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On September 27, 2003, the destructive Chuya (Altai) earthquake (magnitude 7.3), the strongest earthquake among those recorded in 1991–2022 in continental Russia, occurred near the southwestern border of the Chuya and Kurai basins in southeastern Altai. We have analyzed the seismic activity of this area and compiled the scheme of neotectonic stresses of the Chuya–Kurai Basin and adjacent structures. The modern and neotectonic stresses of southeastern Altai have been compared. The neotectonic stresses are retrieved using the structural-geomorphological method of shear stress reconstruction. It has been found that the orientation of the subhorizontal compression axis of the first rank changes from meridional to NNE and NE. The earthquake occurred in the region within which the largest faults of rank I (Kurai and Shapshal), SSW part of the prerecent Charysh–Terekta fault (we combine it at the neotectonic stage with the fault bounding the Chagan–Uzun block from the southeast in a common regional fault of rank I) were activated at the neotectonic stage in an setting of additional compression. A larger-scale stress field has been also reconstructed for the Chagan–Uzun rigid block that delineates the Chuya and Kurai basins. The neotectonic fault of the NW strike bounding the Chuya and Kurai basins from the southwest has been identified. The fault, we notionally called Bezymyannyi, is divided by smaller faults of SE orientation into three segments and is included in the large South Chuya fault zone. According to the data on strong earthquake focal mechanisms in the investigated region, a stable subhorizontal position of the compression axis has been established (as well as at the neotectonic stage). It is accepted that strong earthquakes occur in a compression setting. The mechanisms of earthquakes (mostly, aftershocks) related to the Chuya event indicate the concentration of thrust mechanisms within the Chagan–Uzun elevated block. This may be due to the ongoing uplift of the Chagan–Uzun block at the current stage. Along the Bezymyannyi fault, the earthquake focal mechanisms are mostly shear-type. This is the result of formation of a shear field under the shear displacement along the fault during the earthquake. Major seismic events occur mainly in a compression setting. Thus, the Chuya earthquake is caused by a subhorizontal compression setting (both regional and local), which is determined by comparing the neotectonic and modern stresses. [ABSTRACT FROM AUTHOR]

A set of tectonophysical methods has been applied to reconstruct the neotectonic stresses of the southeastern Altai Mountains within the Chuya–Kurai depression and its framing structures. It is suggested that at the neotectonic stage, the tectonic structures of the Altai Mountains underwent a transformation of geodynamic conditions—a situation of sublatitudinal horizontal compression with predominant reverse-fault and transpressional movements along faults that existed up to the Neogene was replaced by horizontal shear conditions with a NNE and NE subhorizontal compression axis and WNW and NW extension axis. With such a stress field, the dominant NW-trending faults in this territory are mainly characterized by dextral strike-slip displacements, and NE-trending faults, by sinistral. Submeridionally trending faults that formed at the neotectonic stage show clear signs of extensional structures. A feature of the faults of the Chuya–Kurai depression according to these studies is a fairly small number of megafractures (indicators of shear displacements) in shear zones. This indicates dominant reverse-fault and transpressional movements over large disjunctive structures in the region in the Paleozoic and Mesozoic versus neotectonic shear movements. The results of the research are of practical importance in studying the regional seismicity. [ABSTRACT FROM AUTHOR]

This paper is concerned with spatial relationships between seismic zones in the eastern Priamurye region (М ≥ 5) on the one hand and, on the other, regional faults and zones of hidden faults as indicated by axes of gravity and magnetic anomalies. The seismoactive zones where М ≥ 5 earthquakes occurred are mostly confined to regional faults, while there are two cases where no such relationship has been detected. Seismic zones are observed both at intersections of regional faults and at intersections of regional faults with hidden faults of different rank. The seismic zones comprise inclined and subvertical deep-seated faults as identified by deep seismic sounding (DSS), the earthquake converted wave method (ECWM), and magnetotelluric sounding (MTS) surveys. The seismoactive zones were studied by geophysical techniques to identify indication of fluid saturation, as follows: the seismoactive faults frequently control low velocity and low resistivity inhomogeneities in the Earth's crust and upper mantle; some seismoactive faults have displaced the Moho interface; there are dome-shaped flexures of crustal interfaces and of the Moho interface; and the crust is found to contain numerous conversion interfaces. The deep occurrence of seismoactive faults and the indications of fluid saturation suggest that the seismoactive faults in the eastern Priamurye region provide supply of fluids from the mantle to the crust. [ABSTRACT FROM AUTHOR]

