This publication is aimed at formalization of the notions of «seismic zone» and «seismic belt». A seismic zone (SZ) is a territory defined and contoured in a technically active area. Within the limits of this territory, more than 10 seismic event with М>3 (К>9) occurred in the specified period of time (typically, 50 years), or the number of seismic event is not below a certain statistically relevant value. The external contour of SZ should be drawn according to the isolines of the corresponding density of registered earthquakes with М≥3, pending no less than three events within the given square area. In each case, selection of contours of SZ should be determined so that it can provide for classification of SZs. SZ should correspond to one or several tectonic structures. The interior structure of SZ can be zoned according to densities of earthquake epicentres.

A seismic belt (SB) is a structure with a uniform geodynamic regime, wherein seismic zones are closely spaced. Typically, such structures are margins of plates or large intra­plate blocks. In real time, SB is generally characterized by a permanent state of lithospheric stresses. Stress vectors in local segments of SB may differ from the dominant type of stresses. They can be variable due to changes in strike of local and regional faults which control seismicity and also due to various directions of zones of the recent lithospheric destruction.

The Earth’s SBs and SZs are mapped. SBs and a number of most important SZs are briefly described. Main parameters of SBs and SZs are tabulated. Based on the available data on SBs and SZs and taking into account the common geodynamical settings and elongated localities of earthquake foci, we suggest that it is required to evaluate structural factors controlling the seismic process and its components (locations of earthquake foci) at all the hierarchic levels, i.e. seismic belts, seismic zones, fault zones wherein stresses are concentrated, and structures wherein earthquake foci are located. 

Due to differences in the structural factors of control and scaling of SB and SZ manifestation, criteria for occurrence of earthquakes of various magnitudes are significantly different. Rare catastrophic earthquakes in SB result from the evolution of inter­plate and large inter­block margins in the geochronological scale intervals and/or disturbances of the evolution regularities due to catastrophic seismic event in the adjacent SB. Developing tectonophysical models of SBs is a future challenge.

In SZ, earthquakes of medium magnitudes and rare strong seismic events results from the impact of strain waves on the mega­stable state of the recent lithospheric destruction zones which comprise the SZ structure. Time spans between seismic events in SZ are estimated in real time scales (decades, years, months) and thus can be considered instant in relation to periods of the geological evolution of inter­plate margins and other large structural margins (hundred thousand years, million years). In terms of the given time evaluation, the mega­stable state of the recent lithospheric destruction zones in SZ can be disturbed by factors of external impact in real time intervals, rather than by ‘the geological evolution’ factors.

In this publication, the Baikal SZ is selected for analyses and testing as one of the best studied zones. In future studies, similar tests can be done for other seismic zones. 

Spatial and temporal regularities of earthquake locations in the areas of dynamic influence of faults in SZ and results of studies to provide for tectonophysical modeling of SZ can be applicable for expanding possibilities of mid­term seismic forecasting. The research data in the present publication confirm strong arguments in favor of transition to quantitative classification of SZs, identification of faults which are active in real time and function as concentrators of earthquake foci, and evaluation of parameters of fault zones which determine space­and­time locations of earthquake foci.

This publication demonstrates the need to develop tectonophysical models of SPs and apply such models to gain a more comprehensive understanding of interactions/correlations between seismic zones in cases of catastrophic earthquakes and/or closely spaced SBs with similar states of stresses.

The stick­slip process of bending faults with one angle change of 5° at the connection location between the two line fault segments is investigated in this paper. The dynamic process and physical field evolution were observed in the laboratory, and measurements were recorded with application of fault displacement measurement, strain tensor analysis and acoustic emission (AE) techniques. The experimental results from the study of bending faults give grounds for the following primary conclusions: (1) It is clearly revealed that there is a negative correlation between the logarithms of the stick­slip cycle and loading rate; (2) With different loading rates, most of the instabilities from bending faults occurred as earthquake doublets. It means that an instable event can contains two sub­events which occur with an interval of time from 100 ms to 200 ms. (3) With application of different observation approaches, regardless of the fact that the sampling rates were the same, differences of the co­seismic response were observed. For instance, strain measurements indicated significant strain weakening stage, but fault displacement was not significantly changed before fault instability. (4) Experimental studies of high sampling rates contribute to a better understanding and awareness of earthquake precursors and the seismogenic process; such studies are helpful in analyzing the mechanism of strong earthquake processes and parameters of aftershocks.

