A Controlled in-situ Fault Activation Experiment at Meter-scale Shows that High-pressure Fluid Injections Mostly Drive Aseismic Motion

Understanding how fluids pressure produce seismic or aseismic motion along faults is an important goal for seismic hazard assessment and for geological reservoir monitoring. Seismicity rate increase in fluid injection areas where some events may reach magnitude greater than 5. Besides, the microseismicity is one the few tools used to follow fluid migration at depth. In-situ experiments were performed in limestones similar to those observed in the Middle-East. By injecting fluids at high-pressure in the damaged zone of an inactive fault at 280m depth, we study the seismological and hydromechanical responses (recorded by 31 sensors) of different fracture types to a fluid perturbation. Only a few tests have generated seismicity even if ruptures are observed with a displacement sensor at the injection point. Detected earthquakes are characterized by high frequency content (0.6 to 3 KHz) and weak magnitude (-4). The relative and absolute locations (1.5m accuracy) indicate a lack of events in the vicinity of injection borehole. Results show that about 96% of the deformation is aseismic. Finally, our experiment showed that fluid injection mainly drives aseismic motion and the seismicity might be only an indirect effect related to stress transferred from the volume deformed by fluid pressurization.