Comparison of the spectral characteristics of ground vibrations from earthquakes with the level of seismic ambient noise in a wide range of periods makes it possible to assess the possibilities of seismic networks, taking into account the local conditions for the location of seismic stations. Based on the probabilistic approach, which allows one to estimate the statistical parameters of seismic variations in the frequency domain, a model of seismic ambient noise for the southeastern shore of Lake Baikal was developed. The study of the spectral characteristics of seismic waves was carried out on the basis of records of earthquakes in the Baikal rift zone with 3.5 ≤ M ≤ 6.9 that occurred during 2017–2022. The spectral characteristics of shear waves from earthquakes were calculated using bandpass filtering in nonoverlapping octave bands. Comparison of the calculated spectral characteristics of seismic vibrations from earthquakes with seismic ambient noise model made it possible to evaluate the possibilities of three broadband seismic stations. The approach can be used to select optimal equipment in terms of its characteristics, taking into account the local conditions for the location of stations.

Seismic ambient noise in the regional seismic network in the central part of the Baikal rift is studied. The probabilistic approach is used to thoroughly investigate the pattern of diurnal variations in microseisms and to analyze amplitude level and frequency content of spatial anomalies and temporal changes (seasonal and annual). Based on the 2020–2021 data, a regional probabilistic model of the microseismic noise is built in a wide range of periods. The study of microseisms in the frequency band of about 1 Hz revealed a seasonal anomaly against the level of the global minimum in the microseismic noise power spectrum. The anomaly is observed from May to December at seismic stations surrounding Lake Baikal except for the northern part of the lake. The direction of the back azimuth in the frequency range of about 1 Hz indicates the location of the lake, suggesting that these signals can be identified as lake microseisms. The high coherence values suggest a linear relationship between the wind speed and the occurrence of lake microseisms. The detailed analysis of the spectral and polarization parameters of seismic ambient noise revealed two types of lake microseisms with frequencies of 0.4–0.7 and 0.7–1.5 Hz. The first frequency interval is likely to correspond to single-frequency lake microseisms, while the second interval contains the frequency ranges of dual-frequency microseisms.

The research in seismic ambient noise as a tool for geophysical studies must primarily rely on the space-time characteristics of the noise itself. What is important in this research is to characterize the distribution of noise sources both over frequency and in energy content. The present review considers the main mechanisms for the generation of microseisms, including primary and secondary microseisms (0.05–0.3 Hz), low frequency oscillations (0.2–50 mHz), high frequency oscillations (2–60 Hz), and lake-generated microseisms (0.5–2 Hz). We also describe the most popular procedures in use for the processing and analysis of continuous seismic ambient noise arrivals; we demonstrate a wide range of geophysical problems based on recordings of microseismic ground motion.

