Seismisyys Espoon Otaniemen syväreikähankkeen viimeistelyvaiheessa

This report describes induced seismicity related to the Otaniemi deep heat project of the energy company St1 in January-September 2020. The St1 company completed a conductivity test and a subsequent counterstimulation in the 5.8 km deep borehole OTN-2b in Espoo, Finland in May 2020. During these procedures, induced earthquakes occurred, several of which were noticed by residents.

The results presented here are part of seismic monitoring of the Otaniemi enhanced geothermal system conducted by the Institute of Seismology. During the conductivity test and counter-stimulation, induced seismicity clearly remained below the 2018 levels. Nine events with magnitude M_L ≥ 0.8 were observed and two of them exceeded the amber traffic light threshold (M_L ≥ 1,2). Preliminary focal mechanisms indicate that the amber events occurred in a NNW-SSE fracture zone crosscutting the heat reservoir. The largest events were located within 500 m radius from the borehole, mainly at a depth of 5.4-5.6 km near the bottom of the borehole. After the water injection the level of induced seismicity decreased rapidly. Only a few weak induced events were observed in between the first and second stimulation period.

Appendix 1.1 lists induced earthquakes observed instrumentally in the Helsinki region until the end of September 2020, with the threshold magnitude M_L ≥ 0.8. Appendix 1.2 presents all induced earthquakes from the same area with macroseismic felt reports submitted to the Institute of Seismology, regardless of magnitude.