Bulletin of the Geological Society of Finland, Vol. 90, 2018, pp 257–273, https://doi.org/10.17741/bgsf/90.2.009 The active tectonics of the Vuoksi Fault Zone in the Karelian Isthmus: parameters of paleo- earthquakes estimated from bedrock and soft- sediment deformation features Sergey V. Shvarev1,2*, Andrey A. Nikonov2, Michail V. Rodkin3,4 and Anton V. Poleshchuk5 1 Institute of Geography, Russian Academy of Sciences, Staromonetny lane 29, Moscow, 119017, Russia 2 Schmidt Institute of the Physics of the Earth, Russian Academy of Sciences, Bolshaya Gruzinskaya str., 10-1, Moscow, 123242, Russia 3 Institute of Theory of Earthquake Prediction and Mathematical Geophysics, Profsoyuznaya str. 84/32, Moscow, 117997, Russia 4Institute of Marine Geology and Geophysics Far Eastern Branch, Russian Academy of Science, Nauki street., 1B, Yuzhno-Sakhalinsk, 693022, Russia 5 Geological Institute, Russian Academy of Sciences, Pyzhevsky lane 7, Moscow, 119017, Russia Abstract The area under study is located in the south-eastern periphery of the Fennoscandian crystalline shield. At present this is a tectonically quiet region without large seismic events. But it is well known that in post-glacial time the Fennoscandian shield was an arena of active postglacial tectonics and large earthquakes. The evidence for such events was found in various parts of Fennoscandia. The traces left by some paleo- earthquakes show an undisputed character of large post-glacial faults some tens of kilometres long and of a few meters in displacement. However, some other features left by earthquakes are under discussion. Numerous deformations in bedrock and in soft sediments which can be considered as being due to earthquakes were found in the Russian Karelia. Interpretation of some of these deformation structures can lead to different conclusions about their origin, for example, weathering, cryogenic, glacial, and gravitational factors. One possible way to overcome these difficulties is an integrated study of different types of deformations at key sites, comparison of these with each other and with the tectonic features of the region, and the search for common structural and kinematic features. Another problem is the estimation of parameters of paleo- earthquakes. This problem includes the determinations of their location, intensities, magnitudes, and age. 258 S. V. Shvarev, A. A. Nikonov, M. V. Rodkin, A. V. Poleshchuk The key site under study is located in the northern part of the Karelian Isthmus in the re-activated (during post-glacial time) tectonic zone (the Vuoksi Fault Zone), whose signature in the relief is seen in the form of the straight-line valley of the Vuoksi River. We studied different types of post-glacial seismogenic deformations at this locality. There are seismically induced gravitational and vibrational deformations in solid rock, as well as folds and ruptures in loose sediments. The key site of large deformation examined here includes three zones: 1) the main zone of deformations or the Central Fractured Massif (CFM); 2) the seismically induced colluvial zone; 3) the outer zone of deformations in loose sediments. We have established that all types of deformations are kinematically similar in the CFM and around it (at distances of a few kilometres). A detailed examination of deformations and their spatial and temporal relationships allows us to distinguish three generations of earthquake-induced deformations: 1) Late Glacial, 2) Early Holocene, and 3) Middle to Late Holocene. We estimate the intensities of the respective earthquakes as I=IX, IX, and VII-VIII. Clearly, the intensities decrease from post-glacial to present time, but the recent level of seismicity is unclear and may be much higher than hypothesized. In addition, the evidence for shear kinematics of the fault shows that earthquakes were not only caused by post-glacial rebound, but also resulted from a different tectonic mechanism possibly related to plate tectonics. Keywords: Fennoscandian shield, Holocene, paleo-earthquakes, earthquake-induced rock fragmentation, soft-sediment deformations, seismically induced rock-falls *Corresponding author (e-mail: [email protected]) Editorial handling: Pertti Sarala ([email protected]) 1. Introduction Nikonov & Shvarev, 2015; Nikolaeva et al., 2017; Shvarev & Rodkin, 2018). Some locations of paleo- The late- and post-glacial fragmentation of rocks earthquakes are undisputable large post-glacial and the deformation of glacial, lacustrine and faults tens of kilometres long with displacements alluvial sediments was presumably the cause of of some meters. However, other locations with large earthquakes as described by researchers presumable evidence for earthquakes are debatable. for different localities in the western part of the Another