Full-Scale Crustal Interpretation of Kokkola–Kymi (KOKKY) Seismic Profile, Fennoscandian Shield

Tiira, Timo; Janik, Tomasz; Skrzynik, Tymon; Komminaho, Kari; Heinonen, Aku; Veikkolainen, Toni; Väkevä, Sakari; Korja, T.

doi:10.1007/s00024-020-02459-3

Cited by 8 publications

(13 citation statements)

References 32 publications

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“…The Institute of Seismology at the University of Helsinki (ISUH) deployed a temporary network of ∼100 4.5 Hz three‐component geophones—‘Cube’ stations from the Geophysical Instrument Pool Potsdam (GIPP)—between May 7 and August 20, 2018 (Hillers et al., 2020). For this study, we use data from six arrays (Figure 1), three large arrays nominally consisting of 25 stations (SS, EV, and TL) and three arrays of four stations (PM, PK, and RS), that were installed on crystalline bedrock outcrops characteristic of the Fennoscandian Shield (Hillers et al., 2020; Tiira et al., 2020). The array aperture is ∼100 m, and the interstation distance ∼25 m. The geophone orientations follow magnetic compass readings and meet the standards for portable broadband seismometer installations (Wielandt, 2012).…”

Section: Data From the 2018 Espoo/helsinki Geothermal Reservoir Stimu...mentioning

confidence: 99%

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

Taylor

Hillers

Vuorinen

2021

Geophysical Research Letters

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We estimate vertical rotation rates for 204 earthquakes that were induced by the 2018 stimulation of the Espoo/Helsinki geothermal reservoir from wavefield gradients across geophone arrays. The array-derived rotation rates from seismograms recorded at 6-9 km hypocentral distances vary between 10 −9 and 10 −7 rad s −1 , indicating a comparable sensitivity to portable rotational instruments. Using co-located observations of translational and rotational motion, we estimate the local propagation direction and the apparent phase speed of SH waves, and compare these estimates with those obtained by S wave beamforming. Propagation directions generally align with the earthquake back azimuths, but both techniques show deviations indicative of heterogeneous seismic structure. The rotational method facilitates a station-by-station approach that resolves site specific variations that are controlled by the local geology. We measure apparent S wave speeds larger than 5 km s −1 , consistent with steep incidence angles and high propagation velocities in the Fennoscandian Shield. Plain Language Summary Earthquakes generate seismic waves consisting of both translational (back-and-forth) and rotational ground motion. Translational motion is routinely measured by standard seismometers, but the observation of the rotational motion requires relatively expensive and rare instruments. In this study we estimate rotational ground motion caused by earthquakes using groups of translational seismometers. The computation of rotational motion from translational seismometers has been demonstrated before, but the novelty of our study is to use high-quality recordings of earthquakes that were induced by the creation of a geothermal reservoir at 6 km depth in bedrock. We use our measurements of ground rotation to estimate the speed and direction in which the seismic waves are travelling when they reach the seismometers. We find that the direction in which the seismic waves travel usually points back to the earthquake location, but at some seismometers the waves arrive from a different direction. At these locations, it is likely that local geological features are altering the direction of the waves. We expect that our findings will provide access to approaches for determining earthquake characteristics and Earth structure that currently require highly specialized instruments. TAYLOR ET AL.

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Section: Data From the 2018 Espoo/helsinki Geothermal Reservoir Stimu...mentioning

confidence: 99%

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

Taylor

Hillers

Vuorinen

2021

Geophysical Research Letters

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“…This relates to sites where the S wave velocity in the upper 30 m is 2.8 km/s or larger. Shallow bedrock seismic velocities in the Helsinki area fit this criterion (Hillers et al., 2020; Kortström et al., 2018; Tiira et al., 2020), but the v S reduction in the topmost 10 − 30 m (Hillers et al., 2020) imply that at least in the study area an average outcrop must not have necessarily have very hard rock site properties. The resonance frequency of a vertically incident S wave is approximated by f 0 = v S /4 h (e.g., Wegler & Seidl, 1997), which yields compatible relations between the 1–2 km/s v S values in the topmost layer, the inferred layer thickness h , and the frequency range where the strongest site effects are observed.…”

