Understanding post-injection seismicity: Causes and mechanisms of trailing earthquakes in Enhanced Geothermal Systems

Enhanced Geothermal Systems (EGS) employ fluid injection for both reservoir stimulation and circulation, a practice that may sometimes induce earthquakes. Paradoxically, numerous EGS projects have observed the occurrence of the largest-magnitude earthquake (LME, 1.6 < M < 5.5) after injection cessation, posing a significant challenge for the development of geothermal power plants. This paper aims to examine the cases of postinjection LME observed in EGS, and provide a detailed understanding of the coupled mechanisms responsible for seismic activity during and after fluid injection. We report the mechanisms that have been hypothesised for different cases of LME emerging in the post-injection phase of EGS. The analysis of post-injection LME cases reveals two types of seismicity, early and late, differentiated by the time at which pore pressure changes become negligible, i.e., a post-injection time longer than the injection time. Notably, we remark that, for most cases of early post-injection high-magnitude earthquakes, the primary triggering mechanisms involve pore pressure diffusion, thermal effects, poromechanical stress relaxation and static stress transfer resulting from seismic fault slip. Conversely, prolonged fluid injections could lead to the gradual accumulation of stress on faults, mainly due to aseismic slip, causing late post-injection LME. Furthermore, recent studies have also demonstrated the importance of the initial fault stress criticality in influencing the post-injection behaviour of fault aseismic slip and trailing seismicity. Finally, we present recent efforts devoted to develop effective methodologies for mitigating induced seismicity, and the current research perspectives aimed at forecasting and minimizing seismic activity associated with EGS.