The Fennoscandian earthquake catalogue (FENCAT) assembles data on the natural seismicity in Fennoscandia, Northern Europe. We present an updated and standardized version of the catalogue originally published in the early 1990s. New instrumental data are recorded by the seismic networks of Denmark, Estonia, Finland, Norway and Sweden, and analysed by the Geological Survey of Denmark and Greenland, the Geological Survey of Estonia, the University of Helsinki in Finland, the University of Bergen and the NORSAR research foundation in Norway and Uppsala University in Sweden. The updated catalogue provides the available earthquake parameters in a brief, user-friendly version: origin time, source coordinates, focal depth, macroseismic data (maximum intensity and radius of the area of perceptibility), up to three observed magnitudes, seismic moment estimate and a standardized moment-related magnitude, mW(HEL), for each event. The standardized magnitude is defined in this paper and its relation to other magnitude scales is provided. Suspected non-earthquakes (e.g. frost events, explosions, human-induced events) have been removed. The standardized event magnitudes range from mW(HEL) -1.0 to 6.2. To enable the usage of earthquake data in a large variety of seismological, geological and earthquake engineering investigations, the data are not truncated at the low-magnitude end.The updated catalogue, FENCAT (2021), contains about 23 000 earthquakes for the period 1467-2021 in an area bounded by 54-75 degrees N latitudes and 0-45 degrees E longitudes. The completeness and quality of the earthquake solutions is best within the areal coverage of the above-mentioned networks.

We present the deployment of a seismic network in the Helsinki capital area of Finland that was installed to monitor the response to the second stimulation phase of an similar to 6-kilometer-deep enhanced geothermal system in 2020. The network consists of a dozen permanent broadband stations and more than 100, predominantly short-period, temporary stations. This 2020 deployment is characterized by a mix of single stations and arrays with diverse configurations. It covers a larger area and exhibits a smaller azimuthal gap compared with the network that monitored the first stimulation in 2018. We surveyed the outcropping rocks at one of the large array sites to study surface expressions of shear or weakness zones that are possibly connected to the stimulated volume at depth. We link the relatively large number of macroseismic reports received during the stimulation to an increased public awareness of the project together with an increased sensitivity because the second stimulation occurred during the local COVID-19 mobility restrictions. The spatial distribution of the reports seems to be controlled by the radiation pattern of the induced earthquakes and hence by the stress state in the reservoir. The continuous records contain strong energy at high frequencies above 50 Hz that is attributed to anthropogenic processes in the densely populated urban area. However, the exceptionally low attenuation of the bedrock yields good signal-to-noise ratio seismograms of the induced small events, the largest of which was magnitude M-L 1.2. The signal quality of the obtained noise correlation functions is similarly very good. 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 scientific studies should inform the legislation and educate the public for transparent decision making around geothermal power generation.

Four Estonian earthquakes were studied, 1.2-2.0 in magnitude and taking place in 2016-2018. Their focal parameters were determined, providing an opportunity to place them in the context of the regional stress field and make tentative geological interpretation of their connection to faults in the Precambrian basement, which is overlain by a 100-600 m thick sequence of sedimentary bedrock. Such research was made feasible by augmenting the permanent three-station Estonian seismic network with seven temporary stations. The analyzed earthquakes were compared with the largest known Estonian earthquake, the 1976 magnitude 4.5 Osmussaar earthquake. The results indicate that it is possible to consider all the five events as expressions of predominantly left-lateral strikeslip movement on north-northwest-south-southeast subvertical faults in concert with the general stress field of northern Europe, which is dominated by plate movement.

This chapter investigates the Fennoscandian uplift area since the latest Ice Age and addresses the question if glacial isostatic adjustment may influence current seismicity. The region is in an intraplate area, with stresses caused by the lithospheric relative plate motions. Discussions on whether uplift and plate tectonics are the only causes of stress have been going on for many years in the scientific community.

Kouvola area, a part of the Vyborg rapakivi batholith in southeastern Finland, has been experiencing an intraplate earthquake swarm since December 2011. The events have magnitudes ranging from ML -1.2 to 2.8 and they happen in the uppermost two kilometers of the crust. The Vyborg batholith has a long history of earthquake swarms with macroseismic data from 1751 onwards and the first instrumentally recorded swarm in 2003-2004. Inspired by the ongoing activity, Institute of Seismology of University of Helsinki (ISUH) has installed temporary seismic stations in the area to complement seismic stations of the Finnish National Seismic network (FNSN). The detection threshold of FNSN is ML1.0, not sufficiently low to catch the smallest earthquakes of the swarm. Several tailored cross-correlators have been developed at the ISUH to lower the event detection threshold. These can be used to detect even very small seismic events well below the current FNSN detection threshold. The method is especially well suited to swarm events, which generate nearly identical signals due to their common origin. Only the largest events of the swarm can be used to calculate focal mechanisms or other event parameters reliably. One approach to use all data is waveform clustering. Event groups with identical signal can be formed, allowing e.g. calculation of composite focal mechanisms for each event cluster.

A seismic network was installed in Helsinki, Finland to monitor the response to an similar to 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 similar to 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 3.3 km/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.

Jari Kortström, Marja Uski and Kati Oinonen report on the Finnish National Seismic Network for the Summary of the Bulletin of the International Seismological Centre.

The Fennoscandian Shield is situated in a seismically quiet intraplate setting in northern Europe. Intraplate seismicity has been attributed to ridge-push from the North Mid-Atlantic Ridge, post-glacial rebound stresses and to local gravitational potential energy differences associated with compositional differences and crustal thickness variations. An up-to-date estimate of the intraplate seismicity in the Central part of the Fennoscandian Shield and its sources is needed in seismic hazard estimates of nuclear power plant sites and thus a seismotectonic study has been undertaken.