Revisiting the Origin of Seismicity in Fennoscandia

A review of the literature suggests that the seismic process in Fennoscandia (the Baltic Shield) is affected by at least four mechanisms: (1) northwest-to-southeast movement of the lithospheric plate under the Norwegian and Barents seas by spreading of the Mid-Atlantic Ridge from Iceland to Spitsbergen; (2) postglacial isostatic uplift; (3) local recent neotectonic movements; and (4) gravitational bending deformations on continental contact with the sea shelf along the Norwegian coast due to strong erosion from the rising crystalline domain of the Baltic Shield. The current seismicity of Fennoscandia is relatively low. The strongest earthquake in this area over the last 1000 years was the earthquake of 1627 which had a magnitude of M ≈ 6.5 and occurred in the Kandalaksha graben in the White Sea. However, Fennoscandia, including the Kola Peninsula and eastern Karelia, has a reliable history of a significant number of Pleistocene and even Holocene paleoseismic dislocations, whose parameters allow them to be associated with strong earthquakes which occurred at that time with magnitudes of 7–8 and even higher. It is likely that these paleo-events occurred at the last stage of the glacial age (9000–10 000 years ago) during the intense postglacial isostatic uplift of the Fennoscandia domain. Their possible recurrence can be estimated as tens of thousands of years from the time interval between consecutive glaciations. One should therefore recognize that the nature of current seismicity of Fennoscandia is determined by tectonic stresses caused by both the global effect of the northwestern uplifting lithospheric plate under the Norwegian Sea (a constant source of tectonic stress accumulation) and local tectonic uplifts (the north coast of Norway) or lowerings (the Swedish coast of the Gulf of Bothnia), rather than by postglacial stresses. In addition, the increased seismicity of southwestern Norway and the adjacent North Sea shelf is most likely caused by the formation of crest-like structures under the action of tensile stresses revealed here.