The problem of earthquake forecasting remains challenging, especially considering strong seismic events (M≥8). Strong earthquakes occur most often along the fault planes due to large-amplitude displacements of the contacting blocks. In such cases, the physical parameters of the earthquake foci generation process are estimated on the basis of the concepts describing the destruction of solids. In this paper, we present a new tectonophysical model of strong earthquake foci in the continental lithosphere. In this model, an earthquake focus is viewed as a body whose rheological properties are changing over time throughout the entire seismic period, including the moment of the seismic event initiation, its occurrence and the subsequent stress release in of the geological medium. In the period when a future earthquake source develops and grows, the physical properties of the host rocks are assumed to change substantially, and both the viscosity and the relative shear strength decreases. At the moment of time when a strong earthquake takes place, the viscosity of the rocks in its focus is at its minimum value and thus favorable for high-amplitude interblock shearing under the current regional stress and unchanged geodynamic factors. A decrease in the viscosity is facilitated by an increase in the fault length and leads to weakening of the geological medium and decreases its strength properties. When the earthquake occurs, the viscosity of the rocks in its source is assumed significantly lower than the dynamic viscosity of the lithosphere and not less than one or two orders below the viscosity of the interblock seismically active medium containing the source. It is most likely that at the moment of time when an earthquake takes place, the viscosity in its source is 10¹⁷–10¹⁹ Pa·s. In our approach, the parameter of viscosity is introduced into the physics of earthquake foci, and the time factor is taken into account when studying the process of earthquake preparation and occurrence, which can be an important step to gaining more knowledge for forecasting of the strongest seismic events (M≥8).

 

The article presents the results of the tectonophysical approach to the analysis of stress fields and the structure of gas–condensate deposits with the complex platform cover. The discussed case is the Kovykta license area (LA) in Eastern Siberia, Russia. In the upper part of the cross section, the network of fault zones was identified from the relief lineaments and structural data. The dynamic conditions for faulting (compression, extension, and strike-slip) were reconstructed by the paragenetic analysis. The state of crustal stresses in the study area was studied by tectonophysical modeling using gelatin as an optically active material. The applied method was successful in distinguishing between the zones of faults in the platform cover, which differ in the degree of their activity in the specified stress fields. The lower part of the cross section in the NE segment of the Kovykta LA is considered as an example of the tectonophysical interpretation of the electrical and seismic survey data in order to identify the fault zones and reconstruct the corresponding stress fields. Based on the synthesis of the analyzed data, it is revealed that the deposits like the Kovykta gas condensate field (GCF) show the zone-block structure of the platform cover formed under the influence of several stress fields closely associated with the stages of tectogenesis in the adjacent mobile belts. The next objective is to enhance the tectonophysical approach in order to develop a hierarchical model of the GCF zone-block structure, which details need to be known for improving the prediction of sites with the complicated stress-strain state of rocks and mitigating the risks associated with drilling exploration and production wells.

The problem of forecasting seismic hazards is discussed. The stress state data characterizing various aspects of brittle failure are reviewed in detail. It is shown that the most convenient tool for analyzing such data is the Mohr stress diagram and the Coulomb criterion. Noted is the role of a fluid in not only reducing the normal stresses responsible for brittle failure, but also predetermining the major processes in fault zones. In each fault body, a node can be distinguished as a fault part wherein the main structural and material transformations take place. The node contains narrow elongated zones of modification of mylonites, from protomylonites to ultramylonites and blastomylonites, that are related to the localization of continuous and discontinuous shear deformation. Due to the metamorphic processes, fault zones are less strong than the surrounding consolidated blocks of the crust. A theoretical analysis of the mechanism of displacements along the discontinuities of different scale ranks shows differences in their manifestation. Tectonic and seismic displacements along the rupture occupy the entire area at once, while displacements along the fault zone occur in stages along its extent and follow the ‘rolling-carpet’ principle that is also typical of intra-crystal dislocations. The stress state in the vicinity of ruptures and faults has different characteristic features. Based on the seismological and tectonophysical data on earthquake focal parameters and discontinuities, it is possible to identify two or three ranks of stresses, which differ in the laws predetermining their mutual relationships. Actually, this conclusion contradicts the hypothesis of self-similarity of discontinuities in their continuous range, from a dislocation to a fault zone, which length amounts to tens of kilometers. Besides, it imposes a restriction on the use of statistical analysis of seismic data. The seismic data show that in the source of a large earthquake, displacement develops as a running band (‘rolling-carpet’ principle). In the source of a weak earthquake, it occupies the entire earthquake focal area at once. The differences in the types of shearing in the sources of weak and strong earthquakes are related to the relationships between three dynamic parameters of the medium: velocity of seismic wave propagation, rate of rupture propagation, and displacement rate of the sides of the fracture. Using tectonophysical methods, the stress state was reconstructed for the seismically active regions of the planet and the sources of the mega-earthquakes of the 21st century. Based on the reconstructions, the mean strength and stress values were calculated, and the specific features of the stress fields were revealed. It is established that the strongest regional earthquakes ‘avoided’ the areas with increased effective isotropic pressure. The sizes of the sources of the strongest earthquakes were controlled by the size of the region with decreased effective pressure. The sites, wherefrom the earthquake were initiated, were often located in the zones of the highest stress gradients. These regularities support the term “metastability of the state of fault zone” (introduced to seismology from the physics of the states of matter) and justify it by a specific distribution pattern of stress values prior to the mega-earthquake. Based on the tectonophysical definition of the metastable state of faults, the important role is outlined for a stress gradient zone that represents a location wherein a trigger earthquake occurs. The ‘maturity’ of the zone with increased stress gradient values is, in essence, a characteristic of the time interval of metastability of the fault zone.

