Stress State of the Earth’s Crust in the Western Tien Shan in Central Asia (Uzbekistan): A Mathematical Stress Model (original) (raw)
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Geotectonics, 2021
The results of analyzing the data on strong М ≥ 5 earthquakes in Central Asia for 1901–2013 are presented. A statistical method for studying the influence of the strongest earthquakes in the region on the occurrence probability of seismic activation in a certain seismically active zone in Uzbekistan is used. Based on this method, the statistical significance of the difference between the fraction of strongest earthquakes in the region that fall within the alarm intervals of the studied seismically active zone and the fraction of cumulative time of alarms in this zone is assessed. It is established which seismically active zone in the Uzbekistan territory is most likely to manifest seismic activation if a strong earthquake occurs in the Central Asian region. A mathematical model is developed for the deformation of the Central Asia lithosphere, the lateral margins of which are being subjected to lateral compression. The proposed model is used to analyze changes in background stresses in the crust of Central Asia caused by strong earthquakes in the territory bounded by the geographical coordinates from 36° to 46° N and from 56° to 76° E. A numerical experiment is carried out by solving the inverse problem of elasticity theory to determine the background stress in the lithosphere. By sequentially selecting the boundary stresses and varying the physical elasticity parameters, we have constructed the stages of crustal stresses conforming to their empirically known values. The earthquake source is simulated in kinematic dislocation with subdivision into types: a concentrated dipole with a moment (type I), a concentrated double dipole without a moment (type II), and a singular volume. The numerical results of the model have shown that that the variations in stresses caused by a strong earthquake are largely determined by the proximity of orientation of displacement of the strong earthquake source and the axes of crustal structures affected by wave perturbation. In regions where stresses in the crust are close to critical, the stress increment can trigger seismic activation.
Experience of Modeling the Seismotectonic Flow of the Earth’s Crust in Central Asia
Izvestiya-physics of The Solid Earth, 2021
Abstract —Tectonic flow of crustal rock masses (a creep flow) in Central Asia is numerically modeled using the Stokes equations within a region limited by 36° and 46° N, 56° and 76° E. In the study, tectonic and seismotectonic flow of a rock mass is understood as a strain rate and its variation induced by an earthquake. The focal mechanism of an earthquake is modeled in two ways: by a lumped internal moment and by a double couple with no net torque. Three-dimensional continuum equations are averaged over the lithosphere’s thickness with the use of the properties of the geodynamic statement of the problem. The right-hand sides of the averaged creep flow equations contain tectonic stresses and present-day topography as the initial data. For their determination, the inverse problem of elasticity is solved. The possible boundary conditions providing the formation of the present-day surface topography of Central Asia by the interaction between the Indian, Arabian and Eurasian plates are sought. The obtained numerical results fairly well agree with the real GPS data.
Journal of Geophysical Research: Solid Earth, 1995
This paper presents a review of the seismotectonics of the Pamir-Tien Shan collision zone in the Garm region, Tajikistan, based on geological structure, seismicity, and focal mechanism solutions. The region is dominated by horizontal compression, manifested by imbricate, low-angle thrust faults that separate the upper crust into a series of tectonic sheets. These thrust systems verge northward from the Tajik Depression toward the southern Tien Shan and southward toward the northern Pamir. The pattern of seismicity across the region suggests that similar lowangle thrust faults exist within the crystalline basement as well. In order to reconstruct the present-day stress-strain state of the region, we used data from over 15,000 earthquake focal mechanism solutions for small earthquakes (M > 1.0) gathered over the 27-year period, 1963-1989. The method of reconstruction of the seismotectonic deformation (STD) field involves dividing the data set into small spatial windows and summing the individual focal mechanism solutions to form an average earthquake mechanism tensor for each cell. The STD state of that cell can then be represented by the orientation of the principal axes of compression and tension, the intensity (or relative uniformity) of the average mechanism, and the Lode-Nadai coefficient, which defines the relation between the magnitudes of the principal strain components. We observe throughout the study