Статья посвящена исследованию неотектоники и современной сейсмической активности Западного Забайкалья. Регион расположен на юго-восточном фланге Байкальской рифтовой системы (БРС) и является западной границей Амурской плиты, представляя переходную область между влиянием Индо-Азиатской коллизии, определяющей деформации сжатия, и растяжением при рифтогенезе. Целью исследования является геодинамическое районирование Западного Забайкалья методом компьютерного линеаментного анализа (технология LESSA) цифровой модели рельефа (ЦМР). Такой анализ позволил выделить в пределах исследуемой площади три района с различным структурным рисунком: Усть-Селенгинский, Западный и Восточный, которые различаются геодинамической обстановкой, характером сейсмичности, историей развития рельефа, временем и стилем неотектонических деформаций на разных стадиях рифтогенеза. Границы районов маркируются протяженными морфолинеаментами, градиентными зонами плотности малых линеаментов и сгущений структурных линий, совпадающими с геологическими глубинными разломами и зонами локализации деформаций. Выделены наиболее активные фрагменты линеаментов и определена их возможная кинематика. Определена зона сгущения субмеридиональных морфолинеаментов и предполагаемого пластического течения, которая предопределила геодинамическую историю региона и возможное положение западной палеограницы Амурской плиты на раннем этапе рифтогенеза пассивного типа. Выявлена роль протяжённых морфолинеаментов меридионального и ССЗ простирания на локализацию сильных землетрясений. Отмечено влияние Даурского мегасвода на распространение протяжённых линеаментов ССЗ простирания. The article concerns the research of neotectonics and modern seismic activity of the Western Transbaikalia. The region is located on the southeastern flank of the Baikal rift system and is the western boundary of the Amur Plate, representing a transition region between the influence of the Indo-Asian collision, which determines compression deformations and extension during rifting. The purpose of the study is the geodynamic zoning of Western Transbaikalia using computer lineament analysis (LESSA technology) of a digital elevation model (DEM). This analysis made it possible to identify three regions within the study area with different structural patterns: Ust-Selenginsky, Western, and Eastern, which differ in geodynamic settings, the nature of seismicity, the history of relief development, the time and style of neotectonic deformations at different stages of rifting. The boundaries of the regions are marked by extended morpholineaments, gradient zones of density of small lineaments, and thickening of structural lines, coinciding with geological deep faults and zones of deformation localization. The most active fragments of lineaments were identified and their possible kinematics were determined. A zone of concentration of submeridional morpholineaments and supposed plastic flow has been determined, which predetermined the geodynamic history of the region and the possible position of the western paleoboundary of the Amur Plate at the early stage of passive rifting. The role of extended morpholineaments of meridional and NNW strike on the localization of strong earthquakes has been revealed. The influence of the Daurian megaarch on the distribution of extended lineaments of NNW strike is noted.

The results of an seismic variation experiment at the Babushkin test site and monitoring observations of the 234U/238Uactivity ratio in groundwater from a paleoseismogenic dislocation in a zone of the Main Sayan Fault are presented. Found is a similarity between effects obtained in the experiment and data at the monitoring station in 2014, but no similar effects during preparation and realization of the strong Bystraya earthquake in 2020. It is inferred that the monitoring observation of a single station is not enough for a successful prediction of a strong earthquake. It is necessary to support hydrogeochemical monitoring of several stations located on the test site in different structural conditions and provided different information on seismogenic deformations that change over time.

The article provides an overview of vibroseismic studies carried out in the Baikal rift zone using LargeScale Research Facilities – a powerful CVO-100 seismic vibrator, installed at the South Baikal geodynamic test site SB RAS. Research is carried out according to several methods focused on different tasks: study of the structure of the Earth’s crust and upper mantle in the BRZ, active vibroseismic monitoring, and verification of velocity models of the Earth’s crust. To study the structure of the Earth’s crust and the upper mantle, there were done the vibrator-generated wavefield recordings at the stationary regional network of seismic stations in the Buryat and Baikal branches of the Federal Research Center of the GS RAS, as well as the experimental studies involving the mobile networks deployment (ICMMG SB RAS, SIPE RAS, GIN SB RAS). The aim of the work is to carry out deep vibroseismic sounding of the Earth’s crust (vibro-DSS) at the junction of the Siberian platform, the BRZ and the Sayan-Baikal folded area. The methodology is based on the study of vibration seismograms with the determination of arrival times of the main groups of waves and their correlation with the velocity models of the Earth’s crust in the BRZ. A CVO-100 vibrator and a regional network of seismic stations are used to carry out active vibroseismic monitoring of the southern part of the BRZ. The active monitoring area is about 500×200 km. During vibroseismic monitoring, there were done thorough studies of seasonal variations of the vibrator-generated wavefield and the development of techniques for spectral correction of seismograms. A seismic vibrator CVO-100 was used to carry out experimental verification of the velocity models of the Earth’s crust, developed based on the BEST and PASSCAL experimental data. The vibrational deep seismic sounding (vibro-DSS) on the Baikal – Ulan Bator profile was carried out by the ICMMG SB RAS, GIN SB RAS and BB FRC GS RAS (Russia) in cooperation with IAG MAN (Mongolia).

