Consideration is being given to the tectonophysical approach to the reconstruction of structure formation mechanisms and stress-strain state of rocks in hydrocarbon deposits localized in the platform cover, which has a complex structure in terms of rheological layering and disturbance by different-rank fractures. With the Kovykta gas condensate field, largest in Eastern Siberia, there were shown the main methods and ways of using modern achievements in tec- tonophysics for interpreting geological and geophysical information on the upper and lower parts of the sedimentary cover, unique in terms of volume and significance, that was obtained during geological exploration therein. Regularities of changes in the stress-strain state of rocks, found during the research, are combined into a tectonophysical model, which can be used as a base for other hydrocarbon deposits. The model is based on the concept of a zone-block structure of the platform cover, which is formed by a network of subvertical and subhorizontal fault zones that divide it into less faulted blocks. Disjunctive structures are highly fractured zones with concentration of relatively small low-amplitude faults, i.e. represent the early stages of faulting. The zone-block structure is formed mainly by tectonic or gravitational forces; in the first case, the stages and fracture characteristics are transformed onto the platform from the surrounding mobile belts, and in the second case they are determined by the presence of ductile rocks in the section capable of gravitational sliding. The graphic component of the tectonophysical model is 3D datasets that show the zone-block structure and stress state of rocks for the deposit with the degree of detail provided by key geophysical materials and, primarily, by seismic data. By modern GIS, this information can be quickly retrieved for any-size area of the studied rock mass and then used as a basis for solving production issues related to the development of deposits in fracture-pore reservoirs, or for analyzing general problems of their formation and dynamics.

The origins and formation mechanisms of neotectonic structures in a part of the Mongolian-Siberian region were identified by geodynamic zoning based on multivariate statistical analysis of numerical data that describe geological-geophysical and geological-geomorphological processes. These processes in the regional lithosphere were described by a set of 11 geological and geophysical parameters, divided into three main groups using the hierarchical method of cluster analysis. The first group includes the seismic moment, the density of active faults, the recent horizontal strain rates, and the magnitude of the deep heat flow. The second group involves the thicknesses of the earth’s crust and exogenously active layer, the recent horizontal crustal velocities, and the amplitudes of vertical neotectonic movements. The third group includes gravity anomalies and the lithospheric thickness. The spatial grouping of the parameters by cluster analysis (K-means method) yields seven clusters, whose spatial position and composition are determined by the geological history, geological structure, geodynamic evolution of the region, and the recent strain rates. Some clusters characterize large rigid lithospheric blocks, while other clusters describe large active fault systems in the studied region. The search for latent factors that make the greatest contribution to the dispersion of the geological and geophysical parameter values was carried out using the principal component method, which allows minimizing the number of factors. Four main factors were identified for areas that differ in the morphology and origin of neotectonic structures: (i) higher horizontal compressive and tensile strains, (ii) dynamic effect of mantle anomalies, manifested in uplifts and doming, (iii) activation of thinned lithosphere within the boundaries of lithospheric plates or large blocks, and (iv) active shear deformation of the earth’s crust. The results of clustering and factor analysis of numerical data describing geological-geophysical and geological-geomorphological processes within the Mongolian-Siberian region are interpreted in the framework of physical mesomechanics.

The instrumental monitoring reveals an autowave nature of ice deformation behavior prior to ice shocks. A few minutes or the first tens of minutes before the shock, this process shows an increase in the amplitude of oscillations, often with a multi-fold reduction in their period. An autowave dynamics of ice deformations is due to self-organization of a structurally heterogeneous ice environment under critical conditions. The self-organization ability of the deformation process is confirmed by the results of ice deformation time series processing by the structural function curvature analysis method (SFCAM) and by the Lomb-Scargle periodogram method. The results of seismic monitoring of ice showed that autowave processes are characterized by constant frequency of 0.1 Hz. Taking into account the ice deformation and microseismic fluctuation features preceding the ice shocks, spectral analysis was performed on the data of deformation and seismicity monitoring at the Buguldeika geodynamic polygon before the Kudara earthquake. According to the results, 14 hours before the earthquake the seismogram recorded a gradual increase in auto-oscillation amplitudes in the frequency range from 0.01 to 0.1 Hz. The maximum amplitude increase is 19.5 against the background.

The research provides an example of the GPS time series processing for monitoring of horizontal coseismic displacements during the 11 January 2021 M 6.7 Hovsgol earthquake, Mongolia. There has been developed a methodological approach to the study of coseismic displacements at the time of the earthquake. This paper presents the results of determining the values of horizontal coseismic displacements which are 0.6 mm in the junction zone between the Hovsgol and Tunka depressions and hundredths of a millimeter for the Siberian block and Transbaikalia areas. For stations located on the southern margin of the Siberian block and stations in Transbaikalia, the vectors of coseismic displacements are directed to the west. The calculated displacement vectors of the stations near the epicenter (MNDY and BADG) are directed to the southeast.