Our study aimed to find a mechanism that controls preparation and subsequent full seismic activation of large faults that may act as sources of strong earthquakes. A large fault was physically modeled to investigate the dynamics of its deformation. The experiments were conducted on elastoviscoplastic and elastic models of the lithosphere. A digital camera was used to capture images in the course of the modeling experiments. The digital image correlation method (DIC) detected the moments of impulse activation and displacements along the entire fault or its major segment. Between the activation moments, the fault structure consists of segments, including active ones. Activation is directional and involves a few large segments of the fault, then numerous small ruptures, and the latter are gradually degenerating. The long-term deformation dynamics of the fault is represented by a regular sequence of its full activations. In most cases, each moment of activation correlates with a minimum dip angle of the repeatability curve (β) and a maximum value of information entropy (S_i). We analysed in detail the deformation dynamics of the fault and in its wings between two full activation that occurred in a regular pattern, including the phases of regression and progression of the deformation process. The analysis revealed two similar scenarios in the evolution of the active segments and plastic micro slip faults within the active segments. In some intervals of time, deformation takes place considerably differently on the segments and the plastic micro slip faults. Such differences suggest that in the studies attempting to statistically predict and assess a large and potentially seismically hazardous fault zone, this zone should be considered spatially subdivided into a central narrow subzone (including the main fault plane) and two wide subzones framing the fault wings. According to our physical modeling results, the central subzone can be up to10 km wide, and the total width of all the subzones can amount to100 km or more. This study contributes to the development of the concepts of geodynamics of large faults in the seismic zones of the lithosphere and investigates one of the possible mechanisms preparing strong earthquakes in the seismic zones.

The physical effects that may prove useful for developing a new approach to short-term earthquake prediction have been studied in laboratory conditions. In seismology and earthquake foci mechanics, one of the major challenges is searching for indicators of an upcoming seismic event and attempting to reliably record such indicators by available instruments. In this regard, the best result of the laboratory studies of dynamic slip along faults would be the identification of specific macroscopic parameters controlling the deformation process, which are measurable in field. Dynamic stiffness of a fault zone seems to be an appropriate parameter. The recent laboratory experiments have shown that the value of this parameter predetermines the slip mode along the fault (unstable slip, creep, tremor, etc.), and a radical decrease in shear stiffness takes place as the fault zone reaches the metastable state. The effect discovered in the laboratory conditions gives grounds to suggest that changes in the stress-strain state of the fault zone at the final stage of earthquake preparation are detectable from the parameters of microseismic noise in the low-frequency range. Apparently, the noise records during and after the arrival of surface waves from distant earthquakes can provide the best opportunity for determining the parameters characterizing the study area. The wave oscillations with a period of a few dozen seconds have significant amplitudes and duration, which contributes to the excitation of resonance oscillations of the blocks. There are problems requiring additional laboratory experiments: estimating the size of a fault, which predetermines regularities in decreasing of the own frequency of the block-fault system; determining the ratio of the mechanical parameters of the fault in the nucleation zone and on the periphery of the future rupture, etc. Having analyzed the results of experimental studies carried out by other researchers, we conclude that laboratory experiments under normal conditions and low pressures can successfully address a number of fundamental issues on the way to creating a new approach to short-term earthquake prediction. Increasing pressure and temperature to values characteristic of seismogenic depths does not lead to the occurrence of any fundamentally new features in the behavior of the block-fault system at the stage when dynamic slip is being prepared. During slip, friction reduces due to melting, physical and chemical transformations at the micro- and nanoscales and other processes on the slipping surface, but these effects play no role at the stage when dynamic rock failure and the onset of slip are being prepared.