area a general predominance of subhorizontal compression, manifested in a mixture of thrust and strike-slip deformation; normal-fault deformation is observed in only two small portions of the study area. The orientation of principal compression varies from nearly N-S in the northern Pamir and the southern Tien Shan to NW-SE in the Peter the First Range. In general, the STD field has a clearly developed "mosaic" structure, defined by spatial groupings of nearly uniform orientations of compression and tension axes. The boundaries of these groups cannot in most cases be directly correlated with known geological contacts. The STD structure also shows some variation with depth, with more or less uniform STD orientation in the upper crust (0-14 km depth), changing significantly at greater depths. Examination of various subsets of the focal mechanism catalog shows the spatial structure of the STD field to be largely stable with respect to both time and magnitude. The reliability of the redonstructed STD field was verified using three approaches: (1) comparison of individual focal mechanism determinations for a large subset of the data using both Soviet and U.S. algorithms, (2) comparison of STD reconstruction results using the two independent focal mechanism catalogs, and (3) examination of the effect of spatial sampling on the results. The dominance of generally north oriented subhorizontal compression is interpreted to be primarily the result of convergence between the Pamir and Tien Shan ranges, in turn caused by the ongoing collision of India and Eurasia. it can provide an objective criterion with which to evaluate
Geosciences Journal, 2017
We investigate the horizontal stress field beneath crustal structures of central Eurasia. The numerical procedure applied for a simultaneous determination of the sub-crustal stress and the crustal thickness from the global gravity, terrain and crustal structure models is based on solving Navier-Stokes' problem which incorporates the inverse solution to the Vening Meinesz-Moritz's problem of isostasy. The numerical results reveal that a spatial distribution of the sub-crustal stress in this study area closely resembles the regional tectonic configuration comprising parts of the Eurasian, Indian and Arabian lithospheric plates. The maximum shear stress intensity is generated by a subduction of the Indian plate beneath the Tibetan block. The intra-plate tectonic configuration is marked by the stress anomalies distributed along major active strike-slip fault systems and sections of subduction which separate the Tibetan and Iranian blocks from the rest of the Eurasian plate. The most pronounced intra-plate stress anomalies are related with a subduction of the Eurasian plate beneath the Tibetan block. We also demonstrate that a prevailing convergent orientation of stress vectors agree with the compressional tectonism of orogenic formations (Himalaya and Tibet Plateau, Than Shan, Zargos and Iranian Plateau), while the extensional tectonism of continental basins (Tarim, Ganges-Brahmaputra, Sichuan) is manifested by a divergence of stress vectors.
Seismotectonic deformations and recent tectonics of the Tien Shan
Izvestiya, Physics of the Solid Earth, 2008
Seismotectonic strains (STSs) are calculated on the basis of the catalog of focal mechanisms of earthquakes including more than 5000 events compiled at the Institute of Seismology, National Academy of Sciences of the Kyrgyz Republic, in order to investigate deformation processes in the Tien Shan (38 ° -44 ° N, 68 ° -80 ° E). A modern approach to STS classification based on the identification of 11 deformation settings (including four main, two extreme, and five transitional settings) is applied for constructing STS maps. Areal distributions of the Lode-Nadai coefficient, vertical component, and angle of the stress-state type are obtained. The results of the calculations are verified by comparing them with STS calculations using focal mechanisms of 116 strong earthquakes from the centroid moment tensor catalog of Harvard University that occurred in the studied region in -2003 91.45.Bg
Geodynamic Activity of Modern Structures and Tectonic Stress Fields in Northeast Asia
Geodynamics & Tectonophysics, 2017
Based on the analysis of changes in the stress-strain state of the crust at the boundary of the Eurasian and North American tectonic plates, we develop a dynamic model of the main seismogenerating structures in Northeast Asia. We have established a regularity in changes of geodynamic regimes within the interplate boundary between the Kolyma-Chukotka crustal plate and the Eurasian, North American and Pacific tectonic plates: spreading in the Gakkel Ridge area; rifting in the Laptev Sea shelf; a mixture of tectonic stress types in the Kharaulakh segment; transpression in the Chersky seismotectonic zone, in the segment from the Komandor to the Aleutian Islands, and in the Koryak segment; and crustal stretching in the Chukotka segment.
