Izvestiya, Physics of the Solid Earth最新文献

Abstract—It is proposed to consider the processes of surface denudation and magmatism in the Earth’s crust to explain the formation in it of elevated horizontal compressive stresses in excess of lithostatic pressure. Rock exhumation only partially unloads the crust due by removing the weight of the overburden if the crust at depth was above the yield point. This is due to the fact that in the case of exhumation, the unloading occurs elastically. As a result, residual horizontal compressive stresses acquired at the stage of supercritical cataclastic flow arise in the rock. Another mechanism to account for the formation of additional compressive stresses in the crust involves volcanic and magmatic processes. Magma ascent along subvertical crustal faults and networks of fractures is only possible if magma pressure at the propagation front is above the level of horizontal compression in the rock. The result is that below the magma propagation front, the level of horizontal compressive stresses in the rocks rises to the level of magma pressure. Because the pressure in the subcrustal or intracrustal magma chamber is close to the lithostatic pressure of the overburden, fault-orthogonal stresses above the magma propagation front in the fault exceed the level of vertical compression. Thus, crustal magmatization is capable of changing the crustal stress state from horizontal extension to horizontal shear.

Abstract—This paper presents the results of numerical modeling of the permafrost thermal regime and thermobaric conditions of methane hydrates in the north of Western Siberia over the past 70 thousand years. The area of hydrate formation was determined and the rate of accumulation of hydrates was estimated in connection with the migration of fluid from the underlying gas-saturated layers under the conditions of cover glaciation. The estimates obtained for the change in hydrate saturation as a result of fluid migration during the 10 thousand-year glaciation period, depending on the permeability of the soil, are from 6 to 40% in the upper 350 m. Based on quantitative characteristics of the equilibrium and metastable states of methane hydrates, the conditions for the preservation of relict methane hydrates in permafrost under the paleoclimatic scenario were determined, taking into account periods of ice cover and transgression. It is shown that due to the effect of self-preservation at temperatures below –4°C, it is possible to preserve relict methane hydrates in the upper 200 m of soil under non-equilibrium conditions. The effect of lowering the temperature while the hydrates dissociate prevents the complete decomposition of the deposit and leads to an increase in the thickness of the frozen soil.

Abstract—The paper addresses the study of changes in the seismic process associated with a volcanic eruption and evaluation of the energy parameters and structure of the wave field from seismic data. Three types of disturbances in the structure of the wave field are identified. Firstly, this is the Rayleigh surface waves with an average oscillation period of 23 s, which are clearly traceable at distances up to 100 degrees. The group velocity of the Rayleigh waves is 3.6–3.8 km/s. The magnitude calculated from these waves for the stations with mainly oceanic propagation paths is M_s = 5.52 ± 0.18, the corresponding seismic energy is on the order of E_c = (1–7) × 10¹³ J, and the scalar seismic moment is M₀ = 2.4 × 10¹⁷ J. Two other types of oscillations are detected on seismic channels with a passband between 0.0003 and 0.1 Hz. In the first oscillation types, the apparent velocity of the propagation of the disturbance lies between 0.28 and 0.37 km/s and a characteristic period is 268 s. This type of disturbance is associated with the gravitational response of the seismometer to the acoustic perturbation in the atmosphere. For the second type of seismic disturbances, the propagation velocity is 0.21–0.28 km/s with the characteristic period of 450 s on the horizontal components of seismic receivers. This type of seismic disturbance is likely to be caused by the interaction of a gravity wave in the hydrosphere with the coast of the islands where seismic stations are located.

Abstract—A change in the non-stationary electromagnetic (EM) signal over the conducting polarizable Earth covered by sea water on measuring lines located in the axial and equatorial regions of the source—a pulsed horizontal electric line (HEL)—is considered. When the HEL operates under pulsed conditions, it creates a galvanic and eddy current in the medium. If the medium affected by the HEL is heterogeneous, both influences lead to the separation of bound charges. After attenuating the impact of an artificial source, relaxation (depolarizing) processes of various nature appear in such a medium, manifesting themselves, in particular, in the form of an EM signal. As a result, the transient process recorded by the grounded line after the pulsed effect of the HEL is at least a superposition of three components: the transient electromagnetic (TEM) signals, galvanically induced polarization (GIP) and inductively induced polarization (IIP). As the contribution of the TEM field component to the overall signal decreases, the IP signal is manifested in the transient process by a change in the time response of the decay, to the point where the signal reverses polarity. As shown earlier by numerical simulations for the axial region of the HEL, the manifestation of the IIP signal at late transient process times, for most of the geoelectric conditions on land, is invisible against the GIP manifestation (Ageenkov et al., 2020). These calculations also show that in the axial region, the GIP signal manifests itself in the form of a deceleration of the transient process rate, and the IDP signal—an acceleration of the decay rate, to the point where the signal changes its sign. Field measurements performed by the aquatic differential-normalized method of electrical prospecting (ADNME), which uses axial electrical installations, record transient processes with a change in the time response of the decay: it becomes more delayed or, vice versa, runs faster and may be accompanied by a change in the polarity of the signal. In other words, measured signals of different forms are observed, which are presumably associated with the manifestation of the GIP or IIP signals. The relevance of the publication lies in the need to explain the results of field measurements performed offshore, to understand the relationship between the course of the transient process and the geoelectric conditions existing in the water area. And in general, to describe the formation of the transient response of the medium in the axial and equatorial region of the HEL for the conditions of aquatic geoelectrics. The calculated signal for axial and equatorial electrical installations with several spacings under the conditions of the sea shelf water area is studied when the installation is located on the surface of and inside the water layer, and on the seabed of the water area—on geological formations. For axial installations, calculations are made of the quantities used in the ADNME