We show that near–real-time seismic monitoring of fluid injection allowed control of induced earthquakes during the stimulation of a 6.1-km-deep geothermal well near Helsinki, Finland. A total of 18,160 m3of fresh water was pumped into crystalline rocks over 49 days in June to July 2018. Seismic monitoring was performed with a 24-station borehole seismometer network. Using near–real-time information on induced-earthquake rates, locations, magnitudes, and evolution of seismic and hydraulic energy, pumping was either stopped or varied—in the latter case, between well-head pressures of 60 and 90 MPa and flow rates of 400 and 800 liters/min. This procedure avoided the nucleation of a project-stopping magnitudeMW2.0 induced earthquake, a limit set by local authorities. Our results suggest a possible physics-based approach to controlling stimulation-induced seismicity in geothermal projects.

[1] Regional seismic tomography provides valuable information on the structure of shields, thereby gaining insight to the formation and stabilization of old continents. Fennoscandia (known as the Baltic Shield for its exposed part) is a composite shield for which the last recorded tectonic event is the intrusion of the Rapakivi granitoids around 1.6 Ga. A seismic experiment carried out as part of the European project Svecofennian-Karelia-Lapland-Kola (SVEKALAPKO) was designed to study the upper mantle of the Finnish part of the Baltic Shield, especially the boundary between Archean and Proterozoic domains. We invert the fundamental mode Rayleigh waves to obtain a three-dimensional shear wave velocity model using a ray-based method accounting for the curvature of wave fronts. The experiment geometry allows an evaluation of lateral variations in velocities down to 150 km depth. The obtained model exhibits variations of up to ±3% in S wave velocities. As the thermal variations beneath Finland are very small, these lateral variations must be caused by different rock compositions. The lithospheres beneath the Archean and Proterozoic domains are not noticeably different in the S wave velocity maps. A classification of the velocity profiles with depth yields four main families and five intermediate regions that can be correlated with surface features. The comparison of these profiles with composition-based shear wave velocities implies both lateral and vertical variations of the mineralogy.

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Accretionary processes contributed to major continental growth in Fennoscandia during the Palaeoproterozoic, mainly from 2.1 to 1.8 Ga. The composite Svecofennian orogen covers c. 1×106 km2 and comprises the Lapland–Savo, Fennia, Svecobaltic and Nordic orogens. It is a collage of 2.1–2.0 Ga microcontinents and 2.02–1.82 Ga island arcs attached to the Archaean Karelian craton between 1.92 and 1.79 Ga. Andean-type vertical magmatic additions, especially at c. 1.89 and c. 1.8 Ga, were also important in the continental growth. The Palaeoproterozoic crust is the end product of accretionary growth, continental collision and orogenic collapse. Preserved accretional sections are found in areas where docking of rigid blocks has prevented further shortening. The Pirkanmaa belt represents a composite accretionary prism, and other preserved palaeosubduction zones are identified in the Gulf of Bothnia and the Baltic Sea areas. In the southern segment of the Lapland–Savo orogen collision between the Archaean continent (lower plate) and the Palaeoproterozoic arc–microcontinent assembly (upper plate) produced a special type of lateral crustal growth: the Archaean continental edge decoupled from its mantle during initial collision and overrode the arc and its mantle during continued collision.

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 the Helsinki suburban area and discuss the complexity of fracture network development. Within the St1 Deep Heat project, 18 160 m3 of water was injected over 49 d 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 of depth. We expand the original catalog of Kwiatek et al. (2019), including detected seismic events and earthquakes that occurred 2 months after the end of injection, totaling 61 163 events. We relocated events of the catalog with moment magnitudes between Mw −0.5 and Mw 1.9 using the double-difference technique and a new velocity model derived from a post-stimulation vertical seismic profiling (VSP) campaign. The analysis of the fault network development at a 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 a cross-correlation-based technique. Our results indicate that seismicity occurred in three spatially separated clusters centered around the injection well. We observe a spatiotemporal migration of the seismicity during the stimulation starting from the injection well in the northwest–southeast (NW–SE) direction and in the 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-oriented permeable zones, which is interpreted to be responsible for the migration of seismic activity away from the injection well. Fault plane solutions of the best-constrained focal mechanisms and 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.

[1] We present an analysis of the crust and upper mantle in the northern Fennoscandian shield, based on new P and S wave 2-D velocity models of the POLAR and HUKKA wide-angle reflection and refraction profiles and results of a new seismic reflection experiment in Finland (Finnish Reflection Experiment). The profiles are almost collocated and crossed the Proterozoic Lapland-Kola orogen. A substantial difference of the depth of ''the wide-angle Moho'' (40-42 km) and ''the reflection Moho'' (47-50 km) was found. In order to explain this difference, we compared the velocity models to published values of Vp and Vp/Vs for the main types of lower crustal and mantle rocks. We found that the main reason for disagreement is that the wide-angle Moho and the reflection Moho correspond to different petrological boundaries. In the southwest and northeast portions of the profiles, the wide-angle Moho marks contact of either anorthositic or granulitic lower crust with a reflective layer in the upper mantle composed of peridotites and pyroxenites. The reflection Moho represents the bottom of this lower layer. In the center of the profile the wide-angle Moho marks the top of a large eclogitic body in the upper mantle, representing a well-preserved crustal root beneath the Lapland-Kola orogen formed because of the collision of three former Archaean crustal blocks (terranes or microcontinents). Lack of postorogenic tectonic collapse suggests another mechanism for stabilization of the lithosphere in the area. Upper mantle reflectors at depth of 65-75 km may mark the upper boundary of the cold and mechanically strong Archaean upper mantle wedge. Alternatively, these reflectors may represent a top of uplifted asthenosphere that can explain preservation of crustal root.Citation: Janik, T., E. Kozlovskaya, P. Heikkinen, J. Yliniemi, and H. Silvennoinen (2009), Evidence for preservation of crustal root beneath the Proterozoic Lapland-Kola orogen (northern Fennoscandian shield) derived from P and S wave velocity models of POLAR and HUKKA wide-angle reflection and refraction profiles and FIRE4 reflection transect,