Our study was focused on narrow linear zones that penetrate to different depths the crust and have complex infrastructure. Rocks in such zones are more intensively tectonically altered in comparison with the background. ‘Flower structures’ and ‘zones of concentrated deformation’ (ZCD) are the terms to describe these zones. The field study results combined with the data of tectonophysical and computational modeling data and supplemented by the literature analysis gave grounds for the following conclusions. In the experiments, as well as in nature, ZCDs show similar and, in some cases, identical morphological and infrastructural features and have similar stages of their evolution. A ZCD is mainly a reflection of the transpression setting. Its formation is accompanied by 3D plastic shear flow of matter and dilatancy of the deformed volume. A ZCD may be associated with the development of the ‘basement – cover’ system. It may also occur due to the intra-cover tectogenesis that does not influence the basement. Locations of ZCDs are spatially regular and predetermine the tectonic divisibility of the crust and lithosphere.

The theoretical prediction of strain waves in the Earth is one of the most significant achievements in geophysics of the last third of the 20th century. Using the strain wave theory, the physical foundations were developed for the mathematical theory of strain wave propagation, and the search for methods that could detect the strain waves in experiment and simulation has commenced. This article provides an overview of the history of the strain wave theory and describes the observation methods, the main types of geological structures generating strain waves, and the properties of strain waves. It presents the most important results of the theoretical, laboratory and field studies of slow migration of strain. Future studies based on the strain wave theory may initiate a fundamental revision of the current concepts of the seismic process.

Our study aimed at investigating the origin and development of ‘slow’ movements in a solid body/medium under loading and studying the role of such movements in the occurrence of critical states, i.e. sources of destruction in a stable solid medium. Computerized modeling was conducted to simulate the evolution of the stress-strain state and the formation of slow deformation waves in a loaded medium. We have developed and justified a mathematical model of the loaded elastoplastic medium, which demonstrates the joint generation and propagation of ordinary stress waves (propagating with the velocity of sound) and slow deformation waves of the inelastic nature. The propagation rates of the latter are 5–7 orders of magnitude lower than the velocity of sound. The features of slow deformation wave propagation in the solid media are investigated. In the model, slow deformation waves interact under certain conditions as solitons and penetrate each other. Considering the properties, they are similar to both solitons satisfying the solutions of the non-linear Korteweg – de Vries equation and kinks satisfying the solutions of the sin-Gordon equation. Slow deformation fronts are actively involved into the formation of sources of destruction and provide an effective mechanism for the transfer and redistribution of energy in the loaded medium.

The article presents the results obtained by field tectonophysical methods applied to study tectonic stresses of the Northern Eurasia regions, including young and ancient platforms (West European, Timan–Pechora, Turan, West Siberian, East European, and East Siberian) and orogenic frame structures (Caucasus, Northern Tien Shan, Mongolia-Okhotsk system of mesozoids, and Sakhalin Island). Tectonic stress reconstructions provided the basis for analysing the influence of spreading in the North Atlantics and the Arctic on the stress state of the platforms in Northern Europe. A spatial boundary of the influence goes approximately along the margins of the Fennoscandian shield and the Russian plate in the north. Further southwards, the boundary is submeridional and extends from the western wing of the Byelorussian anteclise almost to the Eastern Carpathians. The stress reconstructions for this boundary show the WNW and W-E-trending axes of compression. The boundary line does not coincide with the Teisser-Tornquist line that represents the boundary between the platforms with heterochronous basements. However, it correlates well with heat flow anomalies. The boundary area is confined to the Baltic coast [Sim, 2000. Along the boundary area, near the Baltic Sea, there is an area wherein faulting is mainly caused by extension [Sim, 2000. In this setting, helium permeability is the highest, as shown by the crust map of the European part of the USSR [Eremeev, 1983. Extension in this area is probably related to formation of young grabens in the Baltic shield. Changes in the compression axis orientation may be due to the alternating activations of the grabens in the submeridionalBotnicGulf and the latitudinalGulf of Finland. Reconstructions for individual faults show contradictions in the directions of shear displacements: both right- and left-lateral displacements are possible on the same fault segments, and the axes of compression can have either latitudinal or meridional orientations. The focal mechanisms of the Osmussaar andKaliningrad earthquakes (meridional and latitudinal axes of compression, respectively) give evidence of specific current neotectonic stresses in this area. Another zone is distinguished at 52°N from the above-described area. It is mainly sublatitudinal and detected along the southern flank of the Byelorussian anteclise. Further to the east, its orientation changes to SSW, and it roughly follows the SW boundary of theVoronezh anteclise. Reconstructions for the Ukrainian Shield, located south of this zone, show mainly the unstable orientations of the axes of compression. For the platforms inNorthern Eurasia, the tectonophysical methods reconstructed neotectonic stresses in the structures formed under the influence of intraplatform tectonic stresses. These are the residual gravitational horizontal compression stresses released by long-term denudation and uplifting of the structures, including the Khibiny massif of the Baltic Shield, theOlenek and Munsky massifs of the East Siberian platform. These structures are composed of the ancient Archaean-Proterozoic rock complexes, which have been subjected to predominantly vertical displacements for a long time, from the Paleozoic to the modern stage. Special attention should be given to the tectonic stresses ofSakhalin located at the boundary between the Eurasian and North American lithospheric plates. At the edges of these two largest plates, there are the Amur and Okhotsk microplates separated by theCentral Sakhalin fault, as described in some publications. Neotectonic stress reconstructions forSakhalinIsland show sublatitudinal compression and submeridional extension in the common stress field of shearing. The tectonophysical studies show that the neotectonic stresses differ in large structures: horizontal compression and shearing are typical of the uplifts (Kola Peninsula, Tien Shan, Sakhalin), while horizontal extension and extension with shearing are characteristic of depressions (Kandalaksha graben, depressions of theTatarGulf and theSea ofOkhotsk). Our studies provide the data on spacious ‘white spots’ in the modern stress maps ofNorthern Eurasia. The stress reconstructions for practically all the studied structures show that shearing is the dominant geodynamic regime in the study region.

