Source mechanisms of earthquakes induced by the 2018 and 2020 geothermal stimulations in Espoo/Helsinki, southern Finland

An experimental ~6 km deep enhanced geothermal system in Otaniemi, in the Helsinki capital region, southern Finland, was stimulated in 2018 and 2020. During the two stimulations that lasted seven and three weeks, respectively, signals of the induced earthquakes with a maximum local magnitude of 1.8 were recorded with dense and diverse seismic networks. The intraplate southern Finland setting of the experiment yields an intriguing opportunity to study earthquake and rock failure processes in the precambrian Fennoscandian Shield where the level of natural seismicity is comparatively low. The high confining pressure of 180 MPa at 6 km depth defines the key characteristics of the stress field, together with the previously estimated North-110-degrees-East direction of the maximum horizontal stress. The competent crystalline bedrock has very low attenuation, and yields high signal-to-noise ratio seismograms even at relatively high frequencies. We study the source mechanisms of ~250 induced earthquakes with Mw > 0.5. We perform probabilistic full moment tensor analysis with the Grond package of the software suite Pyrocko. We use data sets from the 2018 and 2020 stimulation experiments. Both experiments were monitored with more than 100 three-component surface stations operated by the Institute of Seismology, University of Helsinki, and 12 three-component borehole stations maintained by the St1 developer company installed at around 300 m depth. The diverse network elements help to evaluate the consistency of the results. We first present results of a detailed analysis of a small event subset characterized by the best data quality and solutions to assess the robustness of the different tensor components to different processing choices. This includes a comparison of surface and borehole sensor data. This allows us to conclude that the majority of the analysed earthquakes have a dominant reverse faulting mechanism and a small subset of events has strike slip mechanisms, which is compatible with solutions reported by the developer group. The predominant fault plane orientations are in agreement with the ambient stress conditions that also seem to control the thrust mechanism. Based on the best quality solutions we discuss the significance of the obtained non-double couple moment tensor components to assess if significant opening or closing elements in the induced earthquake source reflect genuine physical processes or spurious effects associated with imperfect resolution.