Patterns and controls of induced seismicity in geothermal reservoirs: insights from case studies and implications for hazard assessment and mitigation

• Review of over 60 deep geothermal case studies related or not to induced seismicity. • Seismic hazard is very low for mass-balanced circulation in porous aquifers. • Fractured and high-temperature reservoirs show moderate-to-high seismic hazard. • Brittle fractured igneous or sedimentary rocks are more prone to felt seismicity. • Seismic hazard assessment based on reservoir properties and operational conditions. This study reviews over 60 deep geothermal case studies to investigate the occurrence or absence of felt induced seismicity, across different reservoir types and exploitation strategies. The review highlights key patterns in seismogenic potential based on reservoir characteristics and operational methods. The potential for felt seismicity is very low for mass-balanced circulations in porous aquifers, low-to-moderate in deeper faulted or karstified sedimentary formations, while it is moderate-to-high for stimulations or circulations in fractured reservoirs and for production and reinjection operations in high-temperature geothermal fields. The presence of pre-existing faults or fractures, in a critical stress state and favorably oriented in the local stress field, constitutes an aggravating factor with respect to the seismic hazard of geothermal operations. The review further reveals that induced seismicity is poorly controlled by depth and lithology of the target formation but rather linked to rock competence and fractures characteristics. Higher seismic hazard is associated with brittle, low-porosity sedimentary or igneous rocks and with operations involving high-pressure stimulations or significant thermal and volumetric perturbations. The review confirms that the largest magnitude events often occur during the shut-in phase, highlighting that this trend is not only observed after reservoir stimulations but also after injection and circulation operations. These results emphasize that induced seismicity is governed by a complex interplay of geological, mechanical, and operational factors. The findings support the development of a framework for seismic hazard assessment tailored to geothermal reservoir characteristics and injection strategies, essential for mitigating risks in current and future geothermal projects.