No 1 (2019)

The application results of the receiver function technique are briefly outlined. The topography of the main seismic boundaries in the mantle transition zone is evaluated with resolution of about 3 km in depth and about 200 km laterally. The maximal amplitudes of depth variations of the main boundaries reach tens of kilometers. The mantle transition zone thinning 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 during olivine – walesite phase transformation. The increase in the S-wave velocity at the base of the second layer can explain the observations of the so-called 520-km boundary. The traditional approach to studying the structure of the crust and upper mantle is from 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 studies of the lunar crust. The joint P- and S-receiver function inversion provides robust estimates of the parameters of seismic boundaries including weak discontinuities such as the lithosphere – asthenosphere interface of cratons. The parameters determined from receiver functions include the P- to S-wave velocity ratio. In a few regions, a very high (> 2.0) velocity ratio is observed in the lower crust, probably indicating the presence of a fluid with high pore pressure. Receiver functions allow estimating the parameters of azimuthal anisotropy as a function of depth. The changes of the parameters with depth make it possible to distinguish the active anisotropy associated with recent deformations from the frozen anisotropy – the effect of the past tectonic processes.

The paper is devoted to the application of geoinformatics and systems analysis methods for processing and interpretation of geospatial data in geophysics and geodynamics. The modern uses of observations with Global Navigational Satellite Systems as a main source of geospatial data are discussed. The advances in the interpretation of geomagnetic data are described and basic points of systems analysis are presented in this context. The systems analysis in geophysics and geodynamics is illustrated by the approaches to estimating and forecasting the stability of structural-tectonic blocks of the Earth’s crust aimed at geoecologically safe burial of high-level radioactive waste in the Nizhne-Kanskii granitoids massif (Krasnoyarsk krai).

The evolution of the views on the subject and methods of recent geodynamics over the past fifty years is outlined. Metrological provision of the results obtained by repeated observations by means of ground-based and satellite geodesy is discussed. The substantial dependence of the main characteristics of recent geodynamical processes on the degree of spatiotemporal detail of the observational systems is demonstrated.

A possible solution of the paradoxes of large and small strain rates which were detected in the studies at geodynamical sites in seismically active and aseismic regions is proposed. To explain the anomalous deformational activity on the platform faults, the mechanism of parametric excitation is suggested. According to this mechanism, the time fluctuations in the internal parameters of a fault zone (stiffness, pore pressure, friction coefficient) create local strain anomalies under quasi static external loading.

The results of strain monitoring are demonstrated by the example of a shelf oil field. It is substantiated that the geodynamical testing sites are a universal instrument for exploring recent deformational processes which offers a unified framework for establishing the spatiotemporal structure of different-scale geodynamical phenomena addressed in the fundamental and applied studies.

Key results of numerical geodynamic modeling of the structures of the lithosphere at the Institute of Physics of the Earth of the Russian Academy of Sciences are presented. Even in the very first models, the aim of these studies was to describe the time evolution of the boundaries of the layers composing the geological structures which is required for correlating the modeling results to the geological and geophysical data. In 1983, the equation of motion for the upper boundary of the model was complemented by the allowance of sedimentation and erosion. This equation provided the basis for building the geodynamic models of the formation of various types of sedimentary basins and made it possible to mathematically analyze the problem of estimating the rates of paleotectonic movements from thickness, age, and facies composition of sedimentary layers.

New data on the formation and evolution processes of large-scale tectonic structures are obtained in the model of a rheologically stratified Earth’s boundary layer, asymptotically linked to mantle convection model. In particular, the role of the small-scale convection in the formation of lithospheric structures in the tectonic settings of extension and compression has been explored. The numerical results clearly demonstrate the key role of the small-scale asthenospheric convection in sedimentary basin formation (post-rift, on passive continental margins, in foredeep basins). The constructed models served as the basis for interpretation of heterogeneous geological and geophysical data in the context of geodynamic models. The examples of statement of inverse problems are presented and the relevant bibliography is provided.

Laboratory experiments on studying the aftershock regime are carried out with sandstone specimens under different axial loading and uniform compression and constant pore pressure. The aftershock sequences are modeled by the scenario of stepwise increasing axial loading of a specimen with strain control ensuring regular generation of aftershock sequences. The experiments are conducted on intact specimens and on the specimens with preliminarily formed shear macrofractures simulating natural faults. The experiments were conducted with multichannel recording of the acoustic emission (AE) signals which made it possible to locate the AE sources. Several types of the dependence of the acoustic activity relaxation parameters (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 fracture on the preexisting fault, the Omori relaxation parameter p increases with the growth of axial stresses whereas 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 as the axial stresses grow, 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 aftershock activity relaxation is determined by the kinetics of fracture in accordance with the kinetic concept of strength in solids. This hypothesis is supported by exponential dependence of parameter c on stresses and on the effective strength of the medium revealed in the experiments. Under this hypothesis, the dependences of parameter c on the Coulomb stresses can be unified for different effective strength values with the use of Zhurkov’s formula for durability of materials. 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, indicating how far the stress state of the medium is from the critical state corresponding to the ultimate strength.

Global geodynamics is determined by 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 convection intensity the ascending convective flows acquire a plume shape, and phase transformations take place in the mantle. The impacts of many factors on the mantle structure have been thoroughly studied and fairly well understood. It is pertinent to reconcile the new data on phase transformations at depths of 650 to 700 km with the seismic data on 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 paleointensity and paleoinclinations contained in the BOROKPINT paleointensity world database with the Geocentric Axial Dipole (GAD) hypothesis and Giant Gaussian Process (GGP) model describing the geomagnetic field variations in the Brunhes epoch is tested. The calculations are based on the geomagnetic field potential representation by the sum of spherical functions of spatial coordinates with random coefficients, enabling computer simulation of the data with the given statistical characteristics of the coefficients. The estimates show that the Kolmogorov–Smirnov and Anderson–Darling tests reject the GAD hypothesis in its canonical form. The extension of GAD to GGD with nonzero time-average quadrupole and octupole terms makes the paleointensity and paleoinclination data to comply with GGP model realizations; however, these models are mutually exclusive because of mutual inconsistency of their parameters. Testing the paleoinclination data against GDP model shows that a small correction to the purely dipole component of the geomagnetic field should be introduced. At the same time, the paleointensity data analysis suggests that these data highly probably agree with 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 paleoinclination data. This inconsistency is most likely to be due to the artifacts associated with incorrect paleointensity determinations; however, this interpretation does not explain the causes of the strong latitudinal dependence of the intensity of the Virtual Axial Dipole Moment (VADM) which follows from the empirical data.