Interpretations of seismic, gravity and magnetic anomalies and structural data on the coastal zone of southern part of Central Viet Nam (SCVN) and the adjacent Tertiary basins suggest several phases in the tectonic evolution of the study region since the Late Cretaceous to Quaternary. In this paper, we try to clarify the tectonic evolution of SCVN and the adjacent continental margin. The Cretaceous – Paleocene tectonic phase commenced after cessation of the West Pacific plutonic magmatic activity that produced numerous diabases and aplite dykes of mainly sub-meridian orientation. It was characterized by N–S compression and E–W extension. The geomorphology and geology ofSE Asiawere considerably changed during the Neotectonic phases caused by collision between the Indian plate and the Eurasian continent. Two tectonic phases – Early and Late Neotectonic – are separated by a regional unconformity represented by a boundary surface between below strongly deformed strata (synrift) and above less deformed formations (post-rift). The Early Neotectonic phase was related to the left-lateral movement of the Red River Fault Zone (RRFZ) and includes two tectonic sub-phases: Eocene – Oligocene (NW–SE compression), and Oligocene – Miocene (E–W compression). Activity in the Oligocene-Miocene sub-phase gave birth to rift basins in the continental margin of the SCVN. The Late Neotectonic phase began since the RRFZ stopped left-lateral movement and the East Viet Nam (orSouth China) Sea stopped spreading. The Late Neotectonic phase is also divided into two tectonic sub-phases: Late Early Miocene (sub-meridian compression), and Late Miocene – Pliocene (NE–SW compression). The Late Miocene – Pliocene sub-phase is characterized by vertical movements that caused episodic uplifting of the onland terrains, and subsidence of the offshore Phu Khanh basin. Besides, Miocene – Pliocene-Quarternary basaltic eruptions were widespread all over the southern Indosinian terrain and the continental margin.

This paper presents the first results of the geostructural and tectonophysical studies of the crustal stress state in the Catoca kimberlite pipe area at the southwestern flank of the Kasai Shield in the northeasternAngola. In the evolution of the crustal stress state, six main stages are distinguished by analyzing the displacements of markers, fold hinges, long axes of boudins, granite dikes of various intrusion phases and kimberlites, as well as fractures with striations. For each of these stages, a dominating horizontal tectonic stress and its orientation is identified. During stage 1 (NW extension and shearing) and at the beginning of stage 2 (NW compression), structures formed in the host rocks in brittle-plastic conditions. The replacement of plastic deformation by faulting could occur about 530–510 Ma ago, when the continental crust ofAfricahad completely formed. Stage 3 (radial, mainly NW extension) and stage 4 (shearing, NW extension, and NE compression) were the most important for kimberlite occurrence: in the Early Cretaceous, radial extension was replaced by shearing. Both stages are related to opening of the central segment of theSouth Atlantic. The main kimberlite magmas occurred during the break-up of the Angola-Brazilian segment of Gondwana. In the course of all the four stages, stress was mainly released by the NE- and E-NE-striking faults and, to a lesser extent, by the NW-striking and latitudinal faults. The initial stage of kimberlite magmatism is associated with the NE- and E-NE-striking faults due to the presence of the Precambrian zones of flow and schistosity, which facilitated the NW-trending subhorizontal extension. Stage 5 (NE compression) began in the second half of the Cretaceous and possibly lasted until the end of the Paleogene, and compression occurred mainly along the NW-striking faults. Regionally, it corresponds to two stages of inversion movements in the southern regions of Africa, during which theAngoladome-shaped uplift emerged and the shoulders of the East African rifts began to take shape. Stage 6 (horizontal extension, mainly in the N-NE direction) is related to the processes that took place in the southern segment of theTanganyikarift and the eastern coast of theAtlantic. Based on the results of our studies, it became for the first time possible to get an idea of the main stages in the evolution of the studied region. Further geostructural measurements and dating of the host rocks will provide for a more precise definition of the proposed stages.

