This repository contains continuous seismic data in miniseed format recorded in May and June 2020 in the Helsinki, Finland, area, to monitor the response to a second stimulation of a ~6 km deep enhanced geothermal system in the Espoo Otaniemi district that followed on the first larger stimulation in 2018. During the 2020 stimulation a total volume of 2600 cubic meter freshwater was pumped in several stages with maximum pumping pressures of 70 MPa, and the largest induced earthquake magnitude was ML 1.2. The provided data set was collected by 5-Hz three component SmartSolo instruments sampling at 500 Hz operated by the Institute of Seismology, University of Helsinki. The provided 43 stations are a subset of the full network that was operating during the 2020 stimulation. The full 2020 network and with it the deployment of the SmartSolo instruments is described in a Seismological Research Letter Data Mine Column by A. Rintamäki et al. This manuscript has been reviewed at the time of writing. The 2020 network is split into three subnetworks. The SmartSolo data are part of the OX subnetwork described by Rintamäki et al. (e.g., their Figs. 1, 3b). The locations of the SmartSolo stations are indicated in the provided html map that can be looked at variable scales in any web browser. (An accompanying data set of the OT subnetwork collected by short period CUBE stations from the Geophysical Instrument Pool Potsdam is available at https://doi.org/10.5880/GIPP.201925.1 .) The instruments were (mainly) powered by internal batteries. 33 of the 43 SmartSolo instruments were arranged in two nominally 17-station arrays that recorded in a 12-hour on/off cycle that was chosen to extend battery life. The two 180 m aperture, 17-sensor Storhemt (SH) and Munkkiniemi (MN) SmartSolo arrays were deployed 4.8 km and 2.7 km west and north-east of the injection site on a fallow agricultural field and in an open park area, respectively, configured in three circles of three, five, and eight sensors around a central station. The final MN data set consists of 16 stations. Four sensors were deployed on the Koirasaari island (KS array) and six in Lauttasaari (AS array). These were equipped with fence batteries to extend the recording period. The SmartSolo data set size is about 500 GB. The Otaniemi 2020 network can be understood in the context of seismic monitoring systems that evolve along with the different stages of an EGS. The data were collected to underpin a wide range of seismic analysis techniques for complementary scientific studies of the evolving reservoir processes and the induced event properties. These should inform the legislation and educate the public for transparent decision making around geothermal power generation.

2021