Annotation of the results obtained in 2022
As part of the work on the Project in 2022, the key-block models in two-dimensional and three-dimensional configurations were created using finite element modeling software. A software tool for calculating the main parameters of the model has been developed, which allows the user to vary the main parameters of the model and calculate the change in the stress-strain state of the continental margin during the seismic cycle with the display of the output parameters. Based on the simulation results, the possibility of occurrence of earthquakes of various magnitudes during the seismic cycle due to incomplete release of the elastic deformation energy accumulated in seismogenic blocks is shown. The displacements of the day surface of seismogenic blocks calculated in the model are in good agreement with the values of vertical displacements measured by satellite geodetic methods at various stages of the seismic cycle. Also, the algorithm for solving the inverse problem was improved, which is used to invert the series of displacements during the seismic cycle of satellite geodetic stations. The data obtained during the inversion (elastic moduli, effective viscosities of the contact layers, characteristics of the seismic cycle, etc.) were subsequently used as a priori and boundary parameters. The solution of the inverse problem was carried out using averaged time series reflecting the general patterns of displacement of the frontal and rear blocks in the Kuril-Kamchatka, Japan and Chile subduction zones. Refined estimates of geomechanical parameters were obtained for a number of seismogenic blocks related to the indicated subduction zones. At the next stage, a study was made of the features of seismic cycles in the Kuril-Kamchatka, Japan and Chile subduction zones. The study of the preseismic stage is based on the restoration of the degree of interplate adhesion before an earthquake. The construction of models of the distributed seismic movement in the sources was carried out within the framework of the dislocation approach for a spherically symmetric layered Earth model. When analyzing the postseismic stage, the aseismic frictional development of a seismic rupture (postseismic creep), the viscoelastic reaction of the asthenosphere, and the concomitant reconfiguration of interplate adhesion in the contact zone are considered. Direct estimates of the viscosity of the asthenosphere were obtained and the effect of changes in interplate cohesion on the process of viscoelastic relaxation was studied. The analysis made it possible to formulate the following main conclusions: The degree of interplate adhesion can be quite stationary for a long period of time, but immediately before an earthquake there is a sharp increase or, on the contrary, a decrease in the adhesion coefficient. In the first years after the strongest subduction periods, the values of the viscosity of the asthenosphere are significantly lower (by 1–2 orders of magnitude lower than the values established for subduction zones on average). For all the studied events, a spatial-temporal relationship of geodynamic processes occurring at different stages of the seismic cycle was revealed. The fact of general consistency between the configurations of areas of significant seismic displacements in the source during an earthquake and areas of non-zero interplate adhesion at the preseismic stage has been established. In the distribution of postseismic creep, those parts of the source zone are most clearly manifested, in which the stresses remained quite high after the earthquake. The characteristic dimensions of the lithospheric blocks that make up an island arc or an active continental margin are among the guiding factors that determine the features of the seismic cycle, along with rheology and subduction rate. The strongest subduction earthquakes that occurred during a six-year time interval within the Chilean seismogenic zone associated with the unity of tectonic conditions are characterized by similar magnitudes and similar mechanisms. However, the comparative analysis made it possible to reveal a number of significant differences in the development of deformation processes in the vicinity of their centers. These differences are presumably determined by the unique tectonic and geological conditions inherent in the source zone of a particular event. It is shown that distant strong earthquakes can have a decisive influence on the final stage of formation of the source of another event. Thus, it was established that the 2010 Maule earthquake could have contributed to the initiation of the 2015 Ilyapel earthquake. As part of a retrospective analysis of the seismic process, based on numerical calculations, the predictive capabilities of the constructed models of the seismic cycle in the subduction zones under study were evaluated: A) the Kuril island arc. According to the forecast based on the assumption that the effective viscosity of the asthenosphere is unchanged in time, the maximum decay time of viscoelastic relaxation in the asthenosphere after the 2006 event is expected in the central part of the Kuril island arc and is about 10 