The history of tectonophysical studies in Irkutsk began in the 1950s at the initiative of Prof. V.N. Danilovich. Tectonophysics as a new scientific field in geology was enthusiastically supported by research institutes of the actively develo­ping Siberian Branch of the USSR Academy of Sciences, including the Institute of the Earth's Crust (IEC). In late 1950s, V.N. Danilovich, G.V. Charushin, O.V. Pavlov, P.M. Khrenov, S.I. Sherman and other scientists began to conduct large-scale studies of faults and rock fracturing with application of methods of structural analysis of fault tectonics and taking into account types of physical and mechanical destruction of the crust. In 1979, the IEC Scientific Council reviewed the initiative of Prof. S.I. Sherman, who was supported by Academician N.A. Logachev and Doctor of Geology and Mineralogy O.V. Pavlov, and approved the decision to establish the Laboratory of Tectonophysics, that has been and is the only scientific research team of the kind in the territory of Russia eastward of the Urals and, in fact, the second in the Russian Federation. Its studies are based on concepts dealing with physical regularities of crustal faulting that are described in the monograph published by S.I. Sherman [Sherman, 1977], three co-authored volumes of Faulting in the Lithosphere [Sherman et al., 1991, 1992, 1994] and other scientific papers. These publications have consolidated results of studies conducted by the team of researchers from the Laboratory, which can be called the Irkutsk school of tectonophysics. On the eve of the 21st century, the Laboratory successfully extended application of physics of destruction of materials and mathematical methods of analysis to studies of structural patterns of faults varying in ranks in the crust and the upper lithosphere.

We conducted comprehensive studies of tectonophysical regularities of formation of large crustal faults, pioneered in establishing quantitative relationships between main parameters of faults, i.e. length and depth, length and amplitude of displacement, length and density, and estimated the factors determining such parameters. A model showing the fault structure was proposed with account of changes of physical properties of the crust with depth. It was shown that faulting in the crust follows the laws of deformation and destruction of Maxwell body.

With accumulation of the knowledge on regularities of faulting in the lithosphere, analyses the state of stresses in the lithosphere has become prioritised, and this is one of the top challenges in geodynamics and tectonophysics. Tectonophysics from Irkutsk published the first map of the state of stresses of the Baikal rift zone and proposed new concepts for studying crustal stresses by structural geological methods. Based on such concepts, a new map of the state of stresses of the upper lithosphere was constructed.

Studies of faulting included researches of areas around virtual axes of faults and variations of sizes of such areas, and a concept of an area of dynamic influence of large lithospheric faults was proposed. It is established that internal patterns of areas of dynamic influence of faults are composed of zones that can be revealed both laterally and in depth, and such zonal patterns depend on the degree of tectonical and dynamo-metamorphical transformation of the rocks.

The internal structure of continental fault zones was studied, and three main disjunctive stages were revealed, each corresponding to a specific type of deformation behaviour of the medium, its state of stresses, pathogenesis of faults varying in ranks, and variations of parameters in space and time.

Triple paragenesises of fractures were revealed and analysed for a number of regions, and such studies provided the basis to propose a method of specialized mapping of the crust, which provides for determination of locations of fault zones and their boundaries, conditions of their formation and major specific features of their internal structures. This method can be effectively applied within the framework of conventional geological surveys of any scale.

Results of studies of tectonic divisibility of the Earth based on advanced tectonophysical concepts were referred to establish the zone-block structure (ZBS) of the lithosphere. Analyses of faults at various scales showed a strict hierarchy of ranks in the ZBS of the lithosphere in Central Asia, and actual characteristics of 11 hierarchic levels (from global to local) were revealed and described in quantitative terms. With reference to the ZBS concept, the Baikal rift was studied, and the soil radon concentration pattern of Pribaikalie was analysed and its main spatial and temporal regularities were revealed.

Comprehensive geological, structural, tectonophysical and geoelectrical studies were conducted in the Cenozoic and Mesozoic basins of Pribaikalie and Transbaikalie, and results were consolidated and published. The fault-block patterns, the deep structure, the state of stresses and seismicity of the crust were studied in a number of areas in the region.

