Izvestiya, Physics of the Solid Earth最新文献

Abstract—Tectonic faults are characterized by a heterogeneous structure, which determines the spatial variation of their deformation modes from aseismic creep and slow-slip events to dynamic ruptures, which are the sources of earthquakes of varying magnitudes. Based on a comprehensive analysis of geological and geophysical information, the authors investigated the features of localization of the deformations and slip modes along a 160-km section of the collision suture between the Siberian Craton and the Olkhon Terrane from the settlement of Buguldeyka to the village of Kurma. The width of the zone of the most intense deformations within the suture varies from 100 to 500 m in its different segments, while estimates of its width based on electric resistivity tomographic measurements and petrographic studies of rock samples taken from exhumed sections of the suture are comparable. Analysis of the material composition and frictional behavior of the collected samples showed that the fault segments with the narrowest width comprise rocks with the property of velocity weakening and are nucleation zones of strong earthquake foci. The typical length of such segments is about 10 km, and the distance between them is about 60 km. Segments hundreds of meters wide are also distinguished along the fault, comprising rocks with the property of velocity strengthening, where accumulated stresses relax through slow slip and aseismic creep.

Abstract—Until recently, the model of the interior structure of a planet was specified based on the solution of the direct problem with input data on the planetary gravitational field (mass, moment of inertia, tidal Love numbers k₂) and the presumed geochemical composition of the planet. To reconcile the different model parameters with the observed quantities, it is important to solve the inverse problem. One of the goals of this study is to design and implement a computational algorithm that allows for easy and fast addition of new input data. At the first step, a computational algorithm is constructed to determine the radial distributions of the parameters of the planet’s interior from a set of observational data. Using the Bayesian statistics approach, we then formulate the inverse problem and solve it using the Markov chain Monte Carlo (MCMC) method. The probabilistic approach to solving the inverse problem greatly simplifies the matching of model parameters that satisfy the observations and the a priori data. The Bayesian statistics approach allows us to take into account the correspondence between the initial information about the model and the observed data. The developed computational algorithm was tested on the classical model example of gravity data inversion. The results of the numerical experiment are presented graphically. The algorithm for solving the problem has the peculiarity that each Markov chain is computed completely independently of the others. The problem is easily distributed evenly over all the cores of a computer or a cluster. This greatly reduces the running time of the computational algorithm, which is important in the future when the number of input parameters increases. At the second step of the work, it is planned to use the presented computational algorithm to find parameter distributions in the interior of planets from the known observational data.

A generalization of the theory of formation of anhysteretic remanent magnetization (ARM) is generalized for noninteracting randomly spatially oriented uniaxial single-domain particles. It is shown that approximate expressions for the ARM intensity, which have been proposed in (Schcherbakov and Shcherbakova, 1977; Victora, 1989; Egli, 2002), are quite admissible for obtaining estimates. However, our calculations have revealed a striking discrepancy between theoretical conclusions and experimental results. It follows from the theory that the ARM intensity exceeds by several times the thermoremanent magnetization (TRM) intensity, while experiments lead to the inverse relation between ARM and TRM. For resolving this paradox and for explaining the mechanism of ARM formation in rocks, it is necessary to supplement the theory proposed here by including the magnetostatic interactions; as regards experimental verification, it is necessary to carry out experiments with ARM and TRM for ensembles of noninteracting grains (i.e., for their very low concentration in the sample).

Abstract—Ground-based magnetometric measurements were used to study ionospheric disturbances observed from November to December 2023 after a series of fairly strong earthquakes in an area of intense seismic activity in the Philippines. It is shown that a stable pattern of the appearance of magnetic disturbances from events with different magnitudes (from Mw = 6 to Mw = 7.4) is observed, containing short- and long-period disturbances caused by variations in the current systems of the lower ionosphere. It has been established that these variations belong to different branches of atmospheric acoustic-gravity waves: acoustic and internal, respectively. It is shown that the origin of disturbances in the acoustic range may be associated with the arrival of seismic Rayleigh waves, which are a source of acoustic vibrations, while the estimated velocities of atmospheric internal waves correspond to their generation directly at the epicenters of events. Magnetometric measurements have made it possible to record ionospheric disturbances from events with a significantly lower magnitude compared to the radio sounding method using global navigation satellite systems.

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 ({{{mathbf{B}}}_{ bot }}), then for an underground source (left| {{{B}_{z}}} right| > left| {{{{mathbf{B}}}_{ bot }}} right|) in the vicinity of the source. For ionospheric sources, this apparent impedance (i.e., the ({{{{mu }_{0}}left| {{{{mathbf{E}}}_{ bot }}} right|} mathord{left/ {vphantom {{{{mu }_{0}}left| {{{{mathbf{E}}}_{ bot }}} right|} {left| {{{{mathbf{B}}}_{ bot }}} right|}}} right. kern-0em} {left| {{{{mathbf{B}}}_{ bot }}} right|}}) 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 ({{{mathbf{E}}}_{ bot }}). 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.

