially inducing fluid leakage from the injection zone through overlying caprock and eventually triggering shallow seismic events. Here we show preliminary results from a controlled field stimulation experiment (FS experiment dedicated to the hydro-mechanical characterization of in-situ clay fault slip) conducted in a N140°-dipping 50-to-60°SE fault in a clay formation in the Mont Terri Underground Research Laboratory (Switzerland). We measured fault slip and seismicity induced by fluid-injection in a natural fault at a depth of 300 m. We observe multiple dilatant slip events (~ 30 micrometer/s) associated with factor-of-1000 increase of permeability, and a magnitude ~ -2.5 main seismic event associated with a swarm of very small magnitude ones. Seismicity occurs after aseismic slip has been initiated within the fluid-pressurized zone of the fault. Two monitoring points set across the fault allow estimating that, at the onset of the seismicity, the radius of the fault patch invaded by pressurized fluid is ~ 5-to-7m which is larger than the approximate location of the seismic events. The seismic source radius which was estimated to ~ 1.2m indicates that only a fraction of the activated fault patch experienced unstable slip. Significantly different slip/dilation signals at the two monitoring points tend to show that patches of different fault hydromechanical properties could be controlling the slip stability.