ALBERT

Description: This study simulates automatic event detection and location performance of a micro-earthquake network centred around a future power plant site in Finland, Fennoscandian Shield. Simulation of the event location capability is based on a relationship derived between event magnitude and maximum detection distance. Azimuthal coverage and threshold magnitude are computed for different station configurations and the results are presented as contour maps. An optimal configuration of ten seismograph stations is proposed for further on-site survey. The network fulfils the preconditions of automatic event location capability down to ML 0.0 and azimuthal coverage better than 180° within 25 km radius from the study site. Annual number of earthquakes (ML ≥ 0.0) detected by the network is estimated to be 2 and 8 within 25 and 50 km radius from the power plant site.

Description: The European Plate has a 4.5 Gy long and complex tectonic history. This is reflected in the present-day large-scale crustal structures. A new digital Moho depth map is compiled from more than 250 data sets of individual seismic profiles, 3-D models obtained by body and surface waves, receiver function results and maps of seismic and/or gravity data compilations. We have compiled the first digital, high-resolution map of the Moho depth for the whole European Plate, extending from the mid-Atlantic ridge in the west to the Ural Mountains in the east, and from the Mediterranean Sea in the south to the Barents Sea and Spitsbergen in the Arctic in the north. In general, three large domains within the European Plate crust are visible. The oldest Archean and Proterozoic crust has a thickness of 40–60 km, the continental Variscan and Alpine crust has a thickness of 20–40 km, and the youngest oceanic Atlantic crust has a thickness of 10–20 km.

Description: The stations are part of a seismic network in the Helsinki capital area of Finland in 2020. The stations recorded the response to a second stimulation of a ∼ 6 km deep enhanced geothermal system in the Otaniemi district of Espoo that followed on the first larger stimulation in 2018. The second stimulation from 6 May to 24 May 2020 established a geothermal doublet system. The Institute of Seismology, University of Helsinki (ISUH), installed the 70 GIPP-provided geophones in addition to surface broadband sensors, ISUH-owned short-period instruments, and a borehole satellite network deployed by the operating company. The data set consists of raw CUBE-recorder data and converted MSEED data. The data set has been 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 in Finland. The full 2020 network and with it the deployment of the CUBE stations is described in a Seismological Research Letter Data Mine Column by A. Rintamäki et al. (2021).

Description: A seismic network was installed in Helsinki, Finland to monitor the response to an ∼6‐kilometer‐deep geothermal stimulation experiment in 2018. We present initial results of multiple induced earthquake seismogram and ambient wavefield analyses. The used data are from parts of the borehole network deployed by the operating St1 Deep Heat Company, from surface broadband sensors and 100 geophones installed by the Institute of Seismology, University of Helsinki, and from Finnish National Seismic Network stations. Records collected in the urban environment contain many signals associated with anthropogenic activity. This results in time‐ and frequency‐dependent variations of the signal‐to‐noise ratio of earthquake records from a 260‐meter‐deep borehole sensor compared to the combined signals of 24 collocated surface array sensors. Manual relocations of ∼500 events indicate three distinct zones of induced earthquake activity that are consistent with the three clusters of seismicity identified by the company. The fault‐plane solutions of 14 selected ML 0.6–1.8 events indicate a dominant reverse‐faulting style, and the associated SH radiation patterns appear to control the first‐order features of the macroseismic report distribution. Beamforming of earthquake data from six arrays suggests heterogeneous medium properties, in particular between the injection site and two arrays to the west and southwest. Ambient‐noise cross‐correlation functions reconstruct regional surface‐wave propagation and path‐dependent body‐wave propagation. A 1D inversion of the weakly dispersive surface waves reveals average shear‐wave velocities around 3.3km/s below 20 m depth. Consistent features observed in relative velocity change time series and in temporal variations of a proxy for wavefield partitioning likely reflect the medium response to the stimulation. The resolution properties of the obtained data can inform future monitoring strategies and network designs around natural laboratories.