Fizika zemli

Abstract-A seismogenic-trigger mechanism is proposed for the activation of methane emission on the Arctic shelf in the late 1970s, which caused the onset of a rapid climate warming in the Arctic, as well as the intensive destruction of the ice shelves of West Antarctica in the late 20th and early 21st centuries. This process is accompanied by the release of methane from the underlying hydrate-bearing sedimentary rocks and the rapid climate warming in Antarctica. The proposed mechanism is associated with the action of deformation tectonic waves in the lithosphere-asthenosphere system, caused by strong earthquakes occurring in the subduction zones closest to the polar regions: the Aleutian, located in the northern part of the Pacific Ocean, and the Chilean and Kermadec-Macquarie, located in the southeastern and southwestern parts of the Pacific lithosphere. Disturbances of the lithosphere are transmitted at an average speed of about 100 km/year over long distances of the order of 2000–4000 km, and the associated additional stresses that come to the Arctic and Antarctica several decades after earthquakes lead to the destruction of metastable gas hydrates located in the frozen rocks of the Arctic shelf or in the subglacial sedimentary rocks of Antarctica, causing the greenhouse effect of warming. Moreover, transmission of additional stresses causes a decrease in the adhesion of sheet glaciers to the underlying rocks, their accelerated sliding and the destruction of the ice sheet ice shelves in Antarctica. The considered hypothesis leads to the conclusion that in the coming decades, the processes of glacier destruction and climate warming in Antarctica will increase due to an unprecedented increase in the number of strongest earthquakes in the subduction zones of the South Pacific Ocean in the late 20th and early 21st centuries.

It is proposed to consider the processes of surface denudation and crustal magmatism to explain the formation of increased horizontal compressive stresses in the crust, which are excessive relative to the lithostatic pressure. Exhumation of a rock results in only partial unloading of the crust due to the removal of the weight of the overburden, unless the crust is above the yield point. This is due to the fact that in the case of exhumation, the unloading follows the elastic law. As a result, residual stresses of horizontal compression received at the stage of cataclastic flow arise in the rock. Another mechanism of formation of additional compressive stresses in the crust is related to volcanic and magmatic processes. The ascent of a magma along subvertical faults and fracture networks is only possible under the conditions when the magma pressure at the propagation front exceeds the level of horizontal compression in the rock. As a result, below the magma propagation front, the level of horizontal compressive stresses in the rocks rises to the level of magma pressure. Since the pressure in the subcrustal or intracrustal magma chamber is close to the lithostatic pressure of the overburden, above the magma propagation front in the fault, the stresses normal to the fault exceed the level of vertical compression. Thus, crustal magmatization is capable of changing the crustal stress state from horizontal extension to horizontal compression.

The paper considers the role of relatively weak earthquakes as a tool for studying the environment, including of strong earthquake process. The spatial structure of the attenuation field of several seismically active regions (of the Garm prognostic polygon in Tajikistan, Altai, the Caucasus, Eastern Anatolia, the Western Tien Shan), as well as the epicentral regions of a number of strong earthquakes, and the confinement of deepened seismicity to it are considered. It is shown that the attenuation field obtained from the short-period code of weak earthquakes in seismically active regions is inhomogeneous and consists of blocks with a high Q factor and weakened zones of strong attenuation. An uneven distribution of deepened earthquakes is noted. It is associated with the block structure: in weakened zones, their share is greater than in blocks with a high Q factor. Examples of variations in the of deepened seismic activity in weakened zones are demonstrated. It varies over time, increasing before strong earthquakes. Facts are presented that testify to the existence of a relationship between the Earth’s rotation rate and the activity of deepened seismicity. Examples are given of the activation of weak seismicity in the form of seismic swarms (series of weak earthquakes concentrated in space and time) in connection with strong events. A characteristic feature of these swarm series is the isometry of the earthquake localization areas in plan view and vertical elongation. As a rule, they coincide with weakened zones of strong absorption of S-waves. Intense localized seismicity, confined to one-dimensional volumes, is most likely associated with increased conductivity channels through which deep fluids migrate. The activation of swarm series is the result of active migration of deep fluids and an increase in fluid saturation of weakened zones. Fluids, in turn, are a catalyst for processes that lead to a decrease in the strength of rocks and the destruction of blocks in epicentral zones. In this case, the clusters to which the swarm series belong can be considered as local seismogenic zones. The appearance of compact isometric in plan and nearly vertical in section clusters of weak seismicity is often observed outside the epicentral zones of strong earthquakes. Such zones may simply be indicators of the seismotectonic situation in the region as a whole. It is assumed that a sharp change in the dynamics of atmospheric pressure during the preparation of a strong earthquake at hydrometeorological stations located in such areas is a consequence of the intergeospheric interaction of the lithosphere and atmosphere. Deep degassing seems to be one of the main mechanisms of the anomalous behavior of atmospheric pressure during the implementation of strong events. It is most active in weakened zones. The mechanisms of the impact of deep degassing on the outer geospheres remain the subject of discussion.