Section: Discussionmentioning

confidence: 90%

Induced Earthquake Source Parameters, Attenuation, and Site Effects From Waveform Envelopes in the Fennoscandian Shield

et al. 2023

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We analyze envelopes of 233 and 22 ML0.0 to ML1.8 earthquakes induced by two geothermal stimulations in the Helsinki, Finland, metropolitan area. We separate source spectra and site terms and determine intrinsic attenuation and the scattering strength of shear waves in the 3–200 Hz frequency range using radiative transfer based synthetic envelopes. Displacement spectra yield scaling relations with a general deviation from self‐similarity, with a stronger albeit more controversial signal from the weaker 2020 stimulation. The 2020 earthquakes also tend to have a smaller local magnitude compared to 2018 earthquakes with the same moment magnitude. We discuss these connections in the context of fluid effects on rupture speed or medium properties. Site terms demonstrate that the spectral amplification relative to two reference borehole sites is not neutral at the other sensors; largest variations are observed at surface stations at frequencies larger than 30 Hz. Intrinsic attenuation is exceptionally low with Qi−1 ${Q}_{\mathrm{i}}^{-1}$ values down to 2.4 × 10−5 at 20 Hz, which allows the observation of a diffuse reflection at the ∼50 km deep Moho. Scattering strength is in the range of globally observed data with Qnormalsnormalc−1 ${Q}_{\mathrm{s}\mathrm{c}}^{-1}$ between 10−3 and 10−4. The application of the employed Qopen analysis program to the 2020 data in a retrospective monitoring mode demonstrates its versatility as a seismicity processing tool. The diverse results have implications for scaling relations, hazard assessment and ground motion modeling, and imaging and monitoring using ballistic and scattered wavefields in the crystalline Fennoscandian Shield environment.

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“…First and foremost, mention should be made of the Fennolora [4,5] and Quartz [6] profiles. In the Fennoscandian Shield, over 50 deep seismic profiles were obtained in order to study the deep lithospheric structure of this major unit of the East European Platform [7][8][9][10][11][12][13][14][15][16][17]. The Earth's crust of the region was shown to display a heterogeneous 'mosaic' structure, with no persistent seismic boundaries across the entire shield.…”

Section: Seismic Heterogeneities Of the Lithosphere 21 Crustal Struct...mentioning

confidence: 99%

Seismic model of the Earth's crust and upper mantle beneath the Fennoscandian Shield

Lebedev

Шаров

2023

E3S Web Conf.

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Well-founded concepts of the crust and upper mantle structure of the Fennoscandian Shield were developed. Quantitative evidence for horizontal and vertical lithospheric heterogeneities was obtained. Highly detailed integrated geological and geophysical investigations, supported by superdeep drilling, have been conducted in the eastern part of the shield in recent years. Seismogeological crustal models of individual geotectonic provinces were constructed, which indicate that the structure of the crystalline crust displays a pattern of blocks, arranged hierarchically, with no persistent seismic boundaries across the entire shield. Both low- and high-velocity zones, correlatable with geological bodies, occur locally. The structural plans of contours and the velocities of various deep sections were shown to be inconsistent. We conclude that the upper mantle is laterally complex and heterogeneous, and that there is no asthenosphere in the classical sense of the word.

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Full-Scale Crustal Interpretation of Kokkola–Kymi (KOKKY) Seismic Profile, Fennoscandian Shield

Cited by 8 publications

References 32 publications

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

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

Induced Earthquake Source Parameters, Attenuation, and Site Effects From Waveform Envelopes in the Fennoscandian Shield

Seismic model of the Earth's crust and upper mantle beneath the Fennoscandian Shield

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