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Summary:
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The study deals with the seismicity of the Olkiluoto area in relation to the general seismicity of the Fennoscandian Shield and the region of the southwestern Finland. The study covers an area within 500 km from Olkiluoto. The area includes altogether over 1100 observations of earthquakes since 1375. About 1/2 of those has been observed by seismic instruments since the years 1965.
The distribution of the seismicity in the southwestern Finland and in the Bothnian Sea area is presented in geological and tectonic maps. The study shows that the Åland-Paldis-Pskov seismic belt has an important role when the seismicity in southern Finland and in Estonia is concerned. All significant earthquakes (M ³ 3.5) in the region of the Gulf of Finland and in the Ålands archipelago are related to this zone.
Two seismically active shear zones are crossing the Bothnian Sea. The Arlanda shear zone in the northern part of the Bothnian Sea runs in the Pori area about 35 km away from Olkiluoto. The Haseela shear zone is part of the Åland-Paldis-Pskov seismic belt. At the closest point this shear zone is about 70 km SW of Olkiluoto.
Within a radius of 100 km from Eurajoki there has been altogether 9 earthquakes. The nearest event (M = 3.1)occurred 1926 in Uusikaupunki, 35 - 40 km from Olkiluoto, as a part of the active period in the Saaristomeri-Paldis-Pskov Zone. Another event about 40 km from Olkiluoto occurred in Laitila in 2007 (ML=1.9). The earthquakes of the area are small (M £ 3.1) and their recurrent periods are long. This means that the potential impacts on the ONKALO due to seismicity are likely indirect changes in the surrounding bedrock structure, stress field and groundwater conditions.
In addition to the seismic zoning, the aim of the seismotectonic interpretation was to identify individual active shear or thrust zones. Two nearest active zones from Olkiluoto seem to be NW-SE oriented. The southern one of those runs along the contact of the Laitila rapakivi massif, about 15 km SW from Olkiluoto. The other one runs along the edge of the Satakunta sandstone formation, about 35 km NE from Olkiluoto. These potentially active zones are possibly related to 1 - 2 small (M = 2.7 - 3.1) earthquakes. The 2007 Laitila reverse faulting and few other earthquakes can be associated with mafic dykes in the area. Otherwise, the earthquakes tend to occur on old zones of crustal weakness reactivated by present stress field.
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Keywords:
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Seismicity, tectonics, Olkiluoto, geology, stress field, geodesy
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