No 4 (2023)

Elastic bending stresses and strains in the lithosphere are typically calculated based on the Kirchhoff–Love (thin) plate theory. The criterion for its applicability is the smallness of plate thickness to length ratio. In oceanic plates, due to the buoyant force of the mantle, the main deformations are not uniformly distributed along the plate but are concentrated in the vicinity of the subduction zone. Therefore, the effective length of the bending part of the plate is a few fractions of the actual length, and the criterion of a thin plate is partially violated. In this paper, we analyze the possibility of applying thick plate bending equations. The existing variational theories of 3D bending of thick plates are much more complicated than the Kirchhoff–Love theory, as they involve solving three differential equations instead of one, and have limited application due to their complexity. Since geophysical applications frequently use 2D models, in this paper we analyze in detail the potential and accuracy of the thick plate bending theory for 2D models. After the conversion to the 2D plane strain and plane stress approximation, the original 3D Reissner thick plate bending equations are written out in the form similar to the Kirchhoff equations with additive corrections and are supplemented with the explicit formulas for longitudinal displacement. The comparison of the analytical solutions of the 2D Reissner equations with the exact solutions shows that the 2D approximation only provides a correction for the plate bending function. However, this correction refines the Kirchhoff–Love theory by almost an order of magnitude. At the same time, the solution of the equations in this case turns out to be almost as simple as the solution of the thin plate equations.

Records of vertical surface displacement velocities in the vicinity of broadband seismic stations located around the epicenter of the 9.1 magnitude Sumatra earthquake on December 26, 2004 are analyzed. During five years from 1996 to 2000, the COCO station, nearest to the epicenter and located at a distance of 1700 km from it, has recorded a steady daily behavior of seismic noise. In 2001, step-like distortions of the level of the recorded seismic noise appeared at this station. These distortions continued up to the time of the earthquake. The station also detected pulses above the diurnal behavior, with a gradual increase and subsequent decrease in the amplitude of the oscillations which lasted a few minutes. The pulses occurred under quiet weather conditions and geomagnetic activity. No such pulses were observed at stations more than 2000 km from the epicenter. It is hypothesized that before the earthquake, there had been a slip on the geological faults in the lithosphere of the Indian Ocean.

Over the whole area of the Eurasian lithospheric plate considered in the paper —rom the Atlantic to the Pacific Ocean—the velocity vectors of the horizontal eastward displacements of GPS sites form a gentle, smooth, huge arc, convex towards the north. Three relatively narrow bow-shaped bands can be distinguished within the arc. The southernmost band, most provided with measurement data (bow-shaped band A in this paper) is discussed in detail in (Shevchenko et al., 2021). The direct, real results of the published geodetic measurements have shown that the length of this band increases by 5 to 10 mm annually. The arc is lengthening. In the cited paper, we considered and rejected five interpretations of this lengthening, which are based on the idea of external action of the above bow-shaped band on the rocks of the Earth’s crust/lithosphere. The most probable cause of the elongation seems to be the internal process of the increase in the volume of these rocks within the band due to the supply of the additional mineral material into these rocks by the deep fluids and its subsequent crystallization. In this paper, we present the results of similar geodetic measurements in other two bow-shaped bands, B and C. It turns out that bow-shaped band C located to the north of the other bands, is lengthening (and at a similar rate) as band A located to the south of the other bands. At the same time, bow-shaped band B, which has an intermediate position between bands A and C, is moving eastward without lengthening. Geodetic measurements in the axial part of the Mid-Atlantic Ridge in the Iceland island suggest that the general expansion of the Atlantic Ocean and the corresponding eastward motion of the Eurasian lithospheric plate are related to the injection of plastic magmatic wedges (of basic composition) in the axial, rift zone of the ridge. Against the background of this general eastward motiont of the lithospheric plate, its constituent parts (the bow-shaped bands A and C are markers of these parts) are moving in the same eastward direction with a velocity increasing from west to east. We attribute this increase in the velocity and the corresponding lengthening of the two bands to the above inflow of the additional mineral material and its subsequent crystallization.

A series of magnetotelluric and seismic studies have been carried out on profiles covering more than two thousand kilometers within the North Caucasus region. The earlier interpretation of the magnetotelluric observations by means of one– and two–dimensional inversion and three–dimensional mathematical modeling software has helped to construct a series of sections and models which are viewed as test and starting ones for the construction of a three–dimensional geoelectric model of the region. The test models have been used to test how well the software for three–dimensional inversion of the impedance tensor components in the magnetotelluric sounding method can estimate the parameters of conducting blocks in the structures of the Greater Caucasus and the Scythian plate. In the resulting geoelectric model, constructed from the results of three–dimensional inversion of all impedance tensor components, the position of low–resistance blocks correlates with deep faults, volcanoes of various genesis, and seismically active zones characterized by the reduced velocity of seismic waves and their increased absorption. The electrical resistivity of low–resistance anomalies is explained by the degree of their saturation with the fluid water fraction. Its maximum concentration is found within the intersections of fault systems, flexural–rupture zones, and deep faults activated by tectonic processes.

The GALO basin modeling system is used to numerically reconstruct the thermal regime of the West Siberian basin lithosphere in the Koltogor-Urengoi graben in the vicinity of the Tyumenskaya SG-6 superdeep well. The reconstruction explains the features in the formation of the thermal regime of the basin, which were not considered in the previous reconstructions of the region. These features include anomalously high growth of the maturity level of the organics in the Jurassic and Triassic rocks, high temperature gradients observed in the upper basement and Triassic–Permian sedimentary complex, the anomalously low rock temperatures in the upper layers of the recent sedimentary section of the basin. The temporal changes in the tectonic subsidence of the basin are analyzed to estimate the intensity and duration of thermal activation events and the extension of its lithosphere. The thermal impact of the sill that intruded into the upper basement horizons in the Lower Jurassic explains the high degree of maturation of the organic matter in the Lower Triassic rocks. Taking into account the abrupt climatic fluctuations in the Pliocene–Quaternary together with hydrothermal activity in the bottom part of the sedimentary cover in the Upper Pliocene–Lower Pleistocene, we obtained the depth profiles of temperatures and vitrinite reflectance, which agree well with the measured values.

The response of sandy and clayey near-surface soils representing the classes of noncohesive and cohesive soils to seismic loading of various intensities is analyzed from the in situ data —from the records by vertical groups of the Japanese nationwide KiK-net strong motion seismograph network. For the analysis, out of a total of ~800 stations, we selected five stations with near-surface sandy soils and five stations with near-surface clayey soils, most purely represented in the upper layers. Using the method (Pavlenko and Irikura, 2003), we have constructed and analyzed the models of strong ground motion behavior for “sandy” and “clayey” stations, showing the distributions of earthquake-induced stresses and strains in the soil layers. Close estimates of the amplification of seismic waves in sands and clays at weak seismic ground motion and close stress-strain relationships characterizing the behavior of the near-surface soils at moderate seismic ground motion are obtained. The liquefaction of sandy soils under strong shaking (the 2011 Tohoku earthquake with Мw ~ 9.0) is analyzed. The effects of the extended seismic sources (directivity of their radiation pattern) on the behavior of sandy and clayey soils and the amplification of seismic waves in these soils is studied. Differences in the behavior of sandy and clayey soils are noted only at strong seismic motions: liquefaction in sandy soils is possible if the groundwater level is on the order of a few meters from the surface, while in clayey soils there is no liquefaction.