Complex tectonophysical studies were initiated in the Yakutian diamond-bearing province to reveal structural factors that control the kimberlite locations, and the first results were reported. By applying tectonophysical methods, it was established that periods of formation of kimberlite bodies are related to stages of formation and activation of the fault pattern of the platform cover. A pioneering conclusion was stated that in the structural control over kimberlite magmatism of the Siberian platform, the dominant role is played by fault zones of the orthogonal network, which were activated in the regime of alternating-sign displacements at different stages of the platform's development in the Paleozoic and Mesozoic.

Physical modelling experiments using an original installation were conducted, and, among its main achievements, an important result is modelling of the process of formation of the Baikal rift zone (BRZ) by an elasto-plastic model in conformity with criteria of similarity. The Shanxi rift system was also modelled, and its physical modelling study was conducted jointly with scientists from China under the Russian-Chinese project supported by the Russian Foundation for Basic Research.

Besides, the article informs about commencement of original experimental studies of deformation waves in elasto-plastic mediums and describes objectives of tectonophysical studies for the nearest future.

 

This publication consolidates and analyses experimental data in a wide range of scales in seismotectonics and geo­mechanics, from a micro-size seismic event (an earthquake focus of a few centimetres) to a mega earthquake. It reviews regularities in changes of geometric parameters of faults and fractures in various ranks, their mechanical properties, linear sizes of earthquake foci, time of preparation of dynamic events, and seismic energy.

Averaging through the whole range of scales yields ratios close to the law of geometrical similarity. A more detailed consideration gives grounds to conclude that several hierarchic levels can be distinguished and, in different scale, changes of parameters of events follow laws that differ and often deviate from the laws of similarity.

It is shown that linear sizes, L from ~500 to 1000 m comprise a transitional range that is a border between two ranges characterized by significantly different scale ratios. Seismicity with shallow foci which is associated with mining operations should be noted separately.

According to energy calculation with reference to categories of seismic events, it is established that earthquakes of the Baikal rift system show an anomalous trend of strongly increasing specific energy with increasing scales. In the range of moment magnitudes from 5 to 6.3, an average specific value of seismic energy is higher than an average global value for the same range at least by a factor of 25. It should be clarified whether such an effect is an artefact related to errors in seismic energy calculations or an actual physical effect that is still unexplained.

 

It is generally accepted that crustal earthquakes are caused by sudden displacement along faults, which rely on two primary conditions. One is that the fault has a high degree of synergism, so that once the stress threshold is reached, fault segments can be connected rapidly to facilitate fast slip of longer fault sections. The other is sufficient strain accumulated at some portions of the fault which can overcome resistance to slip of the high-strength portions of the fault. Investigations to such processes would help explore how to detect short-term and impending precursors prior to earthquakes. A simulation study on instability of a straight fault is conducted in the laboratory. From curves of stress variations, the stress state of the specimen is recognized and the meta-instability stage is identified. By comparison of the observational information from the press machine and physical parameters of the fields on the sample, this work reveals differences of temporal-spatial evolution processes of fault stress in the stages of stress deviating from linearity and meta-instability. The results show that due to interaction between distinct portions of the fault, their independent activities turn gradually into a synergetic activity, and the degree of such synergism is an indicator for the stress state of the fault. This synergetic process of fault activity includes three stages: generation, expansion and increase amount of strain release patches, and connection between them.. The first stage begins when the stress curve deviates from linearity, different strain variations occur at every portions of the fault, resulting in isolated areas of stress release and strain accumulation. The second stage is associated with quasi-static instability of the early meta-instability when isolated strain release areas of the fault increase and stable expansion proceeds. And the third stage corresponds to the late meta-instability, i.e. quasi-dynamic instability as both the expansion of strain release areas and rise of strain level of strain accumulation areas are accelerated. The synergism is accelerated when the quasi-static expansion transforms into quasi-dynamic expansion, with interaction between fault segments as its mechanism. The essence of such transformation is that the expansion mechanism has changed, i.e. expansion of isolated fault segments is replaced by linkage of the interacting segments when the fault enters the critical state of a potential earthquake. Based on the experimental results, coupled with data on the temporal-spatial evolution of earthquakes along the Laohushan-Maomaoshan fault, west of the Haiyuan fault zone in northwestern China, the synergism process of this fault before the 6 June 2000 M6.2 earthquake is analyzed.