years. The total duration of the seismic cycle in the central part of the Kuril island arc is about 159 years. B) Japanese subduction zone. According to the forecast, the predominance of postseismic displacements in the vicinity of the source will stop approximately 20 years after the Tohoku earthquake. According to the calculation results, the elastic seismogenic potential required for the realization of an event of such strength will accumulate over 982 years, i.e. the total duration of the seismic cycle in the middle part of the Japanese subduction zone is about 1000 years. C) the Chilean subduction zone. The duration of the postseismic stage of the seismic cycle associated with the Maule earthquake is about 20 years. According to the calculation results, it will take 174 years to accumulate the necessary elastic seismogenic potential. Therefore, the estimated duration of the seismic cycle in the central part of the Chilean subduction zone is 194 years. Sharp deviations of the interplate adhesion regime from the stationary regime can be used as one of the prognostic features in the course of improving the methods of medium-term earthquake prediction. However, for the successful application of this feature, it is necessary to develop criteria for assessing its reliability. Finally, as part of the actualization of the tectonic wave propagation model, which is the most important element of the seismogenic-trigger mechanism proposed by the project manager for the occurrence of sharp phases of warming in the Arctic, the possibility of wave propagation in the lithosphere-asthenosphere system in a non-isothermal mode was studied, taking into account the finite thickness of the phase transition layer containing a small percentage melt (0.1%). The effect of the transition layer between the lithosphere and asthenosphere was taken into account using a generalized boundary condition. It has been established that there are solutions to the constitutive equations with the corresponding boundary conditions in the form of moderately damped deformation tectonic waves, which provide excitation of excess stresses in the earth's crust and lithosphere at large distances from the source of the waves themselves (about a few thousand kilometers). The values of these stresses, which are about 0.1 MPa, are quite sufficient to initiate and develop the processes of induced seismicity and destruction of metastable gas hydrates. Information about the project is available on the page of the Laboratory for Geophysical Research of the Arctic and the Continental Margins of the World Ocean on the website of the Moscow Institute of Physics and Technology: https://mipt.ru/science/labs/arctic-geo-lab/rezultaty-issledovaniy/proekt-rnf/

 

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Annotation of the results obtained in 2020
As part of the project in 2020, theoretical studies were carried out in a number of areas related to the development of understanding geodynamic processes occurring in subduction zones accompanied by the great tsunamigenic earthquakes. Such earthquakes with magnitudes M> = 8 lead to the release of colossal elastic stresses accumulated over hundreds or even over a thousand years. Prediction of such events, causing significant socio-economic and environmental damage, is among the most important and urgent problems of geophysics. One of the models describing the occurrence of the great subduction earthquakes, taking into account the fault-block structure of the continental margin, is the keyboard-block model of the strongest earthquakes in present-day subduction zones, proposed earlier by the project leader Leopold Lobkovsky. This model combined the ideas of possibility of synchronous destruction of several adjacent asperities, mutual slip along a plane with a variable friction depending on velocity, and subsequent healing of destructed portions of the medium under high pressure conditions. This concept made it possible to simulate the displacement of frontal seismogenic blocks at different stages of the seismic cycle. However, the rear massif of the island arc was considered as a single structural element, not divided into separate blocks, and not experiencing its own horizontal displacements. At the same time, the observational data in combination with the results of geological and seismological surveys clearly indicate that the rear part of the arc also has a complex structure and is divided into separate segments by large faults rooted into the contact zone of interacting lithospheric plates. This points to the need of generalization of the original model in order to take into account the discontinuity of not only the frontal, but also the rear part of the island arc. The most important motivation for improving the model is also the fact that geodetic observations of surface’s motion are only possible onshore, that is, within the rear blocks, which does not allow directly interpreting the available data on displacements within the framework of the original single-element model. The model described above was generalized for the case when the block system is represented by two, front and rear, elements. The geomechanical evolution