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ESTIMATION OF SEISMIC HAZARD IN TERRITORY OF SOUTHERN FINLAND

Authors:
  • PVA Engineering Services

Abstract

This paper decribes the work done during 1998 and 1999 for characteristics of extreme seismic ground motion in southern Finland. The investigated window is bounded by cities of Helsinki and Tampere in north-south axis and cities of Turku and Lappeenranta in east-west axis. The aim of the study was to establish the spectral shapes and magnitudes of extreme ground motion accelerations in the investigated area. The probabilistic target for the motions to be determined was 100 000 year return period in median. The task included also the development of the maps for spectral accelerations and peak ground accelerations for the investigated window. The recorded history of earthquakes dates back to the late 15 th century in nordic countries which inthis case include Finland, Schweden, Norway, Baltic countries and Carelian Republic in Russian Federation.. Instrumental recording started in 1956 in Finnish territory. During the instrumental period there have been on an average five events yearly, the magnitudes of which are typically less than three. A magnitude of 4.9 relates to the largest earthquake in the vicinity of Finland that occurred off the Eastonian coast about 120 km from Helsinki in 1976. On the whole, ten Finnish earthquakes with greater magnitude than 4.5 are known. None of these had an epicentre in Southern Finland. It can be said that Finland is one of the most quiet seismic regions in the world. No recorded strong motion accelorogrms are available in Finland. An attenuation of Fennoscandian earthquakes is based on historical intensity observations. There exist no local data to estimate the relationship between intensity and ground acceleration. A local seismic monitoring network consisting of three strong motion accelerograms was installed in the area Loviisa nuclear power plant in 1985. It operated five years before the monitoring was discontinued because of lack of financing. Observed events have been very small, the magnitude of the largest one has been 1.2, and it is impossible to derive the relationship between magnitude and ground acceleration by using the measured values. So far estimations of ground acceleration are based on the results presented in literature. Uncertainties in the various assumptions critical to the analysis of the ground motion characteristics will be taken into account in form of the decision tree. The dependence of ground acceleration on yearly exceedance probability will be presented by a a distribution. The given results are defined as median values for a return period of 100 000 years.
... Further work concentrated on carrying out hazard calculations with focus on deaggregation calculations Saari, 2014a, 2014b). The zoning (Figure 1) was identical to the one used in 2009 (Saari et al., 2009) and the attenuation relationship has been mostly based on the Saguenay and Newcastle earthquakes and used in Finland since 2000 (Varpasuo et al., 2000). The use of the 2013 version (Tiira and Heikkinen, 2014) of the attenuation equation based on earthquake data from the Fennoscandian shield was judged to be premature because work continued in that area and an updated version is expected in 2015. ...
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The paper summarises the main outcomes of the project carried out in the period 2011-2014. It is a continuation of the material published in the Interim Report of the SAFIR 2014 program (L. Fülöp et al., 2013), with a general shift from hazard prediction towards focus on qualification of systems, structures and components (SSC’s). This shift resulted from the progress of the project. Still, some seismology related conclusions are refined compared to the previously published data, and this paper should be considered the definitive conclusion of the project. The paper briefly presents topic areas studied in the project: (1) preliminary description of high quality near-field data recorded in 2012 from the Kouvola earthquake swarm, (2) a ground motion prediction equation (GMPE) proposal for mid magnitude earthquakes and ecommendation of GMPE’s to be used for high magnitude events, (3) results of a deaggregation study focused to north of Finland, (4) results of sensitivity modelling for a generic reactor building subjected to earthquake loads expected in Finland and (5) challenges concerning equipment qualification. Conclusions are given briefly after each topic, but the reader is directed to the project reports for in-depth understanding.
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