 

Long-term studies of shear zones have included collection of data on fractures showing no indication of displacement which are termed as 'blank' fractures. A method aimed at mapping fault structures and stress fields has been developed on the basis of results of paragenetic analysis of measurements of abundant fractures. The method is termed as 'specialized mapping', firstly, due to its specific structural goal so that to distinguish it from the conventional geological mapping of regions in nature, and, secondly, because of the specific procedure applied to refer to fractures as references to decipher fault-block patterns of natural regions. In Part 1, basic theoretical concepts and principles of specialized mapping are described. Part 2 is being prepared for publication in one of the next issues of the journal; it will cover stages of the proposed method and describe some of the cases of its application.

In terms of general organizational principles, specialized mapping is similar to other methods based on structural paragenetic analysis and differs from such methods in types of paragenesises viewed as references to reveal crustal fault zones. Such paragenesises result from stage-by-stage faulting (Fig 2 and Fig. 7) during which stress fields of the 2nd order are regularly changeable within the shear zone. According to combined experimental and natural data, a complete paragenesis of fractures in the shear zone includes a major (1st order) fault plane and fractures of other seven types, R, R’, n, n’, t, t’ and T (2nd order) (Fig. 4 and Fig 8). At the fracture level, each of them corresponds to a paragenesis including three nearly perpendicular systems of early ruptures (Fig. 1), which are based on two classical patterns of conjugated fractures, one of which is consistent with the position of the fault plane (Fig. 3). Taking into account that strike-slip, reverse and normal faults are similar in terms of mechanics (i.e. they are formed due to shearing), standard patterns of fractures systems for their impact zones are members of the above described paragenesis of faults and fractures, which is spatially oriented in such a way that its position and displacements along Y-shears are correspondent to the right- or left-lateral strike-slip faults and also to normal and reverse faults with different dip angles. Under this approach, it has become possible to construct standard circle diagrams / patterns, each containing a complete set of fracture systems of one of the main types of fault zones (Fig. 6). In the process of specialized mapping, the patterns are compared with diagrams based on mass crustal fracture measurements taken on sites in the regions of studies. This procedure yields local solutions showing a presence of fault zones of specific types and spatial orientations; such solutions are shown as points at the corresponding sites on the schematic map of the territory under study, and points with similar paragenesises are then connected by lines so that to outline the boundaries of the revealed fault zones.

Besides construction of a schematic map of a fault structures, specialized mapping provides for identification of stress fields wherein elements of such a fault structure has formed or activated at some stages. With this goal, the identified fault zones are classified by ranks. At the first phase of such analysis, types and orientations of all the initial local solutions are compared with types and orientation of the members of the ‘ideal’ paragenesis of the 2nd order, which corresponds to a strike-slip, reverse (thrust) or normal fault (Fig. 8). This procedure reveals solutions showing the presence of fault zones varying in types and classified in the higher rank, which correspond to the regional stress field known form the history of the region under study. Such regional solutions are used as a basis for further iterations with reference to ‘ideal’ fault paragenesises, until possibilities to classify the fault zones into the fault networks of some specific types are exhausted. A few (typically, three to four) remaining solutions, showing orientations of the fault zone and the dynamic setting of its formation, are indicative of the lowest (regional or geostructural) level of the process of destruction in the region under study. Their simultaneous development is impossible, and therefore they correspond to different stages of faulting in the territory under study. Indirect (statistical) indicators of frequencies and angle ratios of fault systems and direct (apriory) information are used to determine ages and to reveal evolutional stages in time. At a final stage of specialized mapping, a reversed procedure provides for construction of schematic maps of fault zones for every main stage of formation of the structure under study. With this goal, faults that occurred or activated in a specified stress field are distinguished from the fault network.