The knowledge of the neotectonic structures inSoutheastern Mongolia, that is considerably distant from the active plate boundaries, is important for determining a source of tectonic deformation and regular features of activation in the intracontinental setting. Our research was focused on the East Gobi and South Gobi depressions located inSoutheastern Mongolia, which developed since the Mesozoic and were activated to various degrees in the neotectonic stage. The study aimed to assess the paleostress state of the crust inSoutheastern Mongolia, identify the stages, factors and mechanisms of the Cenozoic activation of the regional structures of different strike, and determine the sources of activation. The analysis of the available literature suggests a similar history of their development in the Late Jurassic – Early Cretaceous (rifting) and Late Cretaceous – Paleogene (tectonic quiescence). In the Cenozoic stage, the depressions experienced activation of completely different styles. In theEast Gobidepression, left-lateral strike-slip faults were activated in the Tertiary, and the post-Late Cretaceous thrusting took place along the northeastern faults on the northern slope of the Totoshan uplift. In the Early Cenozoic, the N-S and N-W compression was dominant as evidenced by the deformed Late Cretaceous sediments and the reconstructed stress tensors typical of the compression and transpression regimes. An overview of the published data suggests that the most probable cause of such deformation was the impact of the Western Pacific zone of plate interaction. However, a potential influence of compression at the early stages of the Indo-Asian collision cannot be completely excluded. TheEast Gobidepression was low active in the second half of the Cenozoic. In contrast to the East Gobi depression, theSouth Gobiactivation began in the Late Cenozoic (Late Miocene – Early Pliocene). Young uplifts and forbergs (Gobi Altai eastern termination) developed actively and ‘cut’ the sediments of the basins originating from the Mesozoic. The W-E and N-W strike-slip and thrust faults were active in the Pliocene–Quaternary. The stress field reconstructions show compression, transpression and strike-slip regimes with the NE-trending axis of compression. Deformation in the East Goby Altay (as well as in Western andSouthwestern Mongolia) is driven by the India-Eurasia collision.

Our study was focused on the parageneses of heterogeneous fractures in the large fault zones of West Transbaikalia,Russia. We reconstructed the latest deformation in the fault zones of Transbaikalia, within which paleoseismic dislocations are known and M 4.7 earthquakes take place. To obtain statistically justified solutions on the kinematic types of the largest faults ofWest Transbaikalia, we collected the required data and conducted the structural and paragenetic analysis of the fractures in the study area. In the Chikoi-Ingoda, Khilok, North Tugnui andNorth Zaganfault zones, we created a network of 54 observation points and measured more than 5500 details of local fractures and faults. Recorded were the observed slickensides, the displacements of markers, and other details of rock fracturing. Based on the analysis results, we calculated a ratio of heterochronous dynamic settings for formation of the observed fault group. It shows that NW-SE-trending extension and compression are dominant in the study region. The parageneses of E-NE-striking faults, i.e. regional faults longitudinal to the depressions ofWest Transbaikalia, are abundant in the studied fault zones and generally observed in heterochronous formations, including the Cenozoic sediments. This fact, along with the focal mechanisms of the recently recorded earthquakes, suggests that these faults are young. Besides, in the Tugnui basin and the area southeast of the Chikoy depression, the right-lateral strike-slip setting was reconstructed for E-NE-trending faults. Our study pioneers in the quantitative analysis of the fault parageneses ofWest Transbaikalia. Considering the development of the network of large faults in the study area, we reconstructed the main stages and the kinematic types of the second-order fractures that constitute the internal structure of the studied fault zones at each stage of their tectonic development.

