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 ∼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 ML 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.
<p align="justify">The company ST1 Oy planned to construct an Enhanced Geothermal System (EGS) with two boreholes drilled down to ca. 6 km depth beneath the Aalto University campus in Otaniemi, Espoo, on the border of Helsinki. The company performed two stimulations, in June&#8211;July 2018 and in May 2020, with a goal of opening up a water reservoir and achieving water circulation between the boreholes. The stimulation periods, which induced thousands of earthquakes, and their immediate surroundings were monitored by both permanent and temporary seismic networks with over 100 stations located within a few tens of kilometers of the site. Between and after the stimulations, the site was and is still being monitored by a relatively dense, consisting in total of ca. 20 stations, regional surface station and company installed borehole station networks.</p> <p align="justify">We have developed a cross-correlation based event detector which uses the existing ISUH manually analysed catalogues primarily of the 2018 and 2020 stimulation period seismicity, complemented by automatically picked catalogues from IMS, to detect events from the collected continuous waveform database. The 4-step detector &#8211; templating, detecting, event filtering & relocating &#8211; can run on varying station configuration and is able to detect events down to ML -0.5 &#8211; -1.0 with the station-event geometry around the EGS. We present here the results of the detector run from the beginning of the dense stimulation monitoring in May 2018 to the end of 2022 providing a comprehensive anatomy of the EGS induced seismicity.</p>
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.
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