point of view is that these deformations can Fennoscandian crystalline shield (Kujansuu, be interpreted differently as, for example, glacial, 1964; Lundqvist & Lagerbäck, 1976; Bungum & gravitational, cryogenic, or weathering. Lindholm, 1997; Kuivamäki et al., 1998; Mörner, A reliable evidence of the seismic origin of the 1985, 2003; Mörner et al., 2003; Olesen et al., deformations can result from an integrated study of 1992; Sjöberg, 1994; Lagerbäck & Sundh, 2008; different traces left by the earthquakes, comparison Kukkonen et al., 2011). Similar studies were of their systematic features, and detection of carried out for the eastern Fennoscandia, which common tectonic and geologic properties (e.g., consists of Russian Karelia, the Kola Peninsula, structure, kinematics, and texture). and the Karelian Isthmus (Lukashov, 1995, 2004; However, the confirmation of the seismic origin Nikonov & Zykov, 1996; Zykov, 2001; Nikolaeva, is only the first part of the study. The determination 2006; Nikonov, 2008; 2012; Nikonov et al., 2014; of parameters of the paleo-earthquakes is equally The active tectonics of the Vuoksi Fault Zone in the Karelian Isthmus 259 important. These parameters include the location to the space parameters of the large fault zone and orientation of the rupture zones, the direction that is found in the strictly oriented valley of the of the seismic impulse, the number of earthquakes Vuoksi River (the Vuoksi Fault Zone (VFZ)). All and their ages, the intensities and magnitudes of the these characteristics provide evidence that the events, and the tectonic structure that generated the gravitational and soft-sediment deformations studied earthquakes. here were also induced by earthquakes, and that The key site discussed in detail in this paper is the VFZ was responsible for the earthquakes. We located close to the Vuoksi River near the settlement examined an alternative genesis of rock and soft- of Krasny Sokol (the former Sokkola) and the sediment disturbances, such as an influence of town of Kamennogorsk (the former Antrea) in the glacial or glaciofluvial, weathering, cryogenic, and northern part of the Karelian Isthmus (Fig. 1a). The gravitational processes, and came to the conclusion location was noticed in the 19th century by the that there are specific features in all types of famous Russian geologist A. A. Inostrantsev, who deformation that contradict these assumptions, so described it as follows: “…close to the Sokkola that the seismic origin of the deformations seems to village… rocks dissected by cracks, which have be the most reasonable explanation. the forms of galleries… I’ve entered inside them We have examined the relative positions of the easily, because these cracks have been transformed deformations and 14C dates to infer that three large locally to small rooms” (Inostrantsev, 1870, authors’ seismic events of Late Glacial to Holocene time translation from Russian from pages 39–40). We could generate these features. We have estimated studied the area in 2010–2016, and it is referred to the intensities of paleo-earthquakes (using the “ESI- below as the “Inostrantsev Caves” (Nikonov et al., scale” (Michetti et al., 2007)), and the possible pairs 2011, 2014). of values (magnitudes and the hypocenter distance) The breakage in bedrock is concentrated in from our new approach based on evaluations of a granite massif of a few hundred m2. Below, the particle velocities that could cause the observed massif is referred to as the Central Fractured Massif block displacements (Rodkin et al., 2012). (CFM). Intensive breakage and cleavage, displace- ments of rock have occurred there. Characteristic features of the cracks and block displacements 2. A geological and suggests a seismic origin of the deformations. We geomorphologic could determine the directions of seismic impulses and the sequence of events by a careful examination description of the area of the deformations. We also studied the colluvial plume, folds, and faulting in the loose deposits The area of study is situated along the southern around the massif. We have found that gravitational periphery of the Fennoscandian crystalline shield, landforms and the soft-sediment deformations which controls a typical two-stage geological are closely interconnected with each other, and are structure: the lower stage is composed of basement similar in the massif and in the peripheral zone at rocks, and the upper stage is a thin discontinuous a distance of a few hundred meters to a few kilo- cover of last-stage glacial, late- and postglacial metres. The orientations of crevices, the directions deposits with a thickness of less than a few meters of the block displacements, and the movements of in valleys. The area is located in the Lazurnensky boulders