Large earthquakes during hydraulic stimulations at the geothermal site of Soultz-sous-Forêts

Several deep wells were drilled in the Rhine Graben (Soultz-sous-Forêts, France) to evaluate the geothermal Hot Dry Rock potential of a deep fractured granite reservoir. Three main boreholes, which reached about 5 km depth, intersected a crystalline basement overlain by 1.4 km of Cenozoic and Mesozoic sediments. Stimulations of these three wells were carried out in 2000 for GPK2, 2003 for GPK3 and 2004, and 2005 for GPK4. During these stimulations and other hydraulic activities a seismological surface network was installed in order to monitor the seismicity induced by the massive fluid injection.Here we analyse the seismicity of magnitude larger than or equal to 1.4, which is the lowest magnitude felt by the population. Based on a spectral analysis of the displacement recorded by a Güralp velocimeter at a depth of 200 m, we know that the source dimensions range from tens to hundreds of metres. We analyse several parts of the reservoir where obvious correlation between the fluid path and tectonic features has been pointed out. Based on seismological arguments such as location and focal mechanisms, we show that this activity is linked with tectonic features, or at least with large fractures that control the behaviour of the geothermal reservoir. To constrain the hypothesis, we study 391 events and focal mechanisms to argue in favour of the existence and stability of tectonic features that can be compared to geological data. We show that the largest events recorded on the site occurred after the shut-in. Their spatial distribution appears not to be random within the reservoir, and the focal mechanisms of these events also confirm the non-randomness of their distribution. Given their source dimensions, the largest events can only occur on large structures, such as tectonic ones. Therefore, the behaviour of the reservoir is controlled by these main fractured zones, which either lead the fluid or hinder its path.