Evaluation of seismic wave attenuation parameters (Q-factor and frequency parameter) in the lithosphere of the Tien Shan and the Baikal rift was carried out using coda waves of regional earthquakes. Comparison of obtained quality factor values shows that attenuation in the lithosphere of the Tien Shan is stronger than in the Baikal rift. It can be explained by a higher level of heterogeneity of medium.

Some methodological issues of the improved location of local earthquakes, recorded by the Baykal seismic network, are considered. Hypocenter location improvement is performed by combination of widely used techniques into the integrated technological line, moreover some of these techniques will be used in their new capacity. For example, for the first time for the Baykal area in the Hypoellipse software the following parameters, obtained from Wadati diagrams, will be used: origin time (To), defined with the peak determination factor, is to apply as an objective measure of the location accuracy; Vp/Vs ratio - as a fixed for each of the stations input parameter. In addition, to calculate the travel times in Hypoellipse software a multi-component velocity model (as equivalent of 3D model) have been used also for the first time for the area of investigations.

The displacements of the Earth’s surface before and after the earthquakes that took place near the southern shore of Lake Baikal on September 21, 2020 (Bystrinskoe earthquake with a magnitude of M = 5.5), and August 27, 2008 (Kultukskoye earthquake with a magnitude of M = 6.3), were analyzed using satellite radar interferometry methods. Remote sensing data obtained by the Sentinel-1B (C-band) and the ALOS-1/2 PALSAR-1/2 (L-band) spaceborne synthetic aperture radars were used to study deformation processes. Blocks with different strain rates were found based on the results of analysis of satellite radar interferometric measurements for the period from May 2017 to October 2020 for the Bystrinskoe earthquake and from January 2007 to February 2011 for the Kultukskoe earthquake. The difference in the deformation values before the Bystrinskoe earthquake was 14 mm and that before the Kultukskoe earthquake ranged from 12 to 13 mm.

Whereas the defined velocity model plays a key role in the process of seismic events localization, so selection of the model as much as possible corresponding to the real velocity conditions of the investigated area becomes a crucial task. Basing on the analyses of published results of the Lake Baikal area seismic study a layered P-waves models for two situations defined: For the high velocity consolidated rock on the lake banks and low velocity sediments up to 10 km thick under the lake bottom.

The results of a geophysical field experiment on the ice sheet of Lake Baikal are presented. The experiment was carried out in order to obtain a better understanding of generation and propagation of geo-hydroacoustic waves in the lithosphere–hydrosphere–ice system characterized by a layered structure. For the first time, the recording of signals from a distant vibroseismic source by means of an ice-based seismo-hydroacoustic array consisting of separate elements has been demonstrated.

Глубоководная впадина бассейна озера Байкал является наиболее сейсмоактивной в Байкальской рифтовой зоне. За последние 160 лет в центральной части Байкальского рифта произошло порядка двух десятков сильнейших землетрясений магнитудой 5 и выше, в т.ч. несколько катастрофических. Наиболее сильными землетрясениями, локализованными в районе дельты р. Селенги, являются максимальные по магнитуде Цаганское (12.01.1862 г.; MLH = 7.5) и Среднебайкальское (29.08.1959 г; MLH = 6.8) землетрясения [7]. В результате Цаганского землетрясения произошло опускание тектонического блока земной коры и образование залива Провал [9, 15]. Среднебайкальское землетрясение также сопровождалось тектоническими движениями отрицательного знака – опусканием дна оз. Байкал в эпицентральной зоне на 10–15 м [Солоненко, Тресков, 1960]. Начавшееся проведение в регионе вибросейсмических исследований в связи с развитием Южно-Байкальского геодинамического полигона [2] привело к уплотнению сети сейсмостанций в центральной части Байкальского рифта. Была создана локальная сеть, позволяющая совместить сейсмический мониторинг в пассивном (регистрация землетрясений) и активном (зондирование с управляемым вибрационным источником сейсмических волн) вариантах [13]. Нами излагаются некоторые результаты исследования сейсмичности Центрального Байкала, полученные по данным локальной сети сейсмостанций.