of the model is described by a set of displacements referred to the internal points of the blocks (of both front and rear elements). For each of the blocks, there is an equilibrium equation that relates the change in stress in the direction of subduction with tangential stresses acting at the bottom of the blocks from the side of the viscous contact layer on the surface of the subduction slab, as well as from the side of neighboring blocks. At the edges of the model, boundary conditions are used, implying that the outer edges of the blocks are free, and the rear parts of the blocks of the second element elastically interact with the fixed edge. The condition at the boundary between elements changes depending on the stage of the seismic cycle: during stress accumulation (interseismic stage), the motion of the frontal block is fully transmitted into the rear block, while during the postseismic (aftershock) stage, it is assumed that the elements are separated by some gap formed as a result of the pushing back of the frontal block towards the ocean after the stress is released by the seismic event. In this case, the motion of the front edge of the rear block is subject to the free boundary condition. The duration of the postseismic stage is determined based on the condition of the elements’ convergence after the change in the direction of frontal block’s motion, which indicates the transition to stress accumulation. The system of equations constructed in the above manner is characterized by significant nonlinearity, and the algorithm used to solve it is implemented in the form of an explicit finite-difference scheme that makes it possible to obtain displacements of the nodal points of the blocks, taking into account the location of observation stations for a set of times specified with a certain timestep. On the basis of this generalized two-element geomechanical keyboard-block model of the subduction zone, a computational scheme has been implemented that makes it possible to simulate the evolution of the system of blocks of the front and rear elements, including the timeseries of displacements and stresses at the nodes (points) of the blocks, as well as the elastic energy accumulated by the blocks in the process of cyclic seismotectonic evolution. A software code has been created for calculating the specified parameters and visualizing the geometry of the model and simulated quantities. The code was used to calculate the displacement series in a time interval of 500 years for a set of specific block parameters. The constructed model of the seismic cycle reproduces well the main characteristics of the seismic process in subduction zones, while the obtained average duration of the seismic cycle was about 200 years, which is consistent with the previously obtained values for the Kuril-Kamchatka and Japanese subduction zones. The comparison with the results obtained earlier using the single-element model shows the consistency of the simulation results, taking into account the stochastically changing constraints on the parameters responsible for the accumulated stress release. The obtained estimates of the coseismic displacements of the frontal and rear blocks, as well as the duration of the aftershock stage of the seismic cycle, are in good agreement with the available data of satellite geodetic measurements and the results of independent modeling. Comparison of the constructed model with the data of satellite geodetic measurements will allow not only to clarify the determining mechanical properties of the medium, but also to obtain critically important estimates of the times of transition of the structural elements of the subduction system from one state to another. In particular, realistic estimates of the moment of the end of the stage of consolidation of the contact layer (i.e., the moment of restoring the adhesion of the island-arc block to the plate), as well as the moment of contact of the frontal block with the rear block (i.e., the beginning of the stage of elastic compression of the system, accompanied by the accumulation of stresses) have a large importance in solving problems of long-term and medium-term forecasting. As part of the testing of the constructed numerical models, an analysis of the sensitivity of the simulated time series of displacements and released energy to changes in individual parameters of the model was carried out. The estimates of model sensitivity were obtained from a series of experiments to simulate a sequence of synthetic seismic cycles over a time interval of 1400 years. In each numerical experiment, a specific selected parameter of the model was varied, while other parameters remained unchanged. Sensitivity analysis made it possible to identify the parameters of the model that most strongly affect the main characteristics of its geomechanical evolution. It was found that viscosity values of the underlying contact layer on the surface of the slab, zones of destruction between the blocks and the viscosity of the crustal asthenolayer underlying the rear block significantly affect the behavior of the time series of displacements of the deformation system. In the presented model, with a