Results of Detailed Paleoseismic Studies of the Kindo Peninsula (Karelian Coast of the White Sea)

Abstract

A standard complex of geomorphologic methods, including identification of aerial photos and space images, topographic and structural–geomorphologic area survey, trenching colluvial sediments and their mapping, and sampling of paleosoils and their dating by the radiocarbon method, was used for identification, parametrization, and dating of seismic dislocations of the Karelian coast of the White Sea. A set of kinematic indicators of paleoearthquakes (mass displacements and systematic rotations of fragments of rock ledges), which make it possible to interpret the directions of maximum seismic impact on detailed areas, is elaborated and tested. In the relief of the rock massifs of the Kindo Peninsula, these methods revealed a halo (10 × 6 km) of secondary seismic dislocations with a radiocarbon age of no more than 5.5 ka, which is a zone (4 × 2 km) of extension fractures and numerous displacements of stones surrounded by a belt of seismic gravitation faults. It is shown that some ledges and stepwise faults in the relief of these stones probably resulted from glacial denudation and further erosion of structural heterogeneities. At the same time, displacements of chipped stones versus inclination and their systematic rotation in rock ledges of different strike suggest intense seismic impacts after the formation of the stepwise surfaces and termination of their abrasion after glacial rebound of the territory. It is found that high-frequency seismic oscillations with high values of peak accelerations (0.4–0.8 g) and velocities (100–300 cm/s) are necessary for the formation of stone displacements. Kinematic indicators are used to reconstruct the directions of maximum seismic impact and determine the position of the epicenter of a paleoearthquake at several points. The zones of intensity of 7 and 8 are contoured to estimate the depth of the focus (H = 1.9 ± 0.2 km) and magnitude (M = 4.4 ± 0.2) of a seismic event using the macroseismic field equation. Typical WNW elongation of the first isoseist along the northern coast of the Kindo Peninsula is indicative of a seismogenic fault at the southern end of a micrograben of the Velikaya Salma Strait, which feathers the southeastern wall of the Kandalaksha Graben. The Holocene activity of this fault is confirmed by normal fault displacements of young sediments, which have been revealed in a series of transverse seismoacoustic profiles. These results quantitatively showed for the first time that the zone of the Kandalaksha Graben could provide conditions for low-magnitude "shallow-focused" earthquakes with high seismic intensity.