 

The article describes the factor analysis procedure ensuring its correct usage for identifying the processes that cause formation of fold structures and the main layers of the continental crust in mobile belts. The proposed approach to this problem of geodynamics is specific: it aims at solving the inverse (rather than direct, which is common) problem of identifying the processes that led to the occurrence of a natural structure characterized by quantitative indicators varying within a certain range of values. The objectives of the study were to specify the number of main processes/factors, describe their nature and calculate their relative ‘loading’ values. The database included detailed structural profiles across the fold structure of the Greater Caucasus. A special method was applied to construct a balanced model of the sedimentary cover, considering ‘structural cells’ which are 5–7 km long along the profile. Each of the 78 ‘cells’ studied was characterized by six parameters: the depth of the basement top at three stages of development (pre-folded, post-folded, and post-mountain-building), the amount of shortening, the amplitude of neotectonic uplifting, and the difference between the depths of the basement at the first and final stages. The parameters, that are directly related to the evolution of the blocks of the continental crust in the study area, constituted the initial data array for the factor analysis. In the first step, the Kaiser criterion was used to determine the number of factors, and it was equal to two. This number was specified for the main study using the methods of principal components with rotation. Factor 1 (Isostasy) amounted to 46 % of loading value, with high loads of the parameter of the depth of the basement top at stages 1 and 3. Factor 2 (Shortening) amounted to 40 %, with high loads of the indicators of shortening values and the amplitude of neotectonic uplifting. Factor 1 is related to the process of ‘isostasy’: after folding and orogeny is complete, the basement top of the ‘structural cells’ tends to return to its depth which was obtained on the pre-folded stage. Factor 2 is related to the process of shortening of the structure. The Chiaur zone was chosen as an example to analyze the Alpine-type development of the structures using the isostatically balanced model. The analysis shows that this zone formed as the density of the crystalline crust gradually increased to the ‘mantle’ values. Geodynamic modeling still fails to properly take such transformations into account. In the discussion of the results, attention is drawn to the fact that the established process of ‘isostasy’ is natural, i.e. not pertaining only to a theoretical model. It is noted that a geodynamic model can be correctly constructed if it considers the impacts of both processes revealed in this study. The obtained results can be used for improving the geodynamic modeling of fold-thrust mobile belts.

The field of earthquake epicentres of Pribaikalie (Russia) is reconstructed from the data of historical and instrumental monitoring of earthquakes. The analysis shows that seismic events in the study area are distributed irregularly in space and time. The seismic process in Pribaikalie is investigated through the prism of seismic structures in the lithosphere; the irregular occurrence of seismic events in time is considered with reference to seismic weather and climate; and the causes of periodic activations of the seismic process are discovered in relation to the external effects on the Earth's physical fields from cosmic and solar processes. It is proposed to classify the seismic structures as specific geometric objects located in the lithosphere. Such objects are viewed as abstract structural elements. Some regular features are noted in the occurrence of seismic events. It is revealed that the seismic process in time shows a similarity with the course of hydrometeorological processes, which is reflected in the periodicity of elastic energy release, if only the change in the number of earthquakes (of different energy classes) in time is analysed by years. An evidently regular time pattern of seismically active periods suggests that the seismic process is influenced by some external factors. In this study, we apply the concepts of heliogeodynamics, space climate and weather to investigate such factors.

The spatial analysis was conducted to analyze the positions of earthquakes hypocenters in the transit zone of the upper mantle and the focal mechanisms of the strongest earthquakes in the subduction slabs of theOkhotskSeasegment of the Kuril-Kamchatka island arc and theJapanSeasegment of the Japanese island arc. It revealed a significant difference in the morphology of these slabs, as well as in the positions of the earthquake hypocenters relative to the active and stagnating parts of the slabs and the forces that caused the earthquakes. Based on the seismic data presented in the article, it is confirmed that there are two types of subduction of the oceanic lithospheric plates in the mantle. The article discusses relationships between the subduction and various geological processes at the upper–lower mantle boundary. It considers possible causes (including those related to phase transitions) of deep-focus earthquakes, in case of which splitting of the oceanic lithospheric plates takes place at depths near the upper–lower mantle boundary. Subduction of the oceanic lithospheric plates and their splitting predetermine a possibility for the crustal elements to penetrate into the lower mantle and deeper into the D″ layer, wherein new plumes arise and transport the deep magma together with the recycled substance into the crust. Deep-focus earthquakes are a necessary link in the mechanism providing for the recycling of chemical elements in the crust – mantle – D″ layer system and thus leading to the formation of a wide range of mineral deposits.

Various definitions of the kinematic types of faults are reviewed. The degree of symmetry in the distribution of anomalous displacements of the ground surface is proposed as a morphological criterion for identifying the types of faults based on geodetic observations. Local anomalies of vertical displacements identified by geodetic observations in the fault zones are analysed by types. It is revealed that 88 % of the analysed anomalies show local symmetrical subsidence of the ground surface near the faults. Morphologically, these anomalies correspond to the subvertical tensile faults. Mechanisms of three types (block, dislocation, and parametric) are discussed considering the formation of the observed displacements in the zones of activation of the tensile faults. A comparison of the calculated and observed displacements of the ground surface shows that the best consistency between the theory and the observations is achieved using the model of local parametric excitation of deformation under quasi-static regional loads and the theory of strain nuclei (soft inclusions).

