Paleoearthquakes of Russian Karelia

The Russian Karelia is a region that stretches along the Eastern periphery of the Fennoscandian crystal shield in the transition zone to the Russian plate. The main features of the topography are due to exposure of basement rocks that were subjected to processes of mainly subaerial denudation throughout almost the entire Phanerozoic. General irregularities of the relief are connected to the geologic structure and associated with an arc-shaped stage along the edge of the shield (“Polkanov flexure”) on which radially superimposed Riphean grabens, rejuvenated in the process of neotectonic activation and remain active until now. Structural and tectonic features of the terrain were the basis for an exogenous transformation of the terrain as a result of Pleistocene glaciation and post-glacial development of the territory: the distribution of zones of glacial denudation and accumulation, the spread of marine and lake basins, the formation and reorganization of the river network. Periodical changes of glacial and interglacial epochs determined vertical fluctuations of the surface under glacial load and after its removal with an amplitude of several hundred meters. The vertical movement vector is supplemented by the horizontal one, probably under the influence of spreading processes in the Atlantic Ocean floor. Reaching the surface of a rigid basement under conditions of vertical and horizontal stresses caused the reflection of the block-and-break structure in the topography and differentiated displacements along the faults, which were manifested in the form of strong earthquakes. Evidence of strong earthquakes are observed throughout the territory of Fennoscandia, although the problem of post-glacial seismicity, its spatial and temporal parameters and causes remains debatable. Numerous paleoseismic manifestations were found in the relief and underlying substrate-rock and loose deposits of the Russian Karelia: cracks, ruptures with vertical and horizontal displacements, rockfalls and landslides, lateral displacements of rock blocks, textures of liquefaction, injection dykes, traces of fluidization, mud-, and water-stone flows. The age of the discovered paleoseismic deformations varies mainly from the Late Glaciation to the Late Holocene, although there are traces of earlier earthquakes. The impact of earthquakes on the development of nature in the Late Neopleistocene and Holocene is underestimated: strong earthquakes could trigger significant transformations of the hydrographic network and landscapes.