Earth Core's stresses variation in Central Asian earthquakes region
Geodesy and Geodynamics, 2020
Tectonic stresses of Central Asia (limited by geographic coordinates 36e46 N, 56e76 E) over the recent times are modeled based on moment elasticity, taking into account the focal mechanism of earthquakes. Numerical results obtained by the method of boundary integral equations. The relief of the earth's surface, built on the map of Central Asia used to verify the solution of the inverse elasticity problem. The moment elasticity equations are simplified by assuming u k ¼ ε ijk u i,j , which makes it possible to return to classical theory of elasticity, with the difference that the stress tensor ceases to be symmetric s ij ss ji . Based on the specifics of the geodynamic formulation of problems, the three-dimensional model reduced to a two-dimensional model for averaged stresses and displacements. According to the results, a displacement field was constructed, which is fully consistent with the movements of the earth's surface, established by the GPS method. The model makes it possible to determine stresses variations in the region from earthquakes occurring in Central Asia.
Izvestiya, Physics of the Solid Earth, 2006
Investigation and understanding of the present-day geodynamic situation are of key importance for the elucidation of the laws and evolution of the seismic process in a seismically active region. In this work, seismic moments of nearly 26000 earthquakes with K p ≥ 7 ( M LH ≥ 2) that occurred in the southern Baikal region and northern Mongolia (SBNM) (48 °-54° N, 96 °-108° E) from 1968 through 1994 are determined from amplitudes and periods of maximum displacements in transverse body waves. The resulting set of seismic moments is used for spatial-temporal analysis of the stress-strain state of the SBNM lithosphere. The stress fields of the Baikal rift and the India-Asia collision zone are supposed to interact in the region studied. Since the seismic moment of a tectonic earthquake depends on the type of motion in the source, seismic moments and focal mechanisms of earthquakes belonging to four long-term aftershock and swarm clusters of shocks in the Baikal region were used to "calibrate" average seismic moments in accordance with the source faulting type. The study showed that the stress-strain state of the SBNM lithosphere is spatially inhomogeneous and nonstationary. A space-time discrepancy is observed in the formation of faulting types in sources of weak ( K p = 7 and 8) and stronger ( K p ≥ 9 ) earthquakes. This discrepancy is interpreted in terms of rock fracture at various hierarchical levels of ruptures on differently oriented general, regional, and local faults. A gradual increase and an abrupt, nearly pulsed, decrease in the vertical component of the stress field S v is a characteristic feature of time variations. The zones where the stress S v prevails are localized at "singular points" of the lithosphere. Shocks of various energy classes in these zones are dominated by the normal-fault slip mechanism. For earthquakes with K p = 9, the source faulting changes with depth from the strike-slip type to the normal-strike-slip and normal types, suggesting an increase in S v . On the whole, the results of this study are well consistent with the synergism of open unstable dissipative systems and are usable for interpreting the main observable variations in the stressstrain state of the lithosphere in terms of spatiotemporal variations in the vertical component of the stress field S v . This suggests the influence of rifting on the present-day geodynamic processes in the SBNM lithosphere.
Tectonic analysis of shikhan earthquakes
The March 11-13, 2013 (ML 4.9-4.6) earthquakes were the biggest events that occurred in the Shaikhan area near the northeastern boundary of the Arabian plate in the Kurdistan Region over the last 20 years. The events were felt over a wide area of the Duhok and Mosul governorates. They were well detected and located by local and global seismic networks. The Shaikhan earthquakes offer a good opportunity to study the active tectonics of the Kurdistan Region. The analysis of earthquake focal mechanisms clearly shows a neotectonic reactivation of N-dipping thrust and NWdipping sinistral conjugate strike-slip faults with focal depths in the range 6-7 km. The rotational optimization method (Windows-Tensor, Version 5.0.2) was used to obtain the stress state from earthquake focal mechanisms. The stress inversion results show that the average trend of maximum compressive stress in the Shaikhan area is N20°E. This present-day stress orientation supports the well-documented N-S convergent motion between Arabia and Eurasia. The Shaikhan earthquakes created considerable alarm in the Duhok Governorate and highlighted the fact that damaging earthquakes can happen in this region.