The event described here can be limited to specific settings and might not be encountered in every geothermal projects. More details about this topic are presented in a report that can be found on GEOENVI website.
Many geothermal areas are associated with geological structures that are characterized by natural seismicity. Geothermal development tends to modify the characteristics of a reservoir by withdrawing and injecting hot and/or cold fluid from/into the underground. In particular, circulating water through the geothermal reservoir creates pressure changes that can cause micro-seismic events. Production and injection rates and pressures, fluid volumes, and injection duration are factors that affect the likelihood and magnitude of an induced seismic event. If the reservoir is fractured (i.e. fluid moves principally within fractures and not within the porous media), the forced fluid circulation can cause induced seismicity by lowering the fracture resistance to slip or by thermal cracking. Some other effects, like perturbations due to drilling, or redistribution of stress due to variations in fluid volume within the reservoir, can also cause induced events.
Micro-seismicity is sometime associated to geothermal development. It refers to seismic events with magnitude below 2-3 that are detected by seismometers but are not, or are slightly, perceived by population (depending on many factors including subject sensibility). In a few geothermal projects felt, although minor, seismic events have also been reported. However, of all the geothermal-related injection and extraction activities conducted in many decades of production in many places in the world, only a very small fraction experienced induced seismicity at levels noticeable to the public or caused any harm or damage (e.g. Charléty et al, 2007; Deichmann et Giardini, 2009; Grigoli et al, 2018). The most impacting phenomenon has occurred in a few projects undergoing hydraulic stimulation, e.g. injection of fluid at high pressure into the reservoir to increase reservoir permeability.
In contrast to natural earthquakes, induced seismicity can, to some extent, be mitigated. First of all, before starting the development of a geothermal project a seismic hazard and risk analysis is conducted enabling to establish the risk level of a given location. Geothermal development in areas having a high risk to trigger seismicity (e.g. close to a main seismogenic fault with history of strong earthquakes) is excluded. Furthermore, in case of actual seismic risk, a contingency plan is defined. Mitigation measures include the adoption of protocols such as traffic light systems (TLS) to monitor seismicity and regulate (or even stop) the operations of geothermal plants. Traffic light systems rely on (quasi)real-time signal processing and link it to the SCADA (Supervisory Control And Data Acquisition) system of the geothermal plant. Whenever some predefined selected parameters linked to the recorded seismic activity (for example magnitude, seismic rate, ...) are higher than predetermined thresholds, actions are taken accordingly.
The table here below provides an overview of this topic in term of risk and impact assessment i.e. its causes, consequences, the phases concerned, the influencing contexts or the principals monitoring and mitigation measures that can be adopted.