The western part of the Bohemian Massif hosts an intersection of two regional fault zones, the SW-NE trending Ohře/Eger Graben and the NNW-SSE trending Mariánské Lázně Fault, which has been reactivated several times in the geological history and controlled the formation of the Tertiary Cheb Basin. The broader area of the Cheb Basin is also related to permanent seismic activity of ML 3+ earthquake swarms. The Eastern Marginal Fault of the Cheb Basin (northern segment of the Mariánské Lázně Fault) separates the basin sediments and underlying granites in the SW from the Krušné Hory/Erzgebirge Mts. crystalline unit in the NE. We describe a detailed geophysical survey targeted to locating the Eastern Marginal Fault and determining its geometry in the depth. The survey was conducted at the Kopanina site near the Nový Kostel focal zone, which shows the strongest seismic activity of the whole Western Bohemia earthquake swarm region. Complex geophysical survey included gravimetry, electrical resistivity tomography, audiomagnetotellurics and seismic refraction. We found that the rocks within the Eastern Marginal Fault show low resistivity, low seismic velocity and density, which indicates their deep fracturing, weathering and higher water content. The dip of the fault in shallow depths is about 60° towards SW. At greater depths, the slope turns to subvertical with dip angle of about 80°. Results of geoelectrical methods show blocky fabric of the Cheb Basin and deep weathering of the granite bedrock, which is consistent with geologic models based on borehole surveys. [ABSTRACT FROM AUTHOR]

Geodynamic activity in the area of West Bohemia is typified by the occurrence of earthquake swarms, Quaternary volcanism and high flux of mantle-derived CO. The highest swarm activity occurs beneath the eastern edge of the Cheb basin, which is delineated by the NW-SE trending morphologically pronounced Mariánské Lázně Fault (MLF) controlling the formation of the basin. The previous trenching survey across the MLF zone has identified several fault strands with possible Quaternary activity. In this paper we present the results of the geophysical survey focused to trace the faults signatures in geophysical sections and to build an image of near surface tectonics. The method of electric resistivity tomography (ERT) along two profiles parallel to the trench identified a strong resistivity contrast between the bodies of sandy gravels in the middle and conductive clayey sands to the west and weathered crystalline basement to the east. The 2-D ground penetration radar (GPR) sections show direct correlation of reflections with lithological boundaries identified in the trench. As expected, the GPR signal amplitudes increase with the resistivities found in the ERT sections. Two of the four faults identified in the trench are indicated in the resistivity and GPR sections. A 3-D GPR measurement has identified a spot of high amplitudes elongated parallel to the MLF trend, which coincides with the high resistivity body. To improve the signal-to-noise ratio of the time slices we stacked the GPR time slices within vertically homogeneous blocks. This provided a contrast image of the sand-gravel body including its boundaries in three dimensions. The detailed analysis of the 3-D GPR cube revealed additional fault that limits the highly reflective sands and appears to be offset by another younger fault. Our results suggest a complex fault pattern in the studied area, which deserves a further study. [ABSTRACT FROM AUTHOR]

This paper considers the geological structure, geodynamics, and seismicity of the Tan Lu-Okhotsk rift system, which includes the Bohai Wan, Liaohe, Khanka, Middle Amur, and Lower Amur basins. The basins have common regularities typical of rift zones but differ in the deep structure and physical parameters of their crust, in particular, in the distribution of the strength properties, which also affect the seismicity level. It was established that the level and type of seismicity in the rift system is mainly determined by the local geodynamic conditions. [ABSTRACT FROM AUTHOR]