––We present the preliminary results of a study of the Bystrinskoe earthquake, which occurred in the southern Baikal region on 21 September 2020 and was accompanied by shaking with an intensity of VI–VII on the MSK-64 scale in the epicentral area and with an intensity of V in large cities of southern East Siberia (Irkutsk, Angarsk, Usolye-Sibirskoe, Zakamensk, etc.). A preliminary characteristic of the seismic event is given on the basis of a comprehensive analysis of seismological, structural-tectonic, strain, emanation, and hydrogeochemical data obtained during the monitoring of hazardous geologic processes in the Baikal natural territory. We have estimated the seismologic parameters of the Bystrinskoe earthquake, characterized the accompanying phenomena, and identified the effects that are of interest as probable precursors of future strong earthquakes in the Baikal region. The data obtained suggest that the earthquake occurred in the zone of the Main Sayan Fault as a result of strike-slip movement along the W–NW fault. The earthquake focus was apparently located at a shallow depth, as evidenced by the duration of the shocks, macroseismic manifestations, and the strong rumble heard at different directions from the epicenter.

Представлены предварительные результаты анализа сейсмологической базы данных о временах вступлений сейсмических волн землетрясений. Показано, что соотношение между наблюденными (в интервале эпицентральных расстояний до 100 км) и теоретическими (рассчитанными в одномерной модели коры) временами пробега волн на северо-востоке Байкальского рифта характеризуется невязками, достигающими 34 с. Основная причина таких расхождений определяется трехмерной неоднородностью коры и зависимостью от глубины гипоцентров. Цель работы заключается в разработке методики составления априорной трехмерной модели среды, которая позволит получить более определенные сведения о распределении глубины очагов землетрясений и их связи с тектоникой земной коры рифтовой зоны. The preliminary results of the analysis of the seismological database on the times of seismic waves of earthquakes are presented. It is shown that the difference between the observed (in the interval of epicentral distances up to 100 km) and theoretical travel time curves (calculated in a one-dimensional model of the crust) in the North-East of the Baikal rift is characterized by residuals reaching 34 s. The aim of the work is to develop a methodology for compiling a priori three-dimensional model of the environment for more reasonable information about the earthquake foci distribution and their relationship with the tectonics of the rift zone earths crust.

Our detailed study of the crust and upper mantle of the South Baikal basin focused on seismic coda and seismic S-waves attenuation and estimated seismic quality factor (QS and QC), frequency parameter (n), attenuation coefficient (δ), total attenuation (QT), and the ratio of two components the total attenuation: intrinsic attenuation (Qi), and attenuation due to scattering caused by the inhomogeneities of the medium (QSC). We calculated the sizes of inhomogeneities revealed in the block medium, which put their effect on the attenuation of seismic waves in different frequency ranges. The seismic wave attenuation field was analyzed in comparison with the geological and geophysical characteristics of the medium, and a direct relationship was established between attenuation, composition and active processes in the crust and upper mantle of the studied area. According to the estimated intrinsic attenuation (Qi) and scattering attenuation (QSC) contributions into the total attenuation, intrinsic attenuation is generally dominant in the studied area, while the QSC component increases in the areas of large active faults.

The low-frequency deconvolution method makes it possible to convert digital records of electrodynamic seismometers to records of virtual sensors of a similar type, but with a lower natural frequency. The procedure requires only knowledge of the sensor parameters, which can be found from its technical description or obtained by calibration. Deconvolution in the time domain requires attention in choosing a numerical integration method, since use of the simplest methods leads to signal distortion. This is especially noticeable when the sampling rate of the original record is close to a geophone’s natural frequency. A universal approach is presented: realization of a low-frequency deconvolution algorithm in the frequency domain. Testing has shown good accuracy both in synthetic tests and for real seismological records, which were used to demonstrate reconstruction of the low-frequency component of a seismic geophone signal. The results are mainly relevant for problems that use a low sampling rate of recording, and they place high demands on the metrological characteristics of the recording equipment (e.g., local and regional seismicity monitoring).