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https://doi.org/10.5194/se-2020-139
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2020-139
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  17 Sep 2020

17 Sep 2020

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This preprint is currently under review for the journal SE.

Seismicity during and after stimulation of a 6.1 km deep Enhanced Geothermal System in Helsinki, Finland

Maria Leonhardt1, Grzegorz Kwiatek1, Patricia Martínez-Garzón1, Marco Bohnhoff1,2, Tero Saarno3, Pekka Heikkinen4, and Georg Dresen1,5 Maria Leonhardt et al.
  • 1Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Section 4.2: Geomechanics and 5Scientific Drilling, Potsdam, Germany
  • 2Department of Earth Sciences, Free University of Berlin, Berlin, Germany
  • 3St1 Deep Heat Oy, Helsinki, Finland
  • 4Department of Geosciences and Geography, University of Helsinki, Helsinki, Finland
  • 5Institute of Earth and Environmental Sciences, University of Potsdam, Potsdam, Germany

Abstract. In this study, we present a high-resolution dataset of seismicity framing the stimulation campaign of a 6.1 km deep Enhanced Geothermal System (EGS) in Helsinki suburban area and discuss the complexity of fracture network development. Within St1 Deep Heat project, 18 160 m3 of water was injected over 49 days in summer 2018. The seismicity was monitored by a seismic network of near-surface borehole sensors framing the EGS site in combination with a multi-level geophone array located at ≥ 2 km depth. We expand the original catalog of Kwiatek et al. (2019) and provide the community with the dataset including detected seismic events and earthquakes that occurred two month after the end of injection, totalling to 61 163 events. We relocated events of the catalog with sufficient number of available phase onsets and moment magnitudes between Mw −0.7 and Mw 1.9 using the double-difference technique and a new velocity model derived from a post-stimulation vertical seismic profiling campaign. The analysis of the fault network development at reservoir depth of 4.5–7 km is one primary focus of this study. To achieve this, we investigate 191 focal mechanisms of the induced seismicity using cross-correlation based technique. Our results indicate that seismicity occurred in three spatially separated clusters centered around the injection well. We observe a spatio-temporal migration of the seismicity during the stimulation starting from the injection well in northwest (NW) – southeast (SE) direction and in northeast (NE) direction towards greater depth. The spatial evolution of the cumulative seismic moment, the distribution of events with Mw ≥ 1 and the fault plane orientations of focal mechanisms indicate an active network of at least three NW–SE to NNW-SSE orientated permeable zones which is interpreted to be responsible for migration of seismic activity away from the injection well. Fault plane solutions of the best-constrained focal mechanisms as well as results for the local stress field orientation indicate a reverse faulting regime and suggest that seismic slip occurred on a sub-parallel network of pre-existing weak fractures favorably oriented with the stress field, striking NNW-SEE with a dip of 45° ENE, parallel to the injection well.

Maria Leonhardt et al.

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Maria Leonhardt et al.

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