decrease in the viscosity of the asthenolayer below 10^18 Pa ∙ s, an unstable behavior is observed. Based on the analysis results, a good adaptability of the constructed model was demonstrated for describing the seismic cycle in subduction zones of different tectonic and rheological structure. An important element of the research was the analysis of the optimal spatial discretization of the problem (model), taking into account the spatial density of GNSS observations. This analysis included the stability tests, which give a general estimate of the resolution of the original data. The main idea of the method is to simulate a synthetic signal (in our case, the station displacements) from a disturbing source at observation points used in solving the original problem. Using synthetic distributions of alternating (over the area) zero and unit displacements, the displacements for the first Simushir earthquake, Maule earthquake, and Tohoku earthquake were recovered, and the following discretization was determined: the size of the rectangular element of the rupture surface for the Simushir earthquake was taken equal to 57.5 × 50 km, for the Maule earthquake is 60 × 30 km, for the Tohoku earthquake 60 × 50 km. The obtained estimates of the resolution of the GNSS data serve as the basis for the construction and testing of models for the release of elastic stresses in the foci of subduction earthquakes. In the future, these will be used in the construction of three-dimensional keyboard-block models of the generation of subduction earthquakes in the Kuril, Chilean, and Japanese subduction zones. The constructed models were tested taking into account the tectonic structure of the studied subduction zones in the sections passing through the sources of the above events, and the distributions of displacement in the sources of the described subduction earthquakes were obtained, taking into account the regional tectonic structure of the subduction zones. The study of the effect of postseismic processes on the release of elastic energy in the source of the strongest subduction earthquake was carried out for the case of Tohoku earthquake. A model of aseismic frictional development of a seismic rupture in the first six months after the Tohoku earthquake was constructed as a result of solving an inverse problem based on GNSS data. Analysis of the monthly distributions of displacements in the earthquake source shows that the development of a seismic rupture occurred mainly in the direction of the source plane dip and was practically completed by the end of the indicated time interval. The constructed model of the frictional aseismic development of the seismic rupture of the Tohoku earthquake based on satellite measurements, taking into account the short-period viscoelastic response of the asthenosphere, is in a good agreement with a similar model constructed from the initial measurements, at the same time revealing some differences in the spatio-temporal distribution of displacements in the source. The analysis of the results obtained revealed that in the case of the Tohoku earthquake, the fast response of the asthenosphere does not significantly affect the course of the aftershock stage of the seismic cycle, which is reflected in the energy characteristics of the process of released elastic stresses. The obtained data will be taken into account when specifying the physical foundations of the developed geomechanical model in order to correctly model the postseismic stage of the seismic cycle. Another area of research was evaluation of the entanglement parameters associated with mechanical contact of lithospheric plates in the subduction interface, using satellite geodesy data. As a quantitative characteristic of inter-plate deformations, the coefficient of inter-plate cohesion was used, which is defined as the ratio of the displacement rate of the lower edge of the overhanging wedge to the rate of mutual displacement of the plates. The estimation of this value was done by solving the inverse problem, where the time series of positions recorded by the GNSS stations of the GEONET network, available from the Japan Geospatial Information Agency, which were used as input data. To reveal the temporal variations of interplate cohesion in the observed time series of surface displacements, filtration was carried out aimed at cancelling other processes: coseismic displacements, postseismic transient processes, seasonal variations. As a result, the spatio-temporal distribution of interplate cohesion in the Japanese region was obtained; it was compared with the distribution of coseismic displacements in the source of the Tohoku earthquake, revealing some peculiarities. Finally, in addition to the above-described research items, the analysis of the spatio-temporal pattern of the distribution of the sources of the great earthquakes in the subduction zones during the 20th and 21st centuries was carried out, and the correlation between the series of mega-earthquakes and the observed phases of sharp climate warming in the Arctic was traced. The well-known temperature curve for the Arctic clearly shows two phases of a rather sharp increase in the average