SEISMIC BELTS AND ZONES OF THE EARTH: FORMALIZATION OF NOTIONS, POSITIONS IN THE LITHOSPHERE, AND STRUCTURAL CONTROL

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.