The measurement data obtained at the GPS network in the southwestern part of the Baikal Rift System for the period from 1994 to 2020 were analyzed. The spatial relationship between seismic events and tectonic strain rates was estimated. The field of modern horizontal motions and deformations was calculated for the Tunka, South Baikal, and Khubsugul depressions. The rotation pole of the Siberian block was defined. The general geodynamic setting on the western flank of the Baikal Rift System is characterized by low horizontal velocities in the range of 0.5–1.4 mm yr–1. Based on calculation of the relative deformations, the contrast zones of dilatation with narrow zones with the “non-Baikal” type of directions of deformation processes against the background tectonic extension regimes have been determined for the first time in the Bystraya and Khubsugul tectonic depression. Tectonic stresses are accumulated in these narrow zones and released in earthquakes.

According to seismological and geodetic data, the central part of the South Baikal basin is under an extension regime of the Earth’s crust. This study presents for the first time seismological evidence of a transtensive stress field in a local area adjacent to the eastern side of the Central Baikal subbasin. Two independent approaches to determining the orientation of the principal stress axes show a subhorizontal orientation of the axes of stresses of minimum and maximum compression, which results in a strike–slip component in the focal mechanisms of the earthquakes. The area under consideration is a part of the Ust’-Barguzin transfer zone, in which the transfer of deformations between South Baikal and the Barguzin basin takes place. The results obtained are in agreement with the geological and structural data and may indicate the contribution of horizontal displacements in the transfer of deformations between the segments of the rift zone.

Summary In the report the results of application of modern ways in the tectonophysical analysis of geological and geophysical data for identification of regularities in a fault structure of a platform cover on the Kovykta gascondensate field (GCF) are considered. The basis for research of sedimentary cover are the materials of 3D seismic and electromagnetic survey, the structural data and materials of digital relief model processing. As a result of the complex analysis the zone-block structure (ZBS) of the Kovykta GCF is established. It represents hierarchy of blocks which contact with each other on wide zones of short fractures concentration. The ZBS style is defined by domination of subhorizontal fault zones (layer-by-layer stripping) and subvertical zones of northwest and northeast orientations. The network of the fault zones was formed in four stress fields which correspond with the main stages of an adjacent Baykal- Stanovoy mobile belt development in the Paleozoic- Cenozoic. Some fields are reactivated in the Cenozoic under the influence of the gravitational processes connected with local lifted raisings of a relief. The zone-block structure of the Kovykta GCF is a basis for it's zonation on hydrocarbon productivity, overpressure zones, on degree of fracturing and types of stress state of rocks. The solution of this task will allow to receive additional substantiating for the choice of drilling site which can be passed without geohazards and with the prospect of detection of natural gas accumulation.

Based on the results of a laboratory simulation of the seismic fault reactivation by “stick-slip” process, it was shown that the system of two blocks just before an impulse offset goes through the meta-instable dynamic state, with early and late stages of meta-instability [Ma et al., 2012]. In the first stage the offset begins in slow stationary mode with slow stresses relaxation on contact between blocks. In the second stage of the “accelerated synergies” strain rate increases and, subsequently, the deformation process through a process of self-organization came to dynamic impulse offset. The experimental results were used for interpretation of the results of spectral analysis of the deformation monitoring data. The data were held within the southern part ofLakeBaikal, where Kultuk earthquake (27.08.2008, Ms=6.1). took place. Its epicenter was located in the South end zone of the main Sayan fault. Monitoring of deformations of rocks was carried out from April to November2008 in tunnel, located at30 km from the epicenter of the earthquake. The time series data was divided into month periods and then the periods were processed by the method of spectral analysis. The results showed that before the earthquake has ordered view spectrogram, whereas in other time intervals, both before and after the earthquake such orderliness in spectrograms is missing. An ordered view spectrograms for deformation monitoring data can be interpreted as a consequence of the self-organiza­tion of deformation process in the transition of seismically active fault into meta-unstable before the Kultuk earthquake.

We have studied the structural geology and geomorphology of the fault zones in the junction area of the Angara-Lena uplift and the Predbaikalsky trough. We have analyzed faults and folds and reconstructed paleostresses for this junction area named the Irkutsk amphitheatre. Our study shows that syn-fold (Middle Paleozoic) faults include thrusts, reverse faults and strike-slip faults with reverse components, that occurred due to compression from the neighbouring folded region. Recently, contrary to compression, faulting took place under the conditions of extension of the sedimentary cover: most of these recent faults have been classified as normal faults. In the Late Cenozoic, the platform cover was subjected to brittle and partly plicative deformation due to the NW–SE-trending extension that is most clearly observed in the adjacent Baikal rift. Thus, the divergent boundary between the Siberian block of the North Eurasian plate and the Transbaikalia block of the Amur plate is a zone of dynamic influence, which occupies the area considerably exceeding the mountainous region on the Siberian platform. Important factors of faulting are differentiated vertical movements of the blocks comprising the platform. Such vertical movements might have been related to displacements of brine volumes. In the Late Cenozoic basins, movements along separate faults took place in the Late Pleistocene – Holocene.

