In a paper published online in Seismological Research Letters on 5 February 2020 a team led by Seismologists from the Institute of Seismology at the University of Helsinki discusses observations obtained from a large network of seismic stations that was deployed during 2018 around the geothermal reservoir stimulation below the Aalto University campus in Otaniemi.
In June and July 2018 the operating St1 Deep Heat Oy company injected about 18,000 cubic meter of water at 6 km depth which caused many thousands of small earthquakes. The earthquake fractures reaching at most the meter scale underpin the concept of the Enhanced Geothermal System because they enable an efficient water circulation and thus heat exchange.
Map of seismic stations around the drill site in Otaniemi. Red, blue, and black symbols indicate different types of stations. The six insets show the arrangements of seismic antennas or arrays. The inset to the right shows in color the distribution of calculated ground shaking associated with the largest magnitude 1.8 earthquake. The dots show locations of reports from residents. The white ellipse indicates Munkkivuori and the surrounding neighbourhoods. Source: Hillers et al., 2020.
The vast majority of the earthquakes went unnoticed. Even so, a key result of the data analysis shows why residents living to the north-east of the drill site, in Munkkivuori and the surrounding neighbourhoods, most commonly reported ground shaking and thunder- or blast-like noise during the stimulation period.
This uneven spatial distribution of the disturbances is controlled by the so-called source mechanism-the orientation of the activated fracture and the direction of the earthquake slip motion-of the largest earthquakes, which causes some areas to be more affected than others.
The data from more than 100 seismic stations, including borehole sensors operated by St1 Deep Heat Oy, allow a comprehensive analysis of the induced seismic signals, and give researchers in other geothermal stimulation experiments valuable insight for optimizing observation networks.
A second borehole is currently being drilled, and a smaller stimulation is planned for this spring to complete the circulation system. The plant is intended to operate for a few decades. Small earthquakes are expected during the forthcoming injection, and may also occur during later operating stages.
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