Due to the deep socioeconomic implications, induced seismicity is a timely and increasingly relevant topic of interest for the general public. Cases of induced seismicity have a global distribution and involve a large number of industrial operations, with many documented cases from as far back to the beginning of the twentieth century. However, the sparse and fragmented documentation available makes it difficult to have a clear picture on our understanding of the physical phenomenon and consequently in our ability to mitigate the risk associated with induced seismicity. This review presents a unified and concise summary of the still open questions related to monitoring, discrimination, and management of induced seismicity in the European context and, when possible, provides potential answers. We further discuss selected critical European cases of induced seismicity, which led to the suspension or reduction of the related industrial activities.

S U M M A R Y In 1987, two microearthquake sequences, with a duration of about a week each and consisting of 37 and 46 events respectively, occurred in the upper crust below the Jura Mountains of northern Switzerland. The seismograms within each sequence exhibit a high degree of similarity, indicating tight clustering of hypocentres and similar focal mechanisms. Using a cross-correlation technique applied to the seismograms in the time domain, together with a new least-squares adjustment procedure, it was possible to determine relative hypocentre locations of the earthquakes in each cluster with a precision of a few tens of metres. The results show that hypocentres of events with the same focal mechanism lie on a plane which coincides exactly with one of. the nodal planes of the fault-plane solution, and that consequently swarm-like sequences of similar earthquakes are due to repeated slip on the same fault. In one case, slip occurred as right-lateral motion on a steeply dipping plane striking in WNW-ESE direction, whereas the other case corresponds to left-lateral slip on two almost vertical planes striking roughly N-S. In 1988, an additional earthquake triplet occurred on a WNW-ESE oriented normal fault nearby. All three mechanisms are consistent with a general NNW-SSE oriented direction of maximum crustal shortening and a corresponding WSW-ENE oriented extension. Repeated slip on the same fault is indicative of a large degree of heterogeneity and of short-term temporal variability of both frictional resistance and stress distribution on the fault. An explanation for the occurrence of such swarm-like seismic activity in terms of barriers or asperities would require that shear stress on the unbroken patches increase from one event to the next. Since, however, the overall shear stress would be expected to decrease as a consequence of the stress released in each event, a more plausible mechanism involves pore-pressure fluctuations, caused by fluids under suprahydrostatic pressures migrating upward through pre-existing zones of weakness in the crust.

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