We have compiled and analyzed earthquake focal solutions for the territory of Mongolia and its surroundings in order to reveal a spatial variability of stress orientation and stress regimes of the crust. According to the stress inversion results, the SHmax is turning from W-E in the eastern Mongolia to SW-NE in the Gobi Altay and the central Mongolia, and then to S-N in the western part of the region. Comparison with data derived from GPS measurements shows that directions of the strain axes revealed by the geodetic and seismological observations are generally consistent. A contradiction is found for the Bolnai zone where results of GPS estimation indicate the predominance of extension (in the SE-NW direction), whereas earthquake data for the longer period of seismic observations reveal compression. Compression in this zone is mainly due to the Tsetserleg-Bolnai earthquakes contribution; however, a part of the recent data on focal mechanisms fits an extensional stress field with the NNW orientated extension axis. These data are in accordance with some published works which suggest a transtensive field from some structural geology studies in the eastern part of the Bolnai zone.The paper is supplemented with a list of M≥4.5 earthquake fault plane solutions and unpublished focal mechanisms for some M≤4.5 earthquakes of the northern Mongolia and the southern Baikal region.

На примере результатов спектрального анализа данных мониторинга деформаций ледового покрова оз. Байкал и горных пород представлен новый подход к диагностике предсейсмогенного состояния литосферы в пределах очаговой области.

A new approach to diagnostics of the preseismogenic state of the lithosphere within the focal area was considered by the example of spectral analysis of the monitoring data on ice cover deformations in Lake Baikal.

Active faults of the Hangay-Hentiy tectonic saddle region in Central Mongolia are studied by space images interpretation, relief analysis, structural methods and tectonic stress reconstruction. The study results show that faults activation during the Late Cenozoic stage was selective, and a cluster pattern of active faults is typical for the study region. Morphological and genetic types and the kinematics of faults in the Hangay-Hentiy saddle region are related the direction of the ancient inherited structural heterogeneities. Latitudinal and WNW trending faults are left lateral strike-slips with reverse or thrust component (Dzhargalantgol and North Burd faults). NW trending faults are reverse faults or thrusts with left lateral horizontal component. NNW trending faults have right lateral horizontal component. The horizontal component of the displacements, as a rule, exceeds the vertical one. Brittle deformations in fault zones do not conform with the Pliocene and, for the most part, Pleistocene topography. With some caution it may be concluded that the last phase of revitalization of strike slip and reverse movements along the faults commenced in the Late Pleistocene. NE trending disjunctives are normal faults distributed mainly within the Hangay uplift. Their features are more early activation within the Late Cenozoic and the lack of relation to large linear structures of the previous tectonic stages. According to the stress tensor reconstructions of the last phase of deformation in zones of active faults of the Hangay-Hentiy saddle using data on tectonic fractures and fault displacements, it is revealed that conditions of compression and strike-slip with NNE direction of the axis of maximum compression were dominant. Stress tensors of extensional type with NNW direction of minimum compression are reconstructed for the Orkhon graben. It is concluded that the activation of faults in Central Mongolia in the Pleistocene-Holocene, as well as modern seismicity were controlled mainly by additional horizontal compression in the SW direction, which was associated with convergence of the Indian subcontinent and Eurasia. The influence of the asthenosphere flow in the SE direction at the base lithosphere is an additional factor facilitating strike-slip deformation of the crust in the study area and a possible explanation of divergent movements in the Baikal Rift, as well as the SE movement of the Amur plate. The Eastern Hangay crust is deformed under extension associated with a dynamic impact of the local mantle anomaly on the lithosphere. The boundary between the Amur plate and the Mongolian block (according to [Zonenshain, Savostin, 1979]) is fragmentary expressed in the tectonic structure. It represents a rim part of the deformation zone, embracing the Mongolian block and the adjacent uplifts of the Mongolian Altai, Tuva and Eastern Sayan. Along the boundary, compressive and transpressive strain occurred in the Pleistocene-Holocene.

УДК 504+338.49(57) ЗАЩИТА СОЦИАЛЬНО-ЭКОНОМИЧЕСКОЙ ИНФРАСТРУКТУРЫВОСТОЧНОЙ СИБИРИ ОТ ПРИРОДНЫХ ОПАСНОСТЕЙ 1 К.Г.Леви, 1 Е.А

New approach to diagnostics of seismically dangerous state of seismoactive fault is presented based on results of spectral analysis of deformation monitoring data.