A quasi-linear zone of noticeable geological and geophysical changes, which coincides approximately with 102–103° E meridians, is termed by the authors as “geodivider”. Active submeridional faults are observed predominantly along the zone and coincide with its strike. Seismicity is most intensive in the central part of this zone, from the Lake Baikal to the Three Rivers Region at the Sino-Myanmar frontier. Transects with deep seismic sections and energy dissipation graphs show most sharply increasing seismic energy amounts and hypocenter depths in the western part of the geodivider which delimits (in the first approximation) the Central Asian and East Asian transitional zones between the North Eurasian, Indian and Pacific lithosphere plates. The transpression tectonic regime dominates west of the geodivider under the influence of the Hindustan Indentor pressure, and the transtension regime prevails east of it due to the Pacific subduction slab submergence and continuation. The regime change coincides with an abrupt increase in the crust thickness – from 35–40 km to 45–70 km – west of the geodivider, as reflected in the geophysical fields and metallogenic characteristics of the crust. The direction of P- and S-waves anisotropy together with the GPS data show decoupling layers of the crust and mantle in the southern part of the geodivider. According to our investigations, the 102–103° E geodivider is a regional geological-geophysical border that may be compared with the Tornquist Line, and, by its scale, with the Uralian and Appalachian fronts and some others large structures.

The study is focused on the submeridional transregional boundary that stretches as a wide band along 105°E in Central Asia. In modern seismic models, it is traceable to a depth of ~600 km. In the continental area to the west of this boundary, seismic activity is increased. Following the study of the origin of the transregional boundary zone, it becomes possible to assess its contribution to the current geodynamic processes in Asia. This article presents a comprehensive analysis based on comparison of the available data with the results obtained in our study using independent methods. The distribution of earthquakes was analyzed by depth. We revealed a correlation between the characteristics of seismotectonic deformation (STD) reconstructed from earthquake focal mechanisms, the structure of P-velocity anomalies, and the distribution of convection flows in the upper mantle. The pattern of seismic velocity anomalies in the upper mantle was investigated on the basis of the data from the ISC catalogue for the period of 1964–2011. The modeling was carried out for two regional tomographic schemes, using the first arrivals of P-waves from [Koulakov et al., 2002 and PP-phases from [Bushenkova et al., 2002, with the subsequent summation with weight coefficients depending on the distribution of the input data in each scheme. A similar approach was applied in [Koulakov, Bushenkova, 2010 for the territory of Siberia; however, that model only partially covered the submeridional transregional boundary zone and was based on fewer ISC data (until 2001). The parameters of the combined model were used to estimate variations in the lithosphere thickness, which can significantly influence the structure of convection flows in the upper mantle [Chervov et al., 2014; Bushenkova et al., 2014, 2016. The thickness variations were taken into account when setting boundary conditions in the numerical modeling of thermal convection, which followed the algorithm described in [Chervov, Chernykh, 2014. The STD field was reconstructed from the earthquake focal mechanisms (M≥4.6) which occurred in Central Asia in 1976–2017. The analysis shows that the zone, wherein the seismic regime changes, correlates with the band wherein the STD principal axes are turning, the submeridional high/low velocity elongated boundary in the seismotomographic model, as well as with the submeridionally elongated descending convective flow in the upper mantle. Shortening of the STD principal axes is observed in the submeridional direction in the western part and in the sublatitudinal direction in the eastern part of the study area. The directions of the principal axes turn in the 93–105°E zone. It is thus probable that the submeridionally elongated descending convective flow in the upper mantle of this region, which results from the superposition of the lithosphere thickness heterogeneities, is a barrier to propagation of seismically manifested active geodynamic processes caused by lithospheric plates collision.

The Amur region (Priamurie) is located in the NE part of the Amur lithospheric plate and its surrounding territories. Seismic activity is moderate in Priamurie, and the regional earthquakes, including the strongest ones, occur mainly in three seismic belts: Stanovoi (the zone of influence of the eastern flank of the Stanovoi fault), Yankan-Tukuringra-Soktakhan (the eastern flank of the Mongolia-Okhotsk lineament), and Turan-Selemzhinsky (from the Lesser Khingan to the north). The Sakhalin Branch of FRC GS RAS Catalogue of focal mechanisms of 57 regional earthquakes provide the data for a more precise estimation of the parameters of the crustal stress state in the study area. The Cataclastic Analysis Method (CAM) developed by Yu.L. Rebetsky (stage 1) was used to estimate the orientations of the main axes of the stress tensor and the Lode – Nadai coefficient. The analysis shows that the Upper Priamurie is dominated by shearing and compression with shearing. The Amur plate moves relative to the Aldan-Stanovoi block along the South Tukuringra and North Tukuringa faults to the east. Vertical shearing is predominant along the Dzheltulak fault and the western segment of the North Tukuringra fault. The NNE-trending compression takes place in the area located east of the quiescence zone of the Dzhagda ridge. Along the Mongolia-Okhotsk fault system, near the Sea of Okhotsk, the direction of compression changes to the northward one. The tectonic stress field along the Tanlu fault zone is inhomogeneous and comprises the alternating zones of horizontal compression and stretching with varying directions of the main stress axes. To the east of the band characterized by the maximum seismic activity, compression changes its direction to the southeast- and eastward. Probably, the impact of the oceanic subduction on the northern part of the Japan-Korean block begins to manifest itself in this part of the Amur region. The tectonic stress field reconstructed from the seismological data is consistent with the measurements of the modern crustal movements. The results of our study can prove useful for clarifying the tectonics of the region. 

