No 6 (2024)

One of the key problems in the search for electromagnetic precursors of earthquakes is the possibility of separating magnetospheric and seismogenic disturbances. This paper presents the results of using a model that enables us to calculate the ultra-low-frequency (ULF) fields on the Earth’s surface created by a linear horizontal current of finite length. This model simulates the occurrence of mechano-electric transformers during a shift along a fault zone at the final stage of the earthquake preparation. The calculations show several characteristics of the field of the underground source in comparison with the field of ionospheric disturbances. If the vertical component B_z of the magnetic field of an ionospheric disturbance is small compared to the horizontal component $b_0,$ then for an underground source $b^{*}$ in the vicinity of the source. For ionospheric sources, this apparent impedance (i.e., the $b_0^{*}.$ ratio) coincides with the impedance of the Earth’s surface Z_g, while the impedance of disturbances created by the lithospheric source may exceed Z_g, up to order of magnitude in the source vicinity. An underground current source can create a vertical electric field E_z of significant magnitude. This is due to the vertical current continuity at the Earth–atmosphere interface, which acts as a powerful “amplifier” with a coefficient determined by the ratio of the complex conductivities of the Earth’s crust and air. Calculations have shown that these ideas are incorrect. The vertical component E_z on the Earth’s surface is of the same order of magnitude as the transverse component $\vartheta$ There have been suggestions to use short-baseline gradient measurements to reduce the contribution of large-scale ionospheric disturbances. The calculation of the field structure has revealed that amplitude-phase gradients in the vicinity of an underground source are highly variable and may provide ambiguous results.

New generalized data on the attenuation of seismic waves in the lithosphere of the North Caucasus were obtained using the frequency-dependent quality factor of the medium Q_s(f). Knowledge of the heterogeneities of the quality factor distribution as a characteristic of the environment in the region is necessary when carrying out seismic zoning work of varying degrees of detail. The information base for the study comprised digital records of 53 seismic stations of 800 local earthquakes with moderate magnitudes (1.8 ≤ M ≤ 5.5), evenly distributed throughout the North Caucasus. The study used the coda-wave envelope method in the single scattering model (CodaQ). For the territory of the North Caucasus and for seven individual zones, average analytical expressions of the frequency-dependent quality factor of the medium Q_s(f) were calculated and maps of the distribution of quality values at frequencies of 1 and 4 Hz were compiled. It was revealed that the zones with the lowest quality factor correspond to tectonically heterogeneous regions characterized by the presence of strong fragmentation in the crust and an increased level of fluid saturation. The zones of the highest quality factor correspond to zones of lithospheric extension, where earthquakes with normal-fault focal mechanisms predominate.

A technique for quantitative estimation of characteristic grain sizes in laboratory rock samples using the relationship between the frequency and attenuation of longitudinal ultrasonic waves in the samples is proposed and implemented experimentally. This relationship is quantified using broadband optoacoustic spectroscopy with a laser source of ultrasound and piezoelectric registration of nanosecond ultrasonic pulses in the operating frequency range of 1–70 MHz. The application of the theoretical model of ultrasound scattering in single-phase polycrystalline materials to quantitative estimation of the maximum and average grain sizes in multiphase rocks is shown using five samples of metasandstones of zonally metamorphosed Ladoga series of the Paleoproterozoic of the Baltic Shield, which underwent different degrees of structural and textural transformations during ancient metamorphic events. The reliability of the data obtained using broadband optoacoustic spectroscopy was for the first time confirmed by independent scanning electron microscopy of the polished surfaces of all samples. The average and maximum grain sizes were estimated separately using the conventional method of line crossing from optical micrographs of thin sections performed for two selected samples, which also showed good agreement with the acoustic spectroscopy data. The proposed method of broadband optoacoustic spectroscopy for estimation of characteristic grain sizes of laboratory rock samples can be used to analyze the possible relationship between their structural features and thermobaric conditions of formation.

In this study, we consider in detail the July 13, 2023, earthquake occurred of the shelf of the eastern Laptev Sea (Belkov–Svyatoi Nos rift). On the one hand, our interest in this event is due to the location of its epicenter, to the east of which there is a sharp decrease in seismic activity. Conversely, detailed Common Depth Point (CDP) data on the structure o the upper crust are available for its epicentral zone, making it possible to analyze the seismotectonic position of the earthquake source. Focal parameters in the instantaneous point source approximation are calculated from surface waves recorded at teleseismic distances. As a result, we have obtained a scalar seismic moment (M₀ = 9.8*10¹⁶ N · m), corresponding moment magnitude (M_w = 5.3), source depth (h = 8 км), and focal mechanism (a normal fault along a gently dipping nodal plane with a NW–SE strike). Our results are compared with data from seismological agencies. It has been shown that differences between them are most likely caused by various initial data, including their different frequency ranges. Our estimates agree better with the available geological and geophysical information on the tectonics of the study area. Taking into account the data on strike, dip, and penetration depth of faults and our source parameter values, we have concluded that the July 13, 2023, earthquake could have been associated with a major listric normal fault on the western slope of the Belkov–Svyatoi Nos rift.

The article presents the results of a study of the crust and upper mantle velocity structure in the central and Arctic parts of the Kola region from the receiver function and surface wave tomography. Significant heterogeneity of the upper mantle was revealed. An increase in the thickness of the crust from north to south is shown, from values of about 33 km in the Murmansk block to 40 km in the Belomorian block. Within the Kola and Belomorian blocks, a layer of lower shear wave velocities was identified at depths of about 90–140 km, probably marking the mid-lithospheric discontinuity (MLD). This layer has not been identified in the Murmansk block. The obtained two-dimensional maps of the distribution of shear wave velocities at depths up to 500 km do not reveal the sublatitudinal zoning traced in the tectonic structure of the Kola region.

A Bloch mode is a vector field that is the product of a three-dimensional field of the flow periodicity, and a Fourier harmonic $e^{iq\cdot x}$ for an arbitrary wave vector q. Previous computations showed that the modes whose growth rates are maximum over all vectors q are arranged in families, which are smoothly parameterised by the molecular magnetic diffusivity. In some families, the growth rates assume the maximum for the so-called half-integer q, whose all components are integer or half-integer, and q is constant for the entire family. From such families, other families can stem, in which the optimal q of the modes varies smoothly over a family. For the modes comprising such offshoot families, the associated eigenvalues of the magnetic induction operator and the optimal q, we construct here asymptotic expansions in power series in the parameter $\vartheta ={({\eta }_0-\eta )}^{1/2},$ where ${\eta }_0$ is the magnetic diffusivity for which the branching occurs. In this paper, we assume that the modes in the family undergoing the branching involve a constant non-zero half-integer wave vector q. The asymptotic expansions differ significantly from the similar expansions that we constructed earlier for the branching from a family of short-scale (i.e., for $q=0)$ neutral (associated with a zero eigenvalue of the magnetic induction operator) magnetic modes generated by a parity-invariant flow.