In addition to the paragenesis principle applied to reveal fault zones and the evolution-in-time principle used to reveal stages of structure formation, the method of specialized mapping employs statistical methods of data collection and processing, and its application is consistent and computerized through all the work stages. It provides for solution of problems dealing with ‘blank’ fracturing with account of seemingly chaotic fracture patterns, local initial observations, uncertainties of age relations, impacts of structural and material inhomogeneities, and long timelines of statistical data collection and processing. In view of the above, specialized mapping can be proposed as one of the most efficient methods of studying the fault structure of the Earth’s crust.

Part 2 will describe cases of application of the proposed method to map fault zones and to identify fault types and stress fields varying in ages in the regions of faulting, including areas wherein rocks are poorly outcropped. The main results of application of the proposed method of specialized mapping is schematic maps of fault zones, showing the fault zones that were active at various stages of formation of the structure under study. Such maps can be used as a basis for finding solutions to the main problems of endo- and exogeodynamics as well as for assurance of structural control over mineral deposits associated with faulting.

 

The article presents results of tectonophysical methods applied to reconstruct tectonic stress field of the north-western flank of the Pacific Ocean seismic focal zone in the region wherein the 2011 Tohoku earthquake was prepared. The reconstructions are based on earthquake foci data for the time period before the catastrophic seismic event. The field of stresses, wherein the Tohoku earthquake focus was formed, had a high gradient along the dip of the seismic focal zone. It is revealed that the focus developed in the junction area of the crust segments with high and low levels of effective pressure. A wide area of lower effective pressure was located at depths close to 30 km, and it was the most susceptible to brittle fracture. In our opinion, the area impacted by the Tohoku earthquake is large due to a large length of the crustal segments with the high gradient of stresses, which are located along the eastern part of the crust of the Honshu Island.

The stress reconstruction also shows that the axis of the Japan oceanic trough divides the seismic focal zone into areas of horizontal compression (westward) and horizontal extension (eastwards). According to our calculations, lateral compression is the highest at the crustal depths up to 20 km westward of the trough’s axis, where maximum lateral compression axes are oriented orthogonally to the trough’s strike. Eastward of the trough’s axis, minimum horizontal compression axes are oriented orthogonally to the trough’s strike. At the crossing point of the Japan trough, a sharp changing of stress is by a factor of 5 to 8 of internal cohesion of rocks, τf. This sharp changing of stress is lower for the Izu-Bonin trough and varies from 3 to 5 τf.

 

Geophysical fields influenced by tectonics faults were observed, and instrumental observation results are analysed in the article. It is shown that fault zones are characterized by geophysical fields that are more variable than those in midmost segments of crustal blocks, more intense responses to weak external impacts such as lunar and solar tides and atmospheric pressure variations, and intensive relaxation. Transformation of energy between geophysical fields varying in origin takes place mainly in the fault zones.

The article reviews specific features of its fault-block structure of the Baikal-Yenisei fault in view of safe operations of nuclear energy facilities in the Krasnoyarsk region, Russia. The fault is located at the junction of the Siberian platform and the West Siberian plate. Velocities of neotectonic movements in the fault zone and adjacent territories are estimated from data on the current positions of fluvial terraces of the Yenisei river valley, peneplanation planes varying in ages and erosional incision depths. It is revealed that maximum gradients of recent movement velocities vary not higher than 10–8 to 10–9. Average velocities of relative displacements amount to 0.1–0.2 mm per year for intra-fault blocks bounded by regional faults and do not exceed 0.02–0.03 mm per year for intra-fault blocks bordered by local faults. There are grounds to conclude that recent geodynamic activity in the zone of the Baikal-Yenisei fault zone is weak and thus does not affect the safety of nuclear energy facilities operating in the region, including FGUP GKhK (Mining and Chemical Combine).

Field experiments were carried out using TRIBO, a specially designed testing stand including a concrete plate that can be moved at different rates. In our experiment, the plate served as an artificial allochtonous wing placed at the uneven surface of the segment of the Angarsky fault in Pribaikalie. Tribological effects of contact interaction of the uneven surfaces in the zone of sliding movements of the plate were recorded by strain gauges, linear displacement gauges and four Baikal-7HR seismic stations; such stations are commonly used for earthquake recording. The effect of shocks in initiation of seismic oscillation sources was studied with changes of the regimes of destruction of the uneven surfaces (underneath the base of the plate) which differ in size and strength. The study was focused on stages in the process of friction at preparation to transition from quasi-regular decelerated sliding movement of the plate to its breakaway and occurrence of a high-energy seismic impulse.