Deformation structures which occur in rock masses in horizontal shear zones are studied with application of theoretical tectonophysical methods of analysis, including mathematical simulation of stresses. The paper presents results of such studies for cases with the rheology of geomedium represented by an elasto­cataclastic body. Taking into account that above the yield stress (in this case, not true plastic dislocation, but cataclastic, i.e. fracturing), results of deformation and the morphology of fractures are dependent on loading modes, in the present study it is proposed to consider the gravity stress state (hereafter GSS) as the initial state of stresses, maintaining deviatory components. Equations based on criteria of the theory of plasticity are presented; they provide for calculation of depths of GSS transition from pure elastic deformation to elastocataclastic deformation. It is shown that for hard and consolidated rocks of the upper crust outside fault zones, the creeping mechanism is associated with cataclastic flow, rather than with dislocations in crystals and grains; this predetermines the dependence of deviatory stresses on isotropic pressure and maintenance of such stresses at a specific level in the rock mass. For the object under study, fracturing occurs at the initial phase of loading under the impact of GSS. Fractures continue to develop during horizontal displacement which is quazi­homogeneous through depth and laterally. The formation of the structural ensemble of fractures is finalized after completion of a long­term phase of displacement of blocks of the crystalline base, i.e. after the phase of localized displacement. By theoretical analyses of the evolution of the state of stresses and the morphology of deformation structures, it is revealed that numerous fractures with shear component are present deep in the middle part of the rock mass; such fractures occur both at the initial, gravity loading phase and during phases of uniform and localized horizontal displacement. Fractures with shear components are mainly formed in the upper part and close to the axial, deep part of the profile. The analysis presented in this publication is necessary for correct reconstruction of loading mechanisms of geological objects. It can also be applicable in exploration geology for forecasting of high fracturing areas of specific morphology.

Introduction. In this publication, shear zones, being traditional objects of tectonophysical studies, are considered in terms of their strain states. This approach differs from a commonly applied one when shear zones are studied with consideration of stress fields. The difference of a stress field and a field of strain for a simple shearing has been already noted by the researchers (Figure 1). As is known, secondary fractures in natural shear zones and in experiments do not always correspond to structures which are theoretically predicted by stress field studies. The problem under investigation in this publication is which combinations of secondary structures are possible/impossible in specific emerging strain fields?

Initial concept. The theoretical basis is the well­known scheme of secondary fractures proposed by P. Hancock [1985]. His representation of combinations of structures (Figure 2) is arbitrarily compiled: some of the secondary fractures (such as thrusts and normal faults) can not exist simultaneously as this leads to opposite deformation results (Figure 3).

Theoretical consideration of 2D strain in a shear zone. As a priority, all cases of elongation and shortening of the zone are theoretically studied in the constant volume of the zone. In previous studies, the situation was considered with additional compression or tension in the direction perpendicular to the shear zone (Figure 4), but not with elongation or shortening of the shear zone. The analysis of the strain state of the shear zone revealed that development of Riedel shears of R and R′ types (which are paired and identical in the stress field of pure shearing) can lead to opposite results in deformation of the zone. Shear cracks of R type cause elongation of the zone and reduction of the zone’s width (Figure 5). Shear cracks of R′ type can occur with shortening of the zone and increase in its width (Figure 6). Shear cracks of X and P types (which are also paired) demonstrate similar behavior: X cracks occur with lengthening of the zone, while P cracks occur with its shortening. Cracks of Y type, which go parallel to the zone, can be observed in both cases. Influence of increase or reduction of the shear zone’s volume on possible combinations of structures, including tension fractures and stylolithic fractures, is also considered. Combinations of secondary fractures revealed by the theoretical studies are tabulated (Table 1); six cases are distinguished with regard to active, possible and impossible structures. GEODYNAMICS & TECTONOPHYSICS PUBLISHED BY THE INSTITUTE OF THE EARTH’S CRUST SIBERIAN BRANCH OF RUSSIAN ACADEMY OF SCIENCES Tectonophysics

