Izvestiya, Physics of the Solid Earth最新文献

For fifty years, scientists from different countries in different regions of the Earth, using direct and indirect methods, discovered the migration of crustal deformation and earthquakes, and revealed its wave nature, and therefore proved the reality of the existence of slow strain waves in the Earth. This review presents a brief history of the development of the concept of strain waves in the Earth, the observation methods and properties of strain waves, and main types of the geological structures generating these waves, as well as the most prominent results of the theoretical, laboratory, and in-situ observations of slow strain migration.

Abstract—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.

Fracture process in crystals begin from the formation of tiny (“primary”) cracks. Larger cracks are formed when these primary cracks unite. To register primary cracks that appear on the surface of a nepheline crystal during the destruction by diamond microcrystals, the fractoluminescence method is used. The fractoluminescence spectrum consists of three bands: 1.4, 1.68, and 1.98 eV. The 1.98-eV band corresponds to excited free radicals ≡Si–O^•, 1.68 eV corresponds to excited Fe^3+• ions, and the 1.4-eV band appears when empty traps are filled with electrons from the conduction band. These radicals, ions, and traps appear during the fracture of nepheline lattice cells and are located on the surface of “primary” cracks. The time dependences of the fractoluminescence signals are sets of separate signals with a duration of about 86 ns. The interval between the signals varies from 0.1 to 1 μs. The nepheline crystal has a hexagonal system and six systems of dislocation slip planes. At the intersection of these planes, six barriers are formed, which prevent the movement of dislocations. The breaking of each barrier causes the appearance of a primary crack and the formation of a peak in the fractoluminescence signal. When six barriers are broken, clusters are formed from the same number of primary cracks. Therefore, fractoluminescence signals contain six maxima. First, the largest crack appears. Its dimensions range from approximately 9 to 17 nm. The growth time of such crack is about 16 ns. The remaining (smaller) cracks have sizes 1.7 to 3.0 times smaller. The size distribution of cracks follows a power law with an exponent equal to 6.

In light of the ongoing global climate changes, the timely study of cryolithozone objects is crucial to prevent potential natural and man-made disasters. Geophysical methods are also widely used to investigate permafrost strata. The transformation of sounding data into apparent electrical resistivity (AER) is a common procedure for electromagnetic methods for exploring the geological environment and allowing one to quickly obtain general information about its structure. The measurement system for pulsed electromagnetic monitoring of the cryolithozone that is discussed in the article consists of a set of field sources and receivers that are mounted inside nonconductive housings and immersed in two different wells. A method has been proposed for converting the pulsed sounding data into apparent resistivity for all recording times. The transformation algorithm is based on selecting a resistivity of a homogeneous conducting half-space so that the signal for this resistivity corresponds to the measured signal. To develop the algorithm, the behavior of signals was studied and their transformations in half-spaces with an arbitrary resistivity were plotted. Examples are provided to determine apparent resistivity in models of the thawing upper layer of frozen rock at different distances between wells. It has been shown that at early times, when the signal reaches its maximum value and becomes measurable, the apparent resistivity provides a qualitative description of the geoelectric model, while the resistivity of the thawed layer can be accurately determined. The obtained values of the apparent resistivity are necessary for understanding the depth of thawing and allow the development of a reliable starting model for the subsequent inversion of pulsed sounding data with precise spatial localization of the boundary between frozen and thawed rocks.

Abstract—To extend the capabilities of using records of local earthquakes (for constructing regional ground motion prediction equations, assessing seismic hazard, etc.), the classification of seismic stations in the North Caucasus by the ground conditions was performed. A technique has been developed that allows assessment of ground conditions by comparing spectra of weak earthquakes selected in narrow ranges of magnitudes and hypocentral distances, at different stations. The use of machine-learning methods showed the complexity of the problem, but at the same time, the application of logical operations and techniques allowed us to find the most effective approaches to solve it. As a result, 70 seismic stations of the North Caucasus were classified according to the ground conditions; the conditions were characterized by one dimensionless parameter based on the calculation of spectral characteristics. We are planning to refine the estimates in the future.

Abstract—Neural networks (NNs) are successfully used to solve inverse and other problems in geophysics. The aim of this work, which is a continuation of a series of works by a group of authors, is to improve the efficiency of the NN method for solving nonlinear inverse 3D problems of geoelectrics, based on the construction of the author’s convolutional neural network. The network includes a number of additional special transformations (data compression, suppression of the influence of an unknown background environment, etc.) preceding the training of a classical MLP neural network and adapted to the inverse problem that is being solved. This allows us to formally, excluding the human factor, solve inverse problems of geoelectrics of large dimensions without specifying a first approximation based on data measured in areas whose dimensions exceed the dimensions of the network training area. The inversion speed is a few tens of seconds and does not depend on the physical dimensionality (2D or 3D) of the data. The solution to the inverse problem found using a trained neural network can, if necessary, be refined using a random search method. Numerical results of solving 3D geoelectric problems on model and field data are presented, confirming the stated development parameters.

We consider the December 27, 2023, earthquake (mb = 5.4) that occurred on the margin of the Siberian Platform, on the northwestern slopes of the Akitkan Ridge. The earthquake epicenter is spatially associated with a structural suture (deep thrust fault) separating the Siberian Platform and the Baikal fold belt. The seismic event was followed by hardly any aftershocks. Its maximum shaking intensity was IV (MSK-64); it was observed at distances up to 180 km. The December 27, 2023, Akitkan earthquake is localized in a previously aseismic region, far from active areas of the Baikal Rift. It suggests a new look at seismic activity of fault structures bordering the Siberian Platform. The focal mechanism, determined from P-wave first-motion polarities at regional stations, demonstrates normal fault movements on inclined fault planes with a submeridional strike, which agrees with the orientation of the structural suture. This does not contradict seismogeological data indicating that an inversion of tectonic movements can be observed in some segments of the deep thrust fault zones. The December 27, 2023, Akitkan earthquake confirms modern seismic activity of the Akitkan seismic source zone and the fundamental possibility of relatively strong seismic events being generated by marginal structures of the Siberian Platform.

The rheology of the Earth’s mantle is studied on the basis of data on the Love numbers for ten tidal components (M2, Mqm, Msqm, Mtm, Mstm, SN, Mf, Msf, Mm, and Msm). An adaptation of the Preliminary Reference Earth Model (PREM) is used to model the internal structure of the Earth, while the inelasticity is modeled using the Andrade rheology. The Andrade model depends on two empirical parameters (α and ζ) that are unknown for mantle minerals at high pressures and temperatures and very slow deformations. As a result, in various problems of planetary geophysics where inelasticity in the interior of planets or satellites must be taken into account, authors are often faced with the difficulty of which values of the Andrade rheology parameters to use. To address this issue, an Earth-based calibration of the rheology was performed. The Love numbers of the Earth were calculated at ten tidal frequencies for two viscosity distributions and for 1530 different combinations of the parameters α and ζ. The comparison of the model values with the observed ones allowed us to determine a set of values for α and ζ that are suitable for describing the inelasticity of the Earth’s mantle.

In this study, we consider in detail the July 13, 2023, earthquake that occurred on 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 of 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 km), 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.

A new of fundamental approach to solving inverse geophysics problems using the linear integral representation method and discrete gravity potential theories is proposed. This approach makes it possible to reconstruct the masses, which can be regarded as arbitrary distributed points in two- and three-dimensional network spaces.