Intermittent induced seismicity during the multiyear operation of a geothermal reservoir

SUMMARY Understanding the mechanical behaviour of a fractured geothermal reservoir during its operation phase, when a sustained circulation of fluid is taking place, is of crucial importance for the appraisal of this technology. This knowledge is also essential for understanding natural fault systems that exhibit fluid-induced seismicity, as the geothermal reservoir serves as a small-scale analogue to these systems. Here, we analyse the seismicity of a geothermal reservoir in France, which has been the primary target for heat exploitation over the last 8 yr. Fluid circulation in the granite has been maintained along a main fractured zone through pathways with enhanced permeability thanks to the continuous injection of fluid from a single well. We show that the seismicity occurring during the operation of this reservoir exhibits a progressive expansion outpacing the zone initially activated during the hydraulic stimulation. We also show that most recorded earthquakes are clustered in time within discrete bursts that activate different portions of the fault system. The migration of the events included in these bursts indicates that they are likely related to aseismic transients developing over the creeping fault interfaces. It therefore demonstrates that the intermittency of the seismic activity characterizing earthquake swarms can arise naturally as the complex hydro-thermo-mechanical response of a system under continuous forcing conditions.