Seismic monitoring of medium-depth geothermal projects in Tampere and Helsinki, Finland

In Finland, several geothermal projects have recently been launched in urban
areas. Thousands of small, induced earthquakes, ML≲1.8, have been observed during the reservoir stimulations of the Otaniemi 6 km deep geothermal system located in the Helsinki metropolitan area. Furthermore, two induced events, ML≲1.0, have been observed related to a 1.3 km deep geothermal well in Koskelo, also situated within the metropolitan area. Events in Koskelo, as well as tens of Otaniemi events, were felt by the public. These have brought a shift in regulatory policies. Local authorities have begun expecting seismic monitoring also for the medium-depth projects. Institute of Seismology, University of Helsinki, has implemented the monitoring for geothermal projects in Tampere and in Helsinki using 1–4 local on-line broad-band stations, accompanied with off-line geophone stations, complementing the stations of the Finnish National Seismic Network. Thus far, no induced earthquakes have been detected in Tampere or Helsinki. Based on the detected industrial explosions, detection threshold of the networks is well below magnitude zero. We have observed, that in monitoring medium-depth geothermal projects, a single additional on-line station near the site may be sufficient, especially if complemented with off-line geophones.