ИСТИНА

ESC2018-S22-686 PROBLEM OF A LEVEL OF RECENT LONG-TERM SEISMICITY OF FENNOSCANDIA Rodkin Mikhail* (1), Shvarev Sergei (1) (1)RUSSIAN ACADEMY OF SCIENCES, Russia * rodkin@mitp.ru A set of data on the present, historical and prehistorical seismic activity of Fennoscandia is considered. An evident disagreement takes place between the low level of instrumental seismicity (in particular, the estimation of the maximum possible magnitude of an earthquake using the methods of the theory of extreme values) and historical and, especially, paleoseismic data. Paleoseismicity appears to be unexpectedly high that is supported by data from both western Fennoscandia (Norway, Sweden, Finland) and its Russian part (Kola Peninsula and Karelia). Within the latter a dozen of active fault zones mainly of NW direction had been delineated. The assumption of a close relationship between strong paleoseismicity and the process of deglaciation seems to be quite natural according to most traces of strong seismic events found in the Early Holocene. But there are also some data about strong earthquakes which had occur much later. Evidences of strong seismogenic impacts were found from the numerous presumably seismogenic displacements of the rock blocks, the tectonic inclination of the Middle Holocene terrace and seismogenic ruptures found in soft deposits in the valley of the river Vuoksa at the Karelian Isthmus and from the soft-sediment deformations of the bottom deposits of similar age of the Lake Imatra at the Kola Peninsula. Similar features were found at some other places. The possible maximum level of a current seismicity is not clear however. It is not clear also, whether the strong regional paleoseismicity is associated with the deglaciation only, or platetectonic factors such as a push from the midAtlantic ridge also has a significant role. Methodologically, the first arising question concerns the reliability of data on paleoseismic dislocations; to what extent they can be caused by other impacts, such as glacial deformations, weathering, kriogenic processes, etc. In cource of verification of data on paleoseismicity, a comparison of results obtained at several keysites of active development of rock deformations in the Russian sector of Fennoscandia with similar observations performed in focal regions of a few recent strong earthquakes was carried out; the comparison testifies for seismogenic nature of majority of the examined dislocations. Thus, the evidence on the high level of paleoseismicity of Fennoscandia looks convincing enough. Note however, that post-glacial seismogenic faults with lengths and amplitudes of discolations similar to those found at the West Fennoscandia were not found yet in the Russian sector of the Fennoscandia. We suggest that this failing may be connected with a location of a majority of linear active tectonic structures at the Eastern part of Fennoscandia in the lowlands with thick postglacial sedimentary cover or under water of large basins such as Barents and White seas, Onega and Ladoga lakes. The signs of Holocene tectonic activity of these structures can be seen, but their full analysis is not performed yet. Thus, a high level of paleoseismic activity appears to be confirmed both for the eastern and western parts of the shield. The second problem is whether this seismicity is almost exclusively associated with the process of deglaciation, or a significant contribution is provided by a push from the MidAtlantic Ridge also. We present arguments “pro” and “contra” both these suggestions but can not yet offer an answer to this question. Having in mind the problem of seismic assessment it seems suitable to note that according to recommendations of the International Atomic Energy Agency the paleoseismic data should be taken into account in a seismic risk assessment in the case of Nuclear Power Plants and other similar objects. We are not sure however that this would be correct for the case of Fennoscandia because the seismic level can decrease here very essentially due to strong decrease in a rate of postglacial deformations. Determination of veritable current level of seismicity could be done only from the finding of mechanism of regional seismicity. The work was supported by the Russian Foundation for Basic Research (project no. 17-05- 00351, and partly in frames of the IG RAS research topic 0148-2014-00016 and of the IPE RAS research topic 0144-2014-0097).