temperature against the background of its interannual fluctuations: the first phase of a noticeable rise falls within the period from about 1920 to 1940, and the second phase of warming began around 1980, and is continued till now. To explain this phenomenon, L.I. Lobkovsky proposed a seismogenic-trigger hypothesis of the emergence of phases of sharp climate warming in the Arctic as a result of strong mechanical disturbances in the marginal region of the Arctic lithosphere, caused by strong earthquakes in the Aleutian subduction zone. Those stress disturbances are thought to propagate across the lithosphere to the Arctic shelf and adjacent land, and cause the destruction of permafrost rocks and metastable gas hydrates with subsequent emissions of greenhouse (methane) gas into the atmosphere. Project information is available at: https://mipt.ru/science/labs/arctic-geo-lab/rezultaty-issledovaniy/proekt-rnf/

 

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Annotation of the results obtained in 2021
As part of the project in 2021, theoretical studies were continued in a number of fields related to the development of ideas about geodynamic processes occurring in subduction zones that cause strong tsunamigenic earthquakes. Such earthquakes with a magnitude M> = 8 lead to the release of colossal elastic stresses accumulated over hundreds or even over a thousand years. Prediction of such events, causing significant socio-economic and environmental damage, is one of the most important and urgent tasks of geophysics. One of the models for the generation of the strongest subduction earthquakes, taking into account the fault-block structure of the continental margin, is the keyboard model of the occurrence of the strongest earthquakes in modern subduction zones, proposed earlier by the project manager. This model combined the ideas of possible synchronous destruction of several adjacent roughnesses, mutual sliding along a plane with a variable velocity coefficient of friction and subsequent healing of medium defects under high pressure conditions and made it possible to calculate the evolution of displacements of frontal seismogenic blocks at different stages of the seismic cycle. As part of the research carried out at the previous stage of the project, the model described above was generalized for the case when the block system is represented by two links - front and rear. The work carried out in the reporting phase of the project (2021) was mainly aimed at preparing GNSS data and other geophysical materials and geomechanical models, as well as developing algorithms that should form the basis for the interpretation of observed movements in terms of geodynamic processes and seismic cycles. in a number of subduction regions. In order to estimate the amplitudes and rates of displacements of points on the earth's surface in the Chilean, Kuril and Japanese subduction zones, caused by the action of various geodynamic processes manifested at all stages of the seismic cycle, an iterative post-processing algorithm was applied to the original satellite geodetic data (time series of coordinate changes), which allows make a direct assessment of the components of the displacement caused by various geodynamic processes, taking into account the complex stochastic nature of satellite geodetic measurements. The main idea of the applied approach is to construct a linear regression model of the motion of a satellite geodetic observation station taking into account the action of geodynamic processes associated with both large-scale tectonic movements and deformation processes caused by the passage of various stages of the seismic cycle. As a result, the following were obtained: 1) estimates of the stationary rate of deformation of the edge of the continental lithospheric plate in the Kuril-Kamchatka, Japanese and Chilean subduction zones and its variations before and after the strongest subduction earthquakes; 2) the values of coseismic and postseismic displacements associated with the strongest earthquakes in the Chilean subduction zone: Maule on February 27, 2010 (Mw = 8.8), Iquique on April 1, 2014 (Mw = 8.1), and Ilhapel on September 16, 2015 (Mw = 8.3 ), The Japanese subduction zone: Tohoku on March 11, 2011 (Mw = 9.0) and the Kuril subduction zone: Paired Simushir earthquakes on November 15, 2006, January 13, 2007 (Mw = 8.3 and Mw = 8.1, respectively). An analysis of satellite geodesy data in Northeast Asia was carried out, which for the first time made it possible to directly observe the modern movements of lithospheric plates in this region and to estimate the intensity of deformation processes, both at the boundaries of the plates and in their central stable regions. Analysis of the field of horizontal components of the displacement velocities of the earth's surface in East Asia revealed a number of its specific features. Throughout the continental part of East Asia, including the territory of mainland China, as well as the Far Eastern and Arctic regions of Russia, including on about. Sakhalin, there is a well-coordinated movement of observation points to the southeast, in the direction of the Pacific Ocean, while the displacement vectors