Paleoseismodeformations of hard rocks in the Karelian isthmus

Abstract

Within the limits of the Fennoscandian (Baltic) crystalline shield, late and postglacial shattering of hard rocks due to strong ancient seismic effects has been studied at several sites [1–5]. The approach of these studies implied the search for evidence of a seis� mic origin of dislocations and, in some cases, determi� nation of the dislocation age. These studies have also been carried out by the authors in southern Karelia and northern Leningrad oblast. The present work is devoted to elaboration of criteria and distinguishing of several coeval events, and to parameterization of these events in terms of seismology standards. This commu� nication presents the first, basic part of such a study. The source parameters were defined on the basis of models and calculations from the initial values of dis� placements for each of the distinguished events. The remarkable shattering of hard rocks in the out� crops near Krasnyi Sokol settlement, northern Kare� lian Isthmus (Leningrad oblast, near the town of Kamennogorsk), had been noticed as early as the 19th century by the famous geologist A.A. Inostrantsev. Since then, this site, which is unique in the expressive� ness of fresh ruptured structures, had not been investi� gated. In 2010–2012, it was studied in detail by the authors and was named “Inostrantsev’s cave” [6]. The site is located within the limits of the NE wing of the large Vuoksa fault zone [7] bearing recent signs of acti� vation. Owing to intensive longitudinal and transverse fracturing of the hard rock massif, the inner structure and shattering conditions were able to be studied down to 5–7 m depth. Here, a complex of seismotectonic dislocations with displacements and deformations of various character is developed in the hard rocks; in the framing, seismogravitational structures are found. Importantly, both these types are closely coupled and correlate with each other. The present work will con� sider only the dislocation in the hard rock massif proper. A significant feature is the clear predominance of signs of horizontal displacements in the two mutu� ally perpendicular directions at the nearly horizontal basis of the shattered part of the massif. The Vuoksa fault zone, which is marked by the river system of the same name, is represented by a system of