Vol 52, No 6 (2016)

For the first time, an attempt is made to apply the data on the lithospheric magnetic anomalies of the Earth for determining the areas prone to strong earthquakes by means of the pattern recognition algorithms. The Caucasian region with the threshold magnitude of the strong earthquakes M₀ = 6 is considered. It is established that the data on the lithospheric magnetic anomalies are informative from the standpoint of recognizing the strong earthquake prone areas. Application of these data is promising for solving the similar problems for different seismically active regions.

The complex seismotectonic studies of the pleistoseist area of the Ilin-Tas earthquake (M_s = 6.9), one of the strongest seismic events ever recorded by the regional seismic network in northeastern Russia, are carried out. The structural tectonic position, morphotectonic features of present-day topography, active faults, and types of Cenozoic deformations of the epicentral zone are analyzed. The data of the instrumental observations are summarized, and the manifestations of the strong seismic events in the Yana–Indigirka segment of the Cherskii seismotectonic zone are considered. The explanation is suggested for the dynamical tectonic setting responsible for the Andrei-Tas seismic maximum. This setting is created by the influence of the Kolyma–Omolon indenter, which intrudes into the Cherskii seismotectonic zone from the region of the North American lithospheric plate and forms the main seismogenic structures of the Yana–Indigirka segment in the frontal zone (the Ilin-Tas anticlinorium). The highest seismic potential is noted in the Andrei- Tas block—the focus of the main tectonic impacts from the Kolyma–Omolon superterrane. The general trend of this block coincides with the orientation of the major axis of isoseismal ellipses (azimuth 50°–85°), which were determined from the observations of macroseismic effects on the ground after the Uyandina (M_s = 5.6), Andrei-Tas (M_s = 6.1), and Ilin-Tas (M_s = 6.9) earthquakes.

The regularities are revealed which, based on seismic statistics, govern the evolution of the dynamics in the preparation of strong earthquakes within discrete closed volumes of the lithosphere (seismic systems, SSs). This concept was previously formulated as a phenomenological approach based on seismic entropy for real-time monitoring the strong earthquakes. The seismic parameters of cumulative energy and entropy contain the energy–time memory about the previous earthquakes within the considered volume, which allows the real-time description of the nonequilibrium spatiotemporal dynamics of the active volume of the Earth’s lithosphere. The power law relating the information entropy and cumulative energy is theoretically derived, which supports the previously established empirical relationships. In the SSs, the strong earthquakes pertaining to a certain magnitude interval are united into the ensembles forming the seismic cycles which periodically restore the equilibrium state of the system. With the lapse of time, the strong earthquakes of the ensemble tend to fill up the SS. The configuration, the size of the SS, and the minimal parameter of action lead to the selectivity and discreteness of the notion of magnitude of an earthquake, depending on the scale of the system. Each SS has a certain minimal threshold magnitude above which the Gutenberg–Richter law is violated, whereas the indicator earthquakes, governed by the Poisson distribution within the scope of the entropy production law, play a crucial role in the preparation of the strong earthquakes. Some results are illustrated by the example of the recent strong earthquakes in Central Kuriles.

The results of studying the magnetic properties of the components of the natural environment of the Barabinsk forest steppe accommodating the archaeological monuments dated to VI century B.C. to II century A.D. are presented. The rock magnetic model is constructed for a two-layer medium composed of modern soil and an underlying blanket substratum deposits (clay loam, sandy loam, sand). It is shown that the magnetism of the humic horizon of the soil depends on the topographic position and soil type, whereas the magnetic characteristics of the underlying substratum deposits are persistent throughout the studied territory. The ratio of the magnetic susceptibilities of the humus horizon to the substratum is K = 3–6 for the automorphic soil, K = 1–2.5 for semihydromorphic soil, and K = 0.8–1.0 for hydromorphic soil. Based on subdividing the studied territory in accordance with the values of K and the calculated amplitudes of the microanomalies of the magnetic field, the areas where application of the magnetic survey is highly promising, moderately efficient, and unpromising are outlined. The particular cases of the absence of magnetic anomalies above the archaeological objects, the false anomalies, and the chaotic patterns of the low-amplitude anomalies—all the situations when the magnetic survey is inefficient—are explained from the standpoint of the model.

The archaeomagnetic studies of ceramics from the Hermonassa multilayer archaeological monument in the Taman Peninsula provided the data on the intensity of the main geomagnetic field in the past. The data for the interval from VIII to XX centuries A.D. demonstrate a well pronounced decreasing trend in the geomagnetic field intensity during this time. Three stages, each lasting for a few centuries, are distinguished in the variations of the centennial average field which slightly varies within each stage and generally decreases from 70 to ~45 μT during the entire period from VIII to XX centuries A.D. The variations of the geomagnetic field in the interval from XII to XVII centuries A.D. have a form of quasi-harmonic oscillations with a characteristic time of about 300 years.

In the paper, the subduction of the South Caspian microplate beneath the continental lithosphere of Central Caspian is substantiated based on the GPS measurements, crustal density model, seismological data, and seismic section along the Elburs–Apsheron–Balkhan Sill regional profile. In this context, the geodynamical model is suggested for the formation and spatial distribution of the oil and gas fields in the South Caspian Basin (SCB). It is shown that the heterogeneous oil-and-gas saturation of the sedimentary section within the South Caspian Depression (SCD), which was revealed by the long-term prospecting and exploration works, is, according to the suggested model, controlled by the subduction zone. The developed geodynamical approach to the problem of the formation of hydrocarbon deposits in the South Caspian Basin calls for rethinking the strategy of further prospecting for hydrocarbons in this region.

Plate tectonics only allows small deformations in the lithospheric plates. The laboratory experiments with the rock specimens show that the creep is transient when the creep strain is at most 1%. Hence, if we assume that the creep strain in the lithospheric plates is below this threshold, the creep is transient. The present paper addresses the role of the elastic, brittle (pseudo-plastic), and creep rheology of the lithosphere during the accumulation of elastic shear strains on the locked faults in the Earth’s crust, i.e., during the process of preparation of the earthquakes. The effective viscosity characterizing the transient creep is lower than that under the steady-state creep and it depends on the characteristic time of a given process. The characteristic duration of the stress and strain accumulation process in the vicinity of the locked faults is a few dozen years. On these time intervals, the thin upper crustal layer behaves as brittle; the underlying layer behaves as elastic (it is just this layer which accommodates stress accumulation leading to the earthquake), whereas the transient creep is predominant in the lower crust and mantle lithosphere. Transient creep entails nonlinear time dependence of the strains arising in the vicinity of the locked fault in the elastic crust. The perturbations in the magnetic field induced by these strains can be treated as the magnetic precursor of the earthquake.