The velocity structure of the mantle under the Baikal Rift Zone (BRZ) is investigated with the P‑wave receiver functions (PRFs) for a group of 10 seismograph stations. The BRZ presents one of the world’s most active continental rift zones. The peculiarities of the BRZ include Cenozoic magmatism in the upper mantle of the southwestern part of the BRZ, which disappears in the central and northeastern parts. The analysis of seismic data reveals other indications of the significant lateral heterogeneity of the mantle beneath the BRZ. At half of the stations there is evidence of a sharp rise of the S velocity with depth at a depth of around 330 km, similar to the descriptions for the “X” or the “300-km” discontinuity. In the central and northeastern regions at depths from about 350 to 410 km there is a well pronounced low S velocity layer, which is practically missing in the southwestern part. The origin of this layer is apparently related to the upwelling and dehydration of wadsleyite in the transition zone. At depths from 500–600 to 660 km in the central and northeastern regions there is another low velocity layer that may be explained by the accumulation of garnetite in the process of subduction of the lithosphere of the Pacific. This layer is poorly pronounced in the southwestern region. The difference between the travel times of the P410s and P660s seismic phases (differential time) in the southwestern region (23.5 s) is close to the data for the standard model (Kennett, Engdahl, 1991). In the central and northeastern regions, the observed differential time is larger than the nominal time by 1.0 s. The rise of the differential time may be related to the cooling and/or hydration of the transition zone by the slabs of the subducted oceanic lithosphere. The obtained seismic data suggest a large role of processes of hydration and dehydration in the central and northeastern regions, however, this role is comparatively small in the southwestern region.

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.

The velocity structure of the southern edge of the ancient Siberian craton has been modeled to an 80-km depth based on teleseismic records of a P-receiver function method (P–S). The correlation between the deep and the surface structure determined through modeling is indicative of the submeridional convergence of the south-western edge of the Siberian craton with the Central Asian mobile belt. A sublatitudinal crust extension in the contact zone caused by such convergence may initiate Baikal rifting at the craton’s southeastern edge.

The models of the velocity structure of the southern margin of the ancient Siberian Craton are constructed for depths down to 80 km from teleseismic records by the method of longitudinal receiver function (P-to-S). The relationship between depth and surface structures revealed using the modeling indicates a submeridional convergence of the southwestern margin of the Siberian craton and the Central Asian mobile belt. This convergence caused sublatitudinal extension of the crust in the contact zone, and this extension could in turn have triggered the Baikal rifting in the southeastern margin of the craton.

Представлены результаты исследований Хубсугульского землетрясения (MW=4,9), произошедшего 5 декабря 2014 г. близ северной оконечности оз. Хубсугул (Монголия). Для этого землетрясения инфразвуковой станцией “Торы” (Россия, ИСЗФ СО РАН) впервые для Байкальской рифтовой системы был зарегистрирован инфразвуковой сигнал. Его длительность составила ~140 с. На основе определения очаговых параметров и механизма очага землетрясения были смоделированы смещения в эпицентральной области Хубсугульского землетрясения. Было показано, что они не способны сгенерировать инфразвуковой сигнал. Использование допустимых значений групповых скоростей инфразвуковых волн (0,28-0,35 км/с) демонстрирует, что источник сигнала находится приблизительно посередине между станцией “Торы” и эпицентром Хубсугульского землетрясения. Это свидетельствует о существовании вторичного источника на указанном расстоянии. По данным об азимуте и времени прихода акустической волны на станцию “Торы” определено положение вторичного источника инфразвукового сигнала, которым являются северные склоны хребта Хамар-Дабан. В качестве наиболее вероятного механизма формирования инфразвукового сигнала рассматривается взаимодействие сейсмических волн от очага землетрясения с горным рельефом.

We have obtained P-wave and S-wave receiver functions for 10 broadband seismograph stations in the Baikal rift zone (BRZ) and inverted them for seismic velocity models of the crust and upper mantle. The thinnest crust (30–35 km) is found in the Baikal basin, the thickest in the East Sayan uplift (45–50 km). Intermediate values (40 km) are found in the BRZ at distances around 100 km from Lake Baikal. A high (at least 1.8) Vp/Vs ratio is observed in the middle and lower crust. It exceeds 2.0 at some stations. In our opinion, the highest Vp/Vs ratios are due to fluid-filled porosity with a high pore pressure. The seismic lithosphere – asthenosphere boundary (LAB) is manifested by a shear velocity drop from 4.5 km/s to 4.0–4.2 km/s. Beneath the Baikal basin, the LAB is located at a depth not more than 50 km, and the S velocity drop is maximal (10 %). A similar structure is found outside the basin, underneath a segment of the East Sayan uplift. At other locations in the BRZ, a typical depth of the LAB varies from 80 to 90 km. Having considered changes in the depth of the 410 km seismic discontinuity, we cannot find any evidence of an elevated temperature of a hypothetical thermal plume beneath the BRZ.

Summary The authors determined the velocity structure from the Earth’s surface to a depth of 270 km around each point of observation in southern Siberia according to the P-receiver functions of broadband seismic stations. An example of the station Ulan-Ude (UUD) in East Baikal indicates that the velocity deep structure around the station can be graphically represented in the form of a specially designed circular model.

The data from the deep seismic sounding along the 2100 km long FENNOLORA profile are considered. Interpretation is made by the method of homogeneous functions with the Earth's curvature taken into account. A seismic section of up to 200 km depth is the result. The image of a thick northward-dipping lithospheric slab, which is located beneath the lithosphere in the mantle, is obtained. The data we obtained are compared with the results of other authors.

The structure of the Baltic Sheet attracts the great interest of geologists over the world. In 1979, within the framework of the EUROBRIDGE project, the long-range deep seismic sounding experiment FENNOLORA was carried out by international group of scientists. The length of this geotraverse is about 2000 km. New seismic cross section along this line now is obtained with depth more 170 km. The cross section has been computed automatically using the program GODOGRAF, based on method of homogeneous functions. As a result of interpretation of the cross section the lithospheric and mantle structures below the Fennoscandian Shield have been obtained. It is showed the large lithosphere slab by thick of 25-45 km exists in asthenosphere. Also the comparison between the new cross section and former cross section, derived by Guggisberg, has been showed.

Regional seismic works in the area of the Norilsk copper-nickel deposit were made using the seismic refraction method in the 1980s. These data were used to derive new information about the studied area. The data of eight profiles of about 2900 km total length were processed by the homogeneous functions method and reinterpreted.

New processing and new interpretation of the refraction data of 8 long profiles is made. The profiles form an almost uniform grid with a step 50-70 km in region located in area of Norilsk. The profiles, executed per 80 years, cross Yenisei River and Pyasina, Lama, Keta, Khantayskoe, Dyupkun, Severnoe lakes. The traveltime curves has been reinterpreted by a method of homogeneous functions. The obtained sections by depth till 10-20 km include interfaces and map an internal structure of the layers, give a location of steep and gentle faults. Two thick high-velocity layered and folded strata, presumably of Proterozoic age, cover blocks of the crystal base. The continuous representation of new sections has allowed constructing velocity horizontal maps - slices, which give a location of geological structures in space. The location of the large structures, allocated per in the past years, is confirmed. These are the Khantaysko-Rybinsk megaswell and the Dolgansk depression. Besides on the velocity map-slices and the cross sections the image of a series of rift structures is obtained, whose location close coincides with a location of large lakes in region.