In the part 2 of the study [Ruzhich, Kocharyan, 2017, we aimed at identifying the elements of paleoearthquake sources in the crust, which formed at the hypocentral depths in the exhumed Primorsky segment of the ancient collisional suture. The study area covered the southeastern margin of the Siberian craton (Pribaikalie, East Siberia). Slickensides, pseudo-tachyllite (basaltic glass) and other petrological evidence of intensive tectonic movements were sampled. The structure of the deep segments of the collisional suture were reconstructed from on the data on coseismic ruptures and faults, and the PT parameters were estimated. In the past decades, similar research problems were actively investigated (e.g. [Sibson, 1973; Byerlee, 1978; Morrow et al., 1992; Hodges, 2004; Kirkpatrick et al., 2012). In Russia, the interest in studying geological and geophysical features of the deeply denuded areas in ancient faults is still limited [Sherman, 1977; Ruzhich, 1989, 1992, 1997; Savel’eva et al., 2003; Ruzhich et al., 2015; Kocharyan, 2016. The deeply denuded Primorsky segment of the collisional suture of the Siberian Craton underwent the geological evolution of a billion years. In the analysis, we used additional geological data from the petrology studies of the Main Sayan fault zone and other exhumed fault segments, including the seismogenerating faults in the Mongolia-Baikal region [Zamaraev, Ruzhich, 1978; Zamaraev et al., 1979; Ruzhich et al., 2009. From the PT conditions for the occurrence of the slickensides, pseudo-tachylyte, and the Primorsky segment structure, the ⁴⁰Ar/³⁹Ar method estimated the age of the slickensides containing tourmaline at 673±4.8 Ma, which may correspond to the Neoproterozoic stage of the breakdown of the megacontinent Rodinia. Another dating, 415.4±4.1 Ma, obtained for the muscovite sample from a decompressional rupture, refers to the Early Paleozoic stage in the development of the collisional suture, when accretion of the Siberian Craton and the Olkhon terrain took place [Donskaya et al., 2003; Fedorovsky et al., 2010. Based on these ages and other available petrological data, the depths of the heterochronous systems of coseismic ruptures were estimated:18 km in the Neoproterozoic, and12 km in the Middle Paleozoic stage of the seismotectonic evolution of the crust in Pribaikalie. The deep paleoseismological settings need to be further investigated in order to more thoroughly clarify the physical and chemical conditions that contributed to the occurrence of the ancient and recent sources of strong earthquakes in the deep segments of faults in the crust. Such information is a prerequisite for further progress towards resolving the problems of securing seismic safety in various regions.

The geostructural, petrological, geochemical, geochronological and biostratigraphic studies were conducted in the Hentei-Dauria fold system of the Mongolia-Okhotsk orogenic belt. This Paleozoic system is composed mainly of three heterochronous rock associations related to the onset and development of oceanic basins and active margins in the conjugation zone of the Siberian continent and the Mongolia-Okhotsk ocean. This region developed in three stages: (1) Late Caledonian (Ordovician – Early Silurian), (2) Early Hercynian (Late Silurian – Devonian), and (3) Late Hercynian (Carboniferous–Permian). In the Late Caledonian, oceanic seafloor spreading was initiated, deep-sea siliceous deposits were formed, basaltic and andesitic pillow lavas were erupted, and layered and cumulative gabbros, gabbro-dolerite dykes and subduction zones with island-arc magmatism were formed. After a short quiescence period, new zones of spreading and subduction occurred at the active margins of the Mongolia-Okhotsk ocean in the Early Hercynian. In the Late Hercynian, large back-arc sedimentary basins, accretionary prisms and connecting intraplate magmatic complexes were formed in all structures of the Hentei-Dauria fold system. As a result of our studies, we propose a comprehensive model showing the geodynamic development of the Hentei-Dauria fold system that occurred in the area of the Mongolia-Okhotsk Ocean and its margins.