Виталий Евгеньевич Матюков

The “NARYN” geoelectric model, which runs along the 76° E meridian through the Central Tien Shan from the Kazakh Shield to the Tarim Platform, is one of the longest cross-sections in continental Asia. This paper provides a historical overview of geoelectric models based on magnetotelluric (MT) soundings from 1999-2000 and 2005. Two inversion models constructed from these data using different algorithms are presented: the discrete-smoothed NARYN-RLM model and the piecewise continuous NARYN-INV2D model. A local earthquake catalog for 1980­2022 is considered. Hypocenter distributions from four regional and global seismic catalogs are superimposed on a reference cross-section to re-examine the previously identified relationship between crustal conductivity and earthquake depth distribution. It is noted that weak seismicity is mainly concentrated above the upper boundary of the middle and lower crustal conductor, while the strongest events are grouped in resistive, consolidated blocks of the earth’s crust or at their contacts with conductive zones.

When studying the relationship between electromagnetic and deformation processes, according to the results of magnetotelluric monitoring, electromagnetic pulses were detected, which may be associated with the earthquakes that occurred. The observations were carried out at two magnetotelluric stationary monitoring points located in the Chu depression (Aksu 42.60911 °N, 74.00833 °E; Chon-Kurchak 42.62828 °N, 74.60671 °E, Northern Tien Shan), on the territory of the Bishkek geodynamic polygon. To identify earthquake responses, the results of instrumental observations of the electromagnetic effects of a strong earthquake and its aftershocks that occurred in northern China on January 22, 2024 with a magnitude in the range of 4.9–6.9, a weaker one with a magnitude of 5.4 (Kyrgyzstan) and a number of powerful remote earthquakes with a magnitude in the range of 5.2–6.6 at a distance of 500–1200 km from the epicenter were analyzed. It is shown that an earthquake with numerous aftershocks located at distances from ~450 km from the registration points is reflected in all recorded parameters, while a weaker earthquake, but located closer, does not manifest itself in one of the horizontal components of the electromagnetic field. The mechanisms of occurrence of seismoelectric signals and mechanical-electromagnetic transformations in the Earth’s crust are considered. The reality of the appearance of electromagnetic precursors of earthquakes and coseismic signals observed in the first tens of seconds or minutes after an earthquake is shown. The results of the research can be used in the development of methods for monitoring seismic activity in potentially dangerous regions.

The paper discusses various approaches for processing and analysis of synchronous magnetotelluric and magnetovariational data obtained at the Kentor mini test polygon, which is located in the Baytic basin of the Chui region of Kyrgyzstan. The versions of noise reduction of received electromagnetic signals at the stage of robust processing are considered. The materials were processed using a remote reference technique for possible noise detection and calculate the additional components, such as a horizontal magnetic tensor for further interpretation of the obtained data. The dynamics of changes in the geoelectric cross-section for 2 sessions of researches by the tested profiles is considered.

This article consolidates the results of studying the deep structure of the lithosphere of the Central Tien Shan, which aimed to identify the main tectonic elements in its geophysical models. We have compared the structural and geological data with the information on the deep structure obtained by geophysical methods and from the positions of earthquake hypocenters in the study area. According to geological concepts, the Tien Shan orogenic belt is characterized by longitudinal and transverse segmentation. The boundaries of the Northern, Middle, Southern Western and Eastern segments of the Tien Shan are deep-seated fault structures. In deep faults and channels of heat and mass transfer, endogenous processes are localized. High-velocity, geoelectrical and thermal models consider such faults and channels as contrasting objects that can be referred to as indicators of these processes.Our analysis of the locations of earthquake hypocenters from NNC, KNET, CAIIG, KRNET, SOME catalogues shows that seismic events are strongly confined to the fault zones and the boundaries of large blocks. A correlation between the anomalies of geophysical fields suggests the degree of inheritance of tectonic structures and the boundaries of the main tectonic segments of the Tien Shan. To compare the crustal and upper mantle heterogeneities reflected in different geophysical fields, we have analyzed seismic tomographic sections based on volumetric seismotomographic models geoelectric and velocity sections along profiles across the main tectonic elements of the study area. The sections are used to identify the zones with relatively low (i.e. reduced) seismic wave velocities and detect the deep-seated longitudinal segmentation of the folded belt. Objects showing anomalous seismic wave velocities are found in the seismotomographic sections at all the depths under consideration. The most contrasting differences in the velocities of P- and S-waves are typical of the depths of 0-5 km and 50-65 km, showing the most clearly observed Northern, Southern and Western segments of the Tien Shan. In general, the velocities of P- and S-waves at the Northern Tien Shan are higher than those at the Middle and Southern segments. We have analyzed the distribution of geoelectric heterogeneities identified from magnetotelluric sounding data in order to determine the boundaries of the main tectonic elements that are considered as the zones of increased electrical conductivity confined to the boundaries of the fault structures. The distribution of earthquake epicenters clearly reflects the segmentation of the Tien Shan into the Northern, Middle and Southern segments and shows the Western and Eastern Tien Shan relative to the Talas-Fergana fault. Ourstudies of the crust and the upper mantle of the Tien Shan have confirmed that the abovementioned tectonic segments have differences in their deep structures Based on a comprehensive analysis of the study results, we can qualitatively identify a relationship between the distribution of the velocity and geoelectric heterogeneities in the crust and upper mantle, seismicity and the stress-strain state of the crust.

The paper describes application of an approach according to which the endogenous-nature electromagnetic field (EM) is included in the impedance relationships in an additive way, which allows distinguishing the EM field recoded on ground surface by the positions of sources. The case-studies of the endogenous EM field identification in the magnetotelluric sounding records obtained in the Tien Shan region are presented. The characteristics of the identified endogenous EM field are compared with the lunar-solar tides parameters and anisotropic properties of electrical resistivity. The found interconnections point at new options to study irreversible lithospheric deformations in the Tien Shan using magnetotelluric sounding data.

New results of a detailed study of the deep structure of the Central Tien Shan along the Son-Kul magnetotelluric (MT) profile crossing the Son-Kul Lake are reported. Based on the results of magnetotelluric data modeling, the regional and local geoelectric anomalies in the lithosphere are studied and their quantitative characteristics are given. Geological interpretation of the geoelectric cross-section was carried out, which supported the existing ideas about the block–hierarchical structure of the upper part of the Earth’s crust. This corresponds to the tectonophysical concepts of the sequential inserted subordination of large and smaller elements of the zone–block structure consisting of stable blocks and limiting mobile zones, which are distinguished by the high dislocation of the geological substrate. The integral pattern of the distribution and morphology of zones of high electrical conductivity in this segment of the Central Tien Shan crust may reflect discretely localized palm tree–type structures associated with the evolution of transgressive suture zones of localized deformation during the Hercynian and Alpine tectogenesis.