Examples of combinations of secondary fractures in experiments and natural structures. Examples of echelon structures are considered in terms of the strain state of shear zones. In experiments, alternations of domains, wherein shear cracks of R and R′ types are developing along shear zone, are interpreted as a combination of domains with elongation and shortening of the medium (along the strike of the zone), while the total length of the zone remains unchanged (Figure 7). It is assumed that variations of widths of zones of influence of faults, that are observed in natural structure, and changes of amplitudes of displacement in seismogenic faults (Figure 8) are related to this phenomenon of alternation of domains wherein shear cracks of R and R′ types are developing, i.e. there is a relation to elongation and shortening of such domains of a fault zone. Structures of terminations of large faults of ‘horse­tail’ and ‘fish­bone’ types are interpreted as domains wherein shear cracks of R and R′ types develop as secondary faults under conditions of lengthening and shortening of the sides of the main fault (Figure 9). It is shown that shatter zones in the basalt detachment of the Vorontsovsky nappe are related to shear cracks of R type; they evidence elongation of the nappe’s body (Figure 10). In the scale of the given outcrop, a number of specific combinations of share cracks of P type and tension fractures are reviewed (Figure 11, 12, 13, 14, and 15). Structures with development of shear cracks of X type are specified; these are synthetic faults in the body of the landslide and echeloned normal faults in sides of regional shear faults in petroliferous structures of the Western Siberia (Figure 16).

Theoretical research of zones of simple shearing in a massif which is subject to general deformation of pure shearing. Simple shearing zones, which are located in massifs which are subject to pure shearing, are a target of special theoretical studies. Under such conditions of the massif’s deformation, the length of shear zones in the massif will either increase or decrease, depending on  orientations of such zones relative to the axis of shortening (Figure 17). Assumptions of possible combinations of secondary fractures in such shear zones are made.

onclusions. It is established that in a shear zone, cracks of R and R′ types can not develop in one domain as they lead to opposite deformation consequences. However, this has not been taken into account when describing shear zones in terms of stress fields. Concerning emerging deformations of a shear zone, it is revealed that cracks of R and X types are paired (in case of zone’s elongation), and cracks R′ and P types are in the opposite pair (case of zone’s shortening). The table of theoretically possible and impossible secondary fractures is compiled for a variety of deformation conditions of a shear zone. The problem of collecting data on stable combinations of echelon secondary structures, that occur in shear zones, and developing a systematic review of such combinations on the basis of concepts of the strain state of the shear zones is put forward. It is proposed to apply changes of shear zone length in modeling of these structures on equivalent materials.

This publication presents new data obtained with CDP, gravimetry and magnetometry methods, showing the structure of the offshore margin of the Northern Sakhalin further from the Okhinsky bridge and the Schmidt peninsula, and results of geological interpretation of these data. The study coves: 1) The Trekhbratskaya anticline zone and the mega­dyke with the same name; 2) The East­Sakhalin trough filled in with the Late Cenozoic, mainly the Pomyrskie and Deryuginskie sediments, debris from the Amur river and products of abrasion which occurs at offshore folded structures / bench; 3) The near­bottom gas hydrates and indicators of gas saturation of the Pomyrskie and Deryuginskie sediments. New problems were identified concerning tectonic, magmatic and sedimentation history of the offshore margin and the adjacent structures of the Sakhalin cordillera and the bathyal Deryugin basin.