The applied method of large-scale modelling at natural objects in field provides new data that may prove useful for stu­dies of mechanisms causing seismicity, identification of stages in occurrence of earthquakes in fault zones and interpretation of seismic monitoring data. Results of such physical tests can contribute to the development of methods aimed at forecasting of rock shocks and earthquakes and also for the development of new physical models showing formation of earthquake foci of various scales in tectonic faults.

Recent deformation processes taking place in real time are analyzed on the basis of data on fault zones which were collected by long-term detailed geodetic survey studies with application of field methods and satellite monitoring.

A new category of recent crustal movements is described and termed as parametrically induced tectonic strain in fault zones. It is shown that in the fault zones located in seismically active and aseismic regions, super intensive displacements of the crust (5 to 7 cm per year, i.e. (5 to 7)·10–5 per year) occur due to very small external impacts of natural or technogenic / industrial origin.

The spatial discreteness of anomalous deformation processes is established along the strike of the regional Rechitsky fault in the Pripyat basin. It is concluded that recent anomalous activity of the fault zones needs to be taken into account in defining regional regularities of geodynamic processes on the basis of real-time measurements.

The paper presents results of analyses of data collected by long-term (20 to 50 years) geodetic surveys in highly seismically active regions of Kopetdag, Kamchatka and California. It is evidenced by instrumental geodetic measurements of recent vertical and horizontal displacements in fault zones that deformations are ‘paradoxically’ deviating from the inherited movements of the past geological periods.

In terms of the recent geodynamics, the ‘paradoxes’ of high and low strain velocities are related to a reliable empirical fact of the presence of extremely high local velocities of deformations in the fault zones (about 10–5 per year and above), which take place at the background of slow regional deformations which velocities are lower by the order of 2 to 3. Very low average annual velocities of horizontal deformation are recorded in the seismic regions of Kopetdag and Kamchatka and in the San Andreas fault zone; they amount to only 3 to 5 amplitudes of the earth tidal deformations per year.

A ‘fault-block’ dilemma is stated for the recent geodynamics of faults in view of interpretations of monitoring results. The matter is that either a block is an active element generating anomalous recent deformation and a fault is a ‘passive’ element, or a fault zone itself is a source of anomalous displacements and blocks are passive elements, i.e. host medium. ‘Paradoxes’ of high and low strain velocities are explainable under the concept that the anomalous recent geodynamics is caused by parametric excitation of deformation processes in fault zones in conditions of a quasi-static regime of loading.

Based on empirical data, it is revealed that recent deformation processes migrate in fault zones both in space and time. Two types of waves, ‘inter-fault’ and ‘intra-fault’, are described. A phenomenological model of auto-wave deformation processes is proposed; the model is consistent with monitoring data. A definition of ‘pseudo-wave’ is introduced. Arrangements to establish a system for monitoring deformation auto-waves are described.

When applied to geological deformation monitoring, new measurement technologies are associated with result identification problems, including ‘ratios of uncertainty’ such as ‘anomaly’s dimensions – density of monitoring stations’ and ‘anomaly’s duration – details of measurements in time’. It is shown that the RSA interferometry method does not provide for an unambiguous determination of ground surface displacement vectors.

 

Reviewed are aspects of modern geodynamics and methods of mapping of geodynamic processes which have been developed since 1960s in the Institute of the Earth's Crust, Siberian Branch of RAS (specifically since 1980s by the Laboratory of Tectonophysics and now jointly with the Laboratory of Recent Geodynamics). Achievements and prospects of the studies are discussed. The publication is devoted to the 35th anniversary of the Laboratory of Tectonophysics and its main achievements in geodynamics.

The article is devoted to the 80th birthday of Professor Semen I. Sherman, the founder of the Laboratory of Tectonophysics in IEC SB RAS, an expert in faulting, the state of stresses, geodynamic activity and seismicity of the lithosphere, and Deputy Chief Editor of Geodynamics & Tectonophysics.