FENNOSCANDIAN SEISMICITY AND ITS RELATION TO THE ISOSTATIC REBOUND

Beside plate tectonics, isostatic rebound may be a main contributor in the seismogenic process in Fennoscandia. Extensional horizontal strai n, presumably related to land uplift, calculated from mathematical mod eling, geodetic data and curvature of uplift, show higher values than compressional horizontal strain, related to ridge push, estimated from sedimentary deformation in the basins surrounding the shield. The loc ation of the current seismicity of central and northern Fennoscandia a nd the sites of large boulder caves near the Bothnian coast of central Sweden, show high correlation with the maximum curvature of uplift at present and late-glacial time, respectively; the caves may have been created by large earthquakes at the last phase of deglaciation. Large faults in northern Fennoscandia probably also have seismogenic origin dating to the late-glacial period. Differential strain along the Swedi sh coast of the Gulf of Bothnia, caused by a larger rate of uplift und er the Gulf than of the adjacent land, is another seismogenic factor r elated to rebound. Earthquake focal mechanism solutions show a variety of faulting styles and stress orientations. Clearly, not all of them can be accounted for by ridge push. The proportion of small compared t o large earthquakes (b value) is larger in northern than in southern F ennoscandia. The maximum b is in the northern Gulf of Bothnia close to where the maximum rate of uplift is. The implication may be that tect onics and uplift are counteracting forces in the north, preventing lar ge stress accumulation, whereas tectonics act more undisturbed in the south, resulting in larger stress build-up and thus relatively more la rge earthquakes. The temporal correlation between the seismicities of the Fennoscandian and the eastern Canadian shields on one hand and seg ments of the North Atlantic Ridge on the other, gives support to the t he idea of stress propagation under the Atlantic.