The possibility of remote studies of electromagnetic and ionospheric effects caused by the eruption of the Tonga volcano on January 15, 2022 is shown. At distances up to 15 000 km from the source, geomagnetic field variations associated with disturbances of the Schuman resonance (SR), Lamb wave propagation and acoustic-gravitational waves are registered. It is shown that the appearance of a powerful source of thunderstorm activity caused by the eruption produced a significant increase (more than three times) in the amplitude of geomagnetic disturbances at SR frequencies, which correlates with the number of lightning discharges. The effect of the eruption on the frequency characteristics of the SR was not detected.

A laboratory setup was constructed in IDG RAS to investigate the process of shearing the contact of rock blocks of one-meter scale. It was used to investigate deformation processes in a fault with a heterogenous structure of the sliding interface, which contained strong contact patches – analogs of the “asperity” in the model of Hiroo Kanamori. It is shown that when a large slip occurs, the rupture, that starts in the zone of maximal deficit of interblock displacement, cuts the segments of the fault with lower effective strength, the latter being decreased in previous deformation events. Those previous events may be “slow” slips with low seismic efficiency. In nature the events that “prepare” the fault interface for a large slip may be smaller earthquakes – foreshocks, or they can be either low frequency earthquakes or slow slip events, both can hardly be detected in seismic records. Thereupon a promising diagnostic indication is the shift of the spectrum of ambient seismic noise to lower frequencies caused by the decrease of fault stiffness.

A computer model of fracture of the heterogeneous materials (including rocks) based on the Discrete Element Method (DEM) is proposed. We used the bonded particle model (BPM), various modifications of which are widely used in the study the fracture process. The material is modeled by a set of spherical particles (simulating polycrystalline grains) connected by bonds placed at the points of particle contacts (simulating grain boundaries). In BPM model, the initiation of cracks was determined by the bonds breakage, and their propagation is provided by the coalescence of many broken bonds. Computer experiments were carried out for the materials with different features (various grain mechanical properties and sizes, various mechanical properties of the grain boundaries), in order to find out the influence of these parameters on local stresses and the defect formation. Calculations were held in the MUSEN software. Cylindrical samples were filled with spherical particles of the same or different radii. The parameters of materials for grains and bonds (grain boundaries) were taken corresponding to granite, quartz, orthoclase, oligoclase, and glass. The sample was placed in a virtual press, in which the lower plate was stationary, and the upper plate moved towards the lower one at a constant velocity until the sample was destroyed. The calculation of the maximum local stresses showed that the homogeneity of material leads to greater space heterogeneity of local stresses and vice versa, heterogeneity contributes to their greater uniformity. Comparison with the results of laboratory experiments on rock deformation showed that the proposed model of polycrystalline materials realistically describes some features of their destruction when the main processes occur along the grain boundaries. These features include the brittle nature of homogeneous materials fracture and the presence of nonlinear elasticity (plasticity) for ones that were more heterogeneous. For heterogeneous materials, the model demonstrates a two-stage character of fracture process, when at the first stage the accumulation of defects occurs uniformly over the sample, and at the second stage – the formation and growth of the fracture site.