Browsing by Subject "seismology"

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  • Kortström, Jari; Uski, Marja; Tiira, Timo (2016)
    This paper presents a fully automatic method for seismic event classification within a sparse regional seismograph network. The method is based on a supervised pattern recognition technique called the Support Vector Machine (SVM). The classification relies on differences in signal energy distribution between natural and artificial seismic sources. We filtered seismic records via 20 narrow band-pass filters and divided them into four phase windows: P, P coda, S, and S coda. We then computed a short-term average (STA) value for each filter channel and phase window. The 80 discrimination parameters served as a training model for the SVM. We calculated station specific SVM models for 19 on-line seismic stations in Finland. The training data set included 918 positive (earthquake) and 3469 negative (non-earthquake) examples. An independent test period determined method and rules for integrating station-specific classification results into network results. Finally, we applied the network classification rules to independent evaluation data comprising 5435 fully automatic event determinations, 5404 of which had been manually identified as explosions or noise, and 31 as earthquakes. The SVM method correctly identified 94% of the non-earthquakes and all but one of the earthquakes. The result implies that the SVM tool can identify and filter out blasts and spurious events from fully automatic event solutions with a high level of accuracy. The tool helps to reduce the work-load and costs of manual seismic analysis by leaving only a small fraction of automatic event determinations, the probable earthquakes, for more detailed seismological analysis. The self-learning approach presented here is flexible and easily adjustable to the requirements of a denser or wider high-frequency network.
  • Quist, Liina-Maija (2019)
    In Tabasco, in the Mexican Gulf of Mexico, many small-scale fishers follow their catch to prohibited offshore areas set aside for the oil industry's extractive activities. They claim that increased seismic studies and oil extraction displace and kill fish, contributing to a reduction in hauls, which acts as an incentive to the fishers to continue accessing traditional fishing grounds in the recently prohibited areas. The author draws on theoretical ideas from de la Cadena and Ingold to examine the fishers' offshore movement and related knowledge claims as `excess', or beyond conventional political discourses, interrogating the multiple and contested meanings that fishers attach to their sea environment, fish and fishing in the context of increased oil extraction operations. The article shows that these meanings are difficult to articulate within a political frame that constitutes the offshore extraction area as a `sacrifice zone'. However, the respective knowledges of fishers and the oil industry about the industry's impacts on marine life rely on patchy evidence, lack systematicity, and are motivated by political interests. The author argues that scientific indeterminacy about the causes of depleting fish populations and the weakness of environmental legislation exclude fishers' knowledge from politics while recognising the oil industry's knowledge as valid.
  • Vuorinen, Tommi Antton Tapani (2017)
    Institute of Seismology, University of Helsinki, provides several online earthquake maps. EQ-Search Tool (http://www.seismo.helsinki.fi/EQs/query.php) is an online map service which allows the user to search for earthquakes within the North European earthquake catalogue (FENCAT). Data in the catalogue is further supplemented with more recent events found by seismic analysts during the daily processing of seismic events. ISUH also provides a global map of significant earthquakes from a Finnish perspective in the past 7 days. The map, LUOVA-järistyskartta (http://www.seismo.helsinki.fi/EQs/jaristyskartta.php), is based on LUOVAsystem, which is an official natural hazard alert program in Finland. Currently this map is available only in Finnish.
  • Rinne, Lauri (Helsingin yliopisto, 2021)
    Kappa-parameter (κ) is used to estimate the decay of seismic spectral amplitudes with frequency and is the sum of regional kappa (κr) and site-specific kappa (κ0). The site-specific kappa (κ0) parameter in Olkiluoto (Southwestern Finland) is generally small, approximately 0.002 to 0.004. These values, although smaller, are in the same range that have been found in Eastern North America, where kappa is around 0.006. In Western North America kappa is around 0.04. In Europe, e.g., in alpine region, kappa value is around 0.025. The kappa-value was studied by analysing microearthquake recordings gathered by Posiva Oy’s seismic monitoring network from 2016 to 2019. From these microearthquakes 51 microearthquakes were selected and used in the analysis. All these microearthquakes occurred relatively close to the monitoring stations, from tens of meters to few hundred meters. Each of the events were detected by multiple sensors and the total number of microearthquake registrations used in this study was 297. From these recordings the κ0 was calculated for each component (two horizontal and one vertical). Total number of calculated κ0 values was 473. The kappa-method used was the original introduced by Anderson and Hough in 1984. Besides using earthquake data, the site-specific kappa was also calculated from excavation blasts in Olkiluoto for comparison. Blasting related kappa was smaller than the one calculated from microearthquakes, with average values between 0.0012 and 0.0017. The number of blasts used to calculate κ0 was quite small and the results may not be statistically relevant. Results are in line with similar study areas around the world – harder rock has lower κ0 values
  • Taylor, George; Hillers, Gregor; Vuorinen, Tommi (2021)
    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.