are not only coordinated in direction, but also close in magnitude. At the same time, on the continental margin of East Asia, the displacement field is extremely heterogeneous both along the strike of the oceanic trench and in the direction inland. These inhomogeneities are especially noticeable in the data of satellite measurements on the Kamchatka Peninsula: the western coast and the central part of the peninsula are displaced to the southeast in accordance with the general direction of the regional field of displacement vectors, while the east coast shows a significant reversal of the displacement vectors to the southeast. west. In addition, there is a general gradual decrease in the magnitudes of the displacement vectors when moving from the western to the eastern coast of the Kamchatka Peninsula. This pattern of displacements is well explained if the observed displacements are presented as the vector sum of the total vector of the regional displacement to the southeast and the deformation vector caused by the compression of the overhanging continental edge due to subduction of the Pacific plate and can be described by the superposition of the long-term action of a long convective cell in the upper mantle and significantly faster cyclic movements of lithospheric blocks in the subduction zone. The next line of research was focused on the construction of model set describing the distributed effective displacement in the sources of the strongest subduction earthquakes based on data on postseismic displacements of the earth's surface. Within its framework, the rheological structure of the subduction zones was refined and the variations in the values of the effective viscosity of the asthenosphere at the postseismic stage of the seismic cycle were estimated. It was found that the degree of mechanical entanglement immediately before the Tohoku earthquake was relatively high. The interplate adhesion coefficient reaches its maximum values near the trough, which could be one of the factors that influenced the occurrence of such a devastating tsunami. Near the lower face of the future fault zone, areas of weakened cohesion are found, which probably prevented the downward propagation of the seismic rupture. The fact of general consistency of configurations of regions of significant seismic displacements in the source and nonzero interplate cohesion was established. The area of maximum displacements in the seismic source is, as expected, located in the upper part of the fault zone. The areas of the greatest displacements in the effective postseismic source are concentrated near the lower edge of the seismic rupture, apparently marking the areas where residual stresses were not released during the earthquake. The obtained estimates of the effective Maxwellian viscosity of the asthenosphere in northeastern Japan are about ten times lower than the average value established for subduction zones of the continental margin type. According to the forecast, based on the assumption of the constant effective viscosity of the asthenosphere, the prevalence of postseismic displacements in the vicinity of the source will cease approximately 20 years after the Tohoku earthquake. The estimates of the source parameters of the three strongest tsunamigenic earthquakes were obtained: Maule on February 27, 2010 (Mw = 8.8), Iquique on April 1, 2014 (Mw = 8.1), and Ilhapel on September 16, 2015 (Mw = 8.3). The focal mechanisms of all the above earthquakes represent a gentle thrust with a dip towards the continent, which corresponds to the compression conditions typical for the convergent boundary of lithospheric plates. Within the foci of the earthquakes under study, equally strong seismic events were realized before, and their recurrence periods vary from 63 to 175 years. Based on data on coseismic displacements during the 2010 Maule, 2014 Iquique, and 2015 Iquique earthquakes, models of distributed displacement in their sources were constructed (Figs. 14b, 15b, 16b, Appendix 1 to the report). The obtained distributions of displacements along the fracture are the result of solving the inverse problem, which reduces to minimizing the discrepancies between the measured satellite methods and the modeled coseismic displacements. For 2010 and 2015 events. there is a bilateral development of seismic ruptures with a total length of 600 and 250 km, respectively. A distinctive feature of the 2014 event is the predominantly unidirectional propagation of the rupture, the length of which was less than 200 km instead of the expected 600 km. The values of the maximum displacements in the sources in all three cases are in the range from 6 to 12 m.Based on the obtained distributions of displacements in the sources, direct estimates of the scalar seismic moment released during earthquakes were obtained, provided that the shear modulus of the rocks composing the lithosphere of the Chilean subduction zone is equal to 40 GPa. One can note the general agreement between the estimates of the scalar seismic moment and the magnitude of all three earthquakes with similar estimates obtained from seismological data. The algorithm of the inverse problem for the selection of