Vol 55, No 1 (2019)

The results of application of the receiver function technique are briefly reviewed. In the mantle transition zone, the topography of the main seismic boundaries is evaluated with a resolution of about 3 km in depth and a lateral resolution of about 200 km. The variations of the depth of the main boundaries have the maximal amplitude reaching tens of kilometers. Thinning of the mantle transition zone in the hot spots and the respective increase in temperature by ~100°C is established. In several regions, two low-velocity layers are revealed in the mantle transition zone: one directly above the 410-km seismic discontinuity and another at a depth of 450 to 500 km. The origin of the first layer is associated with dehydration in the mantle plumes in the process of the olivine–wadsleyite phase transformation. The increase in the velocity of S-waves in the base of the second layer may explain the observations of the so-called 520-km boundary. The traditional approach to the studies of the structure of the crust and upper mantle is based on using the surface waves. Receiver functions can provide higher resolution at the same depths when a combination of P- and S-wave receiver functions is used. This type of results was obtained for Fennoscandia, Kaapvaal craton, Indian shield, Central Tien Shan, Baikal rift zone, the Azores, Cape Verde Islands, and the western Mediterranean. S-receiver functions were used in the study of the lunar crust. The joint inversion of P- and S-receiver functions provides robust estimates of the parameters of the seismic boundaries, including weak discontinuities such as the lithosphere–asthenosphere interface of cratons. The parameters determined from receiver functions include the P-wave to S-wave velocity ratio. In a few regions, a very high (>2.0) velocity ratio is observed in the lower crust that may indicate the presence of a fluid with a high pore pressure. Receiver functions allow estimating the parameters of the azimuthal anisotropy as a function of depth. The change of the parameters with depth makes it possible to distinguish the active anisotropy associated with recent deformations from the frozen anisotropy—the effect of the past tectonic processes.

The application of geoinformatics and systems analysis methods for processing and interpreting geospatial data in geophysics and geodynamics is considered. The modern capabilities of observations with Global Navigational Satellite Systems as the main source of geospatial data are described. Achievements in the interpretation of geomagnetic data are presented and, in connection with this, some basic points of systems analysis are presented. The application of systems analysis in geophysics and geodynamics is illustrated by the approaches to estimating and forecasting the stability of the structural-tectonic blocks of the Earth’s crust for the geoecologically safe burial of high-level radioactive waste in the rocks of the Nizhne–Kanskii massif, Krasnoyarsk krai.

This paper addresses the evolution of the views on the subject and methods of geodynamics over the past fifty years. The problems of the metrological provision of the results of repeated observations by the ground-based and satellite geodesy are discussed. It is shown that the basic characteristics of the recent geodynamical processes substantially depend on the degree of spatiotemporal detail of the observational systems. A variant of the solution is suggested for the problem of paradoxes of large and small strain rates which was detected during the studies at the geodynamical sites in seismically active and weakly seismic regions. For explaining the anomalous deformational activity of the platform’s faults, the mechanism of parametric excitation of the processes is suggested according to which the time fluctuations in the internal parameters of a fault zone (stiffness, pore pressure, coefficient of friction) under quasi-static external loading form local strain anomalies. The results of strain monitoring are demonstrated by the example of a shelf oil field. The assertion is substantiated that the concept of geodynamical testing sites is a universal form for investigating the recent deformational processes which offers a unified framework for determining the spatiotemporal structure of the geodynamical phenomena of different scales that are used in the fundamental and applied studies.

The main results obtained at the Institute of Physics of the Earth of the Russian Academy of Sciences (IPE RAS) in the numerical geodynamic modeling of the structures of the lithosphere are presented. Even in the first models, the objectives were set to describe the time evolution of the boundaries of the layers composing the geological structures, which is necessary for a detailed comparison of the modeling results with the geological and geophysical data. In 1983, the equation of motion for the upper boundary of the model was complemented with the description of sedimentation and erosion. With the use of this equation, not only was it possible to build geodynamic models describing the formation of various types of sedimentary basins but also to perform a mathematical analysis of the problem of determining the rates of paleotectonic movements from the data on thickness, age, and facies composition of sedimentary layers. New data on the processes of the formation and evolution of large-scale tectonic structures are obtained in the model of a rheologically stratified boundary layer of the Earth, asymptotically liked to the model of mantle convection. In particular, the role of small-scale convection in the formation of the lithospheric structures in the tectonic settings of extension and compression is investigated. The numerical results demonstrate the decisive role of the small-scale convection in the asthenosphere in the formation of sedimentary basins (post-rift, on passive continental margins, and foredeep basins). The constructed models served as the base for stating the problems of the interpretation of different geological and geophysical data in the frameworks of the geodynamic models. Examples are presented of the statement of inverse problems and the respective bibliography is provided.

