The 2018 Geothermal Reservoir Stimulation in Espoo/Helsinki, Southern Finland: Seismic Network Anatomy and Data Features

Abstract

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 3.3 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.

Using Array‐Derived Rotational Motion to Obtain Local Wave Propagation Properties From Earthquakes Induced by the 2018 Geothermal Stimulation in Finland

“…Our tests show that results from different choices of wave type, component, or frequency band can vary due to the sensitivity to different propagation effects. However, our observations generally agree with the vertical component P wave beamforming results obtained by Hillers et al (2020) using data from 6 M ≥ 1.1 events, which supports our comparison of the ADR and beamforming methods applied to the same S waveforms.…”

“…A total of ∼2900 induced seismic events were automatically detected by ISUH routine analysis. A subset of the 204 largest events was selected for magnitude and location estimation (Hillers et al, 2020; Supporting Information S1). We apply the method outlined in Section 2 to these events, which have a local magnitude range between M0.0 and M1.8 and source-receiver distances of 6-9 km.…”

“…We compare the directions and velocities estimated by ADR analysis with those calculated from conventional beamforming, as well as the great‐circle direction indicated by the earthquake locations of Hillers et al. (2020). For the beamforming results, we process the North and East component waveforms exactly as for the rotational analysis, and apply time‐domain delay‐and‐sum beamforming to a 0.5 s long window centered on the manually picked S wave arrival.…”

Seismicity during and after stimulation of a 6.1 km deep enhanced geothermal system in Helsinki, Finland

“…Fluid injection at depth in EGS stimulations and in wastewater disposal is commonly associated with induced seismicity (e.g., Ellsworth, 2013;Majer et al, 2012). Suc-cessful mitigation of an induced seismic hazard is important for public acceptance of geothermal projects, as there is significant concern related to the occurrence of larger induced earthquakes during previous EGS projects, e.g., in Basel and St. Gallen, Switzerland (e.g., Diehl et al, 2017), and most recently in Pohang, South Korea (Hofmann et al, 2019;Ellsworth et al, 2019).…”

A Novel Multidisciplinary Approach for the Thermo‐Rheological Study of Volcanic Areas: The Case Study of Long Valley Caldera

“…Background natural seismicity is a common feature of geothermal systems (Hillers et al., 2020). Many studies revealed how the earthquakes occurring within a geothermal environment are tightly linked to fluid diffusion processes and their principal driving mechanism are transient fluid pressure changes in fractured rock volumes (Buizje et al., 2019; D'Auria et al., 2011; Hainzl et al., 2012; Norbeck & Shelly, 2018; Shapiro & Dinske, 2009; Tosha et al., 1998).…”