the parameters of the two-link key-block model of the subduction zone has been implemented; it was tested on synthetic data arrays. The inversion of the prepared time series of displacements of the GNSS stations for the Japanese subduction zone was carried out, and estimates of a number of geomechanical (rheological) characteristics of the key-block system were obtained. The results of solving the inverse problem on an array of synthetic data imitating the movement of elements of a 5-key two-link model allow us to conclude that it is fundamentally possible to reconstruct the required parameters by rows of displacements in separate points of the earth's surface located on the blocks of the keyboard system. In both cases, the inversion algorithm managed to achieve a good quantitative agreement between the input and calculated displacement series at all considered nodes of the model. The obtained estimates of the elastic moduli and viscosity parameters of the contact layer at the base of the lithosphere under the frontal blocks, as well as the crustal asthenosphere under the rear blocks, show a fairly good agreement with the true values. At the same time, the restoration of the viscosity parameters of the interblock zones is less accurate. An analytical solution is obtained for a thermomechanical problem describing the propagation of small transverse perturbations associated with tectonic deformation waves in a two-layer model of the lithosphere-asthenosphere, taking into account the condition of the phase transition between the layers. In the approximation of small perturbations of mechanical equilibrium, using the linearized heat transfer equation, analytical expressions were obtained for the fields of temperatures, vertical displacements, and velocities. Using the constructed solution for a set of characteristic values of the parameters of the medium, we calculated the curves of the dependence of the velocity of tectonic waves on their length, as well as the dependences of the wavelength, amplitude of surface displacements and horizontal stresses on the viscosity of the asthenosphere under conditions of an equilibrium and nonequilibrium phase transition at the lithosphere-asthenosphere boundary. Analysis of the results obtained, taking into account the characteristic values of the physical parameters of the lithosphere and asthenosphere, allows us to conclude that there are solutions of the constitutive equations with the corresponding boundary conditions in the form of moderately damped deformation tectonic waves, which provide the excitation of excess stresses in the earth's crust and lithosphere at large distances from the source of the waves themselves (about a few thousand kilometers). The values of these stresses, with initial displacements in the source of the order of 1 m, amounting to about 1 MPa, are quite sufficient for the initiation and development of processes of induced seismicity and destruction of metastable gas hydrates. Characteristic velocities of tectonic wave propagation are about 100 km/year, they are found to be able to travel over a few thousand km without significant attenuation. These results support the correlation between the maximum seismic activity of the 20th century in Aleutian arch and abrupt warming phases in the Arctic which followed with 20-year delay.. The analysis of seismic cycles in the Aleutian subduction zone is carried out. It is shown that the direction of the plate convergence vector can have a significant effect on the preparation and implementation of the strongest earthquakes in subduction zones. In particular, as a result of the analysis of the features of the seismogenic process in the western part of the Aleutian subduction zone, it was found that the seismic cycles in the western part of the Aleutian arc are, on average, shorter than in the eastern one. In addition, it was revealed that the strongest earthquakes, repeating in the same areas of the western part of the Aleutian subduction zone, differ both in magnitude and in the length of the source. As a mechanism explaining the reduction in the duration of seismic cycles and noticeable differences in the spatial extent and localization of the sources of events of similar magnitudes in the same segment of the western half of the Aleutian subduction zone, a key-block model of the generation of the strongest earthquakes under conditions of oblique subduction is proposed. Taking into account the proposed concept of generation of the strongest earthquakes in subduction zones with a pronounced tangential component and the seismogenic-trigger hypothesis of the activation of methane emission in the Arctic cryolithosphere, it can be concluded that the shortened time intervals between the series of the strongest seismic events in the Aleutian subduction zone in the Arctic. Information about the project is available at the website of the Laboratory for Geophysical Research of the Arctic and Continental Margins of the World Ocean on the website of the Moscow Institute of Physics and Technology: https://mipt.ru/science/labs/arctic-geo-lab/rezultaty-issledovaniy/proekt-rnf/

 

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