Laboratory experiments on studying the aftershock regime are carried out on sandstone specimens at different levels of axial loading and uniform compression and at constant pore pressure. The aftershock sequences are modeled by the scenario of stepwise increasing axial loading of a specimen with strain control, which ensures the regular generation of aftershock sequences. The experiments are conducted on intact specimens and on those with preliminarily formed shear macrofractures simulating natural faults. The multichannel recording of the signals of acoustic emission (AE) during the experiments allowed locating the AE sources. Several types of the dependence of the parameters of relaxation of the acoustic activity—parameters p and c of the modified Omori law and the Gutenberg–Richter b-value—on the level of acting stresses are revealed. The b-value decreases with the growth of axial stresses at all levels of uniform compression. In the case of a fracture on the preexisting fault, the Omori relaxation parameter p increases with the growth of axial stresses; parameter c—the time delay before the onset of relaxation—decreases with the growth of axial stresses and increases with the rise of the level of uniform compression. In the case of a fracture of an undamaged specimen, parameter p remains unchanged with the growth of axial stresses, whereas parameter c increases slightly. Parameter variations in the case of a complex stress state with both varying deviatoric (differential stresses) and spherical parts (effective pressure) of the stress tensor take on a unified form when expressed in terms of Coulomb stresses. It is hypothesized that the time delay of the relaxation of the aftershock activity is determined by the kinetics of a fracture in accordance with the kinetic concept of strength in solids. This hypothesis is supported by the exponential dependences of parameter c on stresses and the effective strength of the medium which are revealed in the experiments. Under this hypothesis, based on Zhurkov’s formula for the durability of materials, it is possible to unify the dependences of parameter c on the Coulomb stresses at different effective strength values. The obtained parameter estimates for the dependence of c on strength and stresses suggest that the c value is determined by the difference of the strength and the acting stresses, thus indicating how far the stress state of the medium is from critically corresponding to the ultimate strength.

Global geodynamics is determined by the thermal convection in the mantle which manifests itself on the surface by movements, relief, heat flow, and volcanism. Thermal convection in the Earth is complicated by the fact that the lithosphere is broken into rigid plates, the crust is broken into six separate floating continents and a number of islands, on the mantle bottom there are two giant piles of heavy material, at high intensity the ascending convective flows acquire a plume shape, and phase transformations take place in the mantle. The influence of many factors on the mantle structure has been studied in detail and substantially understood. It is topical to reconcile the new data about phase transformations at depths of 650 to 700 km with the seismic data about the positions of these boundaries. The ultimate problem of global geodynamics has not yet been solved; the three-dimensional structure of the whole-mantle flows, consistent with the observations in geophysics, geochemistry, geology, and numerical modeling is not known even on a semischematic level.

The consistency of the empirical data on the paleointensity and paleoinclinations contained in the BOROKPINT paleointensity world database with the Geocentric Axial Dipole (GAD) hypothesis and the Giant Gaussian Process (GGP) model describing the geomagnetic field variations in the Brunhes epoch is tested. The calculation procedure is based on the representation of the geomagnetic field potential by the sum of the spherical functions of a spatial coordinate with random coefficients, which enables the computer simulation of the data that correspond to the given statistical characteristics of the coefficients. The estimation shows that according to the Kolmogorov–Smirnov and Anderson–Darling tests, the GAD hypothesis in its canonical form should be rejected. The extension of GAD to GGD with nonzero time-average quadrupole and octupole terms brings the paleointensity and paleoinclination data into agreement with the GGP model realizations; however, these models turn out to be mutually exclusive because their parameters are inconsistent with each other. Testing the paleoinclination data against GDP model demonstrates the necessity of introducing a small correction to the purely dipole component of the geomagnetic field. At the same time, the analysis of the paleointensity data shows that these data with a very high probability agree with the GGP models with a high quadrupole contribution making up 1/3 of the dipole coefficient, which is strongly at odds with the parameters of the model corresponding to the paleoinclination data. This inconsistency is most probably caused by the artifacts due to the incorrect determinations of paleointensity; however, this interpretation leaves unsolved the question about the causes of such a strong latitudinal dependence of the intensity of the Virtual Axial Dipole Moment (VADM) what follows from the empirical data.