Seismicity related to the hydraulic stimulation of GRT1, Rittershoffen, France

The Rittershoffen deep geothermal reservoir, in Northeastern France, is well characterized and has been extensively studied by a multidisciplinary approach. A hydraulic stimulation for the development of the geothermal reservoir was performed in June 2013. This injection of fluid led to seismic activity which was closely monitored by a dedicated set of seismic stations. The seismic sequence started during the injection but showed an unusual long quiet period of 4 d after shut-in before the occurrence of a second swarm of events. Here we take the opportunity of this well-monitored activity to gain insight into the geomechanical factors favouring the development of induced earthquakes. We apply a template matching approach and a relative relocation procedure to obtain a precise estimate of the geometries of the activated structures. Our approach shows that the induced events during the injection took place on two parallel planar structures. It shows that details of the seismicity generally obtained from borehole seismic network are achievable from surface network when an appropriate analysis is performed. The development of this induced seismicity is in good agreement with the known stress field and failure criterion proposed for the reservoir. In particular, the orientation of the activated structure, associated focal mechanisms and the overpressure needed to initiate the seismic activity are all in line with the geomechanical model of the area. The swarm of delayed events, 4 d after shut-in, can be well explained by considering an aseismic slip on the imaged fault and the related static stress transfer. We therefore suggest that the ability to monitor local slow aseismic movements at depth, in conjunction with precise tracking of the seismicity, is of primary importance to understand induced earthquake activity.