Abstract—An EIGEN-6C4 model for the Altai-Sayan region and northwestern Mongolia constructed using data from satellite gravimetric missions and the results of ground-based measurements with absolute gravimeters and space geodesy receivers is considered. Using the EIGEN-6C4 geopotential (ETOPO1 relief), within the framework of a homogeneous crust model with the involvement of seismic exploration data on the platform part of the study area, an idea was obtained about the changes in the thickness of the earth’s crust in central Asia for the territory extending from 56° to 46° north latitude and from 80° to 100° east longitude, covering Gorny Altai, Kuznetsk Alatau, Western Sayan and Eastern Sayan, Tuva Basin, Tarbagatai Ridge (Kazakhstan), Mongolian Altai (PRC, Mongolia), Great Lakes Basin and Khangai Ridge (Mongolia). Research has shown that the depth of the Mohorovičić boundary increases from the northwest to the southeast of the territory from 40 to 55 km. For the mountainous regions in the south (Mongolian Altai, Khangai Range), the maximum crustal thickness was 55 km. For intermountain valleys and depressions (Tuva Basin, Big Lakes Basin) the depth of the Moho surface is within 45–47 km. In the north, in the flat part of the territory, the thickness of the crust is from 40 to 43 km. The differences between models constructed using gravimetric and seismic data are considered.

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.

Abstract—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.

Obtaining the most accurate and reliable gravimetric data has always been and remains the main task of gravimetry. The purpose of the authors’ long-term research and this work in particular is to determine interference in gravimetric data caused by various external influences and to find ways to take them into account or eliminate them. The proposed method of iteratively taking pressure and tidal correction into account made it possible to increase the accuracy of single gravimetric readings to ±2 µGal. The main instruments for many years of research were relative automated gravimeters of the CG Autograv series from Scintrex; the main results obtained in this work are shown based on their example. In CG-5 and CG-6 gravimeters, the instrumental accuracy is 1.0 and 0.1 µGal, respectively. However, it cannot be said that a single reading will give the gravity increment with the specified accuracy. Relative gravimeters, in addition to the desired value, also record the device response to inertial influence, changes in meteorological factors, and its own hardware errors, which cannot be eliminated without additional information. Under the conditions of the Zapolskoye geophysical observatory in the Vladimir region, continuous gravimetric, seismic, and meteorological measurements were carried out for 8.5 months. The obtained data made it possible to analyze the possibility of partially taking the influence of the atmospheric pressure and determining the correct delta factors for 20 groups of waves with periods of 48 days or less into account. The minimum duration of the gravimetric series to obtain delta factors of waves with periods from 0.02 to 3.38 cycles per day was also estimated at 6 months.

A new approach is proposed to calculate the instantaneous velocity of magnetic poles. The method uses the spatial distribution of the vector of the horizontal component H, calculated from analytical models of the main geomagnetic field for the current and the nearest epochs. The horizontal component was calculated using the coefficients of two models: IGRF13 and COV-OBSx2. The equation for the velocity of pole movement is obtained from the condition that the horizontal field component at the pole point is equal to zero at any moment in time, which allowed us to determine the directions of instantaneous velocity. To find the position of the pole and the velocity of its movement between epochs, it is proposed to use a hermitian spline, which describes a smooth curve, whose tangent coincides with the velocity vector in each epoch. It is shown that the velocity vector of the pole movement depends linearly on the derivative of the horizontal component with respect to time and is inversely proportional to the derivative of H with respect to coordinates. It has been established that higher harmonics are primarily responsible for the acceleration of the pole movement. This is due to their significant contribution to the horizontal component in the polar regions. The obtained instantaneous velocities were compared with the average or interval ones, which are determined from the position of the pole for neighboring epochs. When using the IGRF13 model to calculate the coefficients, artifacts were found in the trajectory of the poles: large deviations in both the directions and magnitudes of the instantaneous velocity vectors compared to interval ones. For the COV-OBSx2 model, no such artifacts were found. It has been assumed that the observed systematic differences in the vectors of instantaneous and interval velocities calculated using the IGRF13 model are associated with the methodological features of constructing this model. In particular, the interval between generations of the IGRF13 model is 5 years, while for the COV-OBSx2 model it is 2 years and splines were used to construct the latter model. It is noted that the direction of interval velocities for these two models can differ by 40°. Limitations on the applicability of the method associated with sudden changes in the trajectory of the pole are determined. In this case, the method may be unstable, since when calculating the time derivatives of the field at a given epoch, models of the nearest epochs are used. In the case of sudden changes in the pole trajectory, the values of these derivatives strongly depend on the chosen method of numerical differentiation with respect to time. For the reliability of the proposed method, it is required to know the geomagnetic field in the vicinity of the pole at time intervals shorter than those in the IGRF13 model.