No 2 (2019)

The group velocity dispersion curves of the Rayleigh and Love waves are constructed along the paths intersecting the European continent in the interval of periods from 10 to 100 s from the records of earthquakes and seismic noise. The radial anisotropy of the European upper mantle is estimated from these data. Primarily, along each path, the average velocity sections of the SV- and SH-waves were calculated from the Rayleigh and Love wave data, respectively. Based on these sections, the average anisotropy coefficient was determined for each path in four depth intervals (the crust + three 30-km upper mantle layers). These results were used for identifying variations in lateral anisotropy in the studied region based on tomographic inversion. This approach eliminates different degree of smoothness of lateral variations of the SV- and SH-waves when these variations are determined separately from the Rayleigh and Love waves: in this case, the anisotropy coefficient can have large errors due to the different sets of the paths. The resolution of the data used for tomography was estimated by a “checkerboard test,” which demonstrated the possibility to resolve structural features with a linear size of 1200 to 1300 km in the central part of the studied area, i.e., approximately 15°–50° in longitude and 40°–65° in latitude. The tomographic inversion of lateral variations in the anisotropy coefficient shows that in the continental part of the studied region, the anisotropy coefficient at all depths in the upper mantle is zero within the error limits, whereas in the region of the Black and Baltic seas, it is positive and equal to 4–4.5% in the subcrustal mantle at the depths of 34 to 64 km. In the underlying layer in the Baltic Sea region, this coefficient is close to zero, whereas beneath the Black Sea Basin it remains positive albeit decreases to 2–3%. In the lowermost layer, anisotropy is not observed anywhere in the entire region; however, this can be due to the lack of the data for the large periods. Positive anisotropy (VSH > VSV) is typical of the oceanic areas, which can testify in favor of the oceanic hypothesis of the origin of the Black Sea basin.

The specificity of applying the approximation approach to solving the linear and nonlinear inverse problems of geophysics, geodesy, and geomorphology is discussed. Within the paradigm proposed by V.N. Strakhov, practically all geophysical problems can be reduced to the systems of linear (and, in some cases, nonlinear) algebraic equations. The method of integral representations is the main one for implementing this approach.
The application of various modifications of the method of linear integral representations in the spaces of arbitrary dimension is analyzed. Based on the combined approximations of the topography and geopotential fields, it is possible to determine the optimal parameters of the method for solving a broad range of inverse problems of geophysics and geomorphology and to most fully use the a priori information about the elevations and the elements of the anomalous fields. The method for numerical solving the inverse problem on the distributions of carriers of mass equivalent in terms of the external field in the ordinary three-dimensional space and in the four-dimensional space is described.

Alongside the determination of the focal mechanism and source depth of an earthquake by direct examination of their probable values on a grid in the parameter space, also the resolution of these determinations can be estimated. However, this approach requires considerable time in the case of a detailed search. A special case of a shallow earthquake whose one nodal plane is subhorizontal is an example of the sources that require the use of a detailed grid. For studying these events based on the records of the long-period surface waves, the grids with high degree of detail in the angles of the focal mechanism are required. We discuss the application of the methods of parallel computing for speeding up the calculations of earthquake parameters and present the results of studying the strongest aftershock of the Tohoku, Japan, earthquake by this approach.

Alternating intervals of normal and reversed polarity are revealed in the sections of two Permian–Triassic trap intrusions of the Ergalakhsky complex, Norilsk region. The near-contact zones of the intrusions are magnetized reversely, whereas magnetization in the central zones has normal polarity. The arguments are presented that this change in the polarity along the intrusions section is not due to the postmagmatic remagnetization or self-reversal of remanence but marks the reversal of the geomagnetic field that occurred during the emplacement of the intrusive bodies. Highly accurate age determination for the Ergalakhsky intrusions – the oldest intrusive trap complex in the Norilsk region – is vital for time correlation of the initial stage of magmatic activity. According to the paleomagnetic data, the studied sills intruded directly at the Permian–Triassic boundary at the very end of the Ivakinsky stage. The existing estimates for the durations of the reversals indicate that the cooling of the intrusions could last a few thousand years. In the future, the examined sills of the Ergalakhsky complex can be used as a unique object for exploring the structure of the geomagnetic field during the reversals, for reconstructing the thermal history of intrusions’ cooling, and as a reference for estimating the total duration of trap magmatism.

The archaeomagnetic studies carried out at the Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences (IPE RAS) provided an important contribution to the international studies of the main magnetic field of the Earth for the past few thousand years. Extensive data on the intensity of geomagnetic field in the past 8000 years were obtained. Four most representative and long time series of the data have been constructed for Eurasia for the Iberian Peninsula, the Caucasus, Central Asia, and Siberia. Unique studies having no analogues in international research have been carried out into rapid variations in the geomagnetic field intensity with characteristic times starting from several tens of years. Based on the analysis of the international data on the ancient geomagnetic field, the spectrum of the variations in the geomagnetic field intensity with the periods ranging from decades to millennia was established and the characteristics of
the variations whose superposition can describe the pattern of the changes of the geomagnetic field intensity were determined. It was found out that variations with different characteristic times have a differently directed drift, and the “main oscillation” with a characteristic time of 8000 years has an eastern drift.

An earthquake with magnitude 4.8 hit the vicinity of Mariupol close to the southern boundary of the East European Platform (EEP) on August 7, 2016. The main event was followed by the aftershocks with magnitudes ranging from 2.2 to 3.9 which lasted for five days. The region experiences external influence from the neotectonically active Alpine zone resulting in intraplate deformations, horizontal and vertical movements of the Earth’s surface, and seismicity. The sources of the main shock and aftershocks are located within the block bounded by the neotectonically active Maloyanisol, Kalmius, and Primorsky faults. A seismogenic structure traced by the submeridional Kalchik lineament zone is identified in the axial part of the block by the combined analysis of geological and geophysical data and visual interpretation of the satellite image. This neotectonically active zone hosts the epicenters of the main event and most of the aftershocks.

Analytical formulas for the tangential components of extremely-low-frequency (ELF) electromagnetic field in the Earth–ionosphere plane waveguide excited by a grounded linear horizontal antenna are obtained. The behavior of surface impedance is studied as a function of electrodynamic characteristics of the waveguide and the distance from the source. It is shown that surface impedance coincides with the plane wave impedance on the Earth’s surface at distances from the source larger than the skin depth provided that the skin layer is thinner than double the waveguide’s height. The influence of the ionosphere on the amplitude of the ELF and lower-frequency magnetic field and, thus, on the impedance at the distances shorter than two ionospheric heights is theoretically substantiated. This type of effect was observed in the experiments conducted on the Kola Peninsula where the low conductivity of the Earth allowed the detection of the effect of the ionosphere on the amplitude of the magnetic field in the low-frequency band.