We consider the December 27, 2023, earthquake (mb = 5.4) that occurred on the margin of the Siberian Platform, on the northwestern slopes of the Akitkan Ridge. The earthquake epicenter is spatially associated with a structural suture (deep thrust fault) separating the Siberian Platform and the Baikal fold belt. The seismic event was followed by hardly any aftershocks. Its maximum shaking intensity was IV (MSK-64); it was observed at distances up to 180 km. The December 27, 2023, Akitkan earthquake is localized in a previously aseismic region, far from active areas of the Baikal Rift. It suggests a new look at seismic activity of fault structures bordering the Siberian Platform. The focal mechanism, determined from P-wave first-motion polarities at regional stations, demonstrates normal fault movements on inclined fault planes with a submeridional strike, which agrees with the orientation of the structural suture. This does not contradict seismogeological data indicating that an inversion of tectonic movements can be observed in some segments of the deep thrust fault zones. The December 27, 2023, Akitkan earthquake confirms modern seismic activity of the Akitkan seismic source zone and the fundamental possibility of relatively strong seismic events being generated by marginal structures of the Siberian Platform.

This paper examines areas of seismic activity in Southern Siberia. It is shown that the potential earthquake source (PES) zone model created for the general seismic zoning does not fully correspond to the data on the seismic activity of the region. For seismic stations at and near hydroelectric dams, a study of seismograms of large earthquakes from different epicentral zones of natural seismicity, as well as records of the largest man-made earthquakes in the region, has been carried out. Attention is paid to platform earthquakes in the areas of hydroelectric power stations. Current power spectra are studied, which show the level of influence of signals against the background of industrial noise and microseisms, and the frequency composition and duration of seismic vibrations of dams during different earthquakes is determined. This work is the first step towards assessing the impacts of earthquakes from the most active epicentral zones on hydroelectric power stations in Siberia, taking into account the real characteristics of the signals. The level of seismic effect of industrial explosions in the region is considered. The research is aimed at clarifying and improving seismic zoning in the areas where hydropower structures are located in Southern Siberia.

We consider a character of the seismic process in the Baikal region and Transbaikalia in 2020. A total number of registered earthquakes with KR≥5.6 was 7711. Most of them (94 %) occurred in the Baikal rift zone where the South Baikal and Baikal-Muya regions were the most active (by number of earthquakes). The strongest seismic events – the September 21, 2020 Bystraya (Mw=5.6) and December 9, 2020 Kudara (Mw=5.5) earthquakes – were localized in the Hovsogol-Tunka and South Baikal regions respectively. Both events in the near-field zone (∆≤19 km) were felt with an intensity of 6–7. Noticeable macroseismic effects with less intensity were observed from another 34 earthquakes with KR≥8.4. Focal mechanisms were determined for 77 earthquakes (KR≥9.1) using two methods: from P-wave first-arrival polarities on regional seismic stations and during calcula tions of a seismic moment tensor (in a double-couple approximation) based on surface wave amplitude spectra. It has been shown that normal fault movements, which are often combined with strike-slips, are dominated in sources of regional earthquakes. In general, the region is characterized by moderate seismic activity in 2020, total seismic energy is estimated as 573∙1012 J that is two times higher than the same parameter calculated in 2018–2019.

This paper is concerned with the influence of the large (ML = 6.9) 2021 Hubsugul earthquake on the seismicity of the block structure in the junction area between the Altai–Sayan mountain region and the Baikal Rift Zone. This study uses data from the networks of seismic stations operated by the Altai–Sayan and Baikal branches of the RAS Geophysical Survey, as well as from Mongolian stations. We show the evolution of the seismic process near the boundaries of the Tuva–Mongolia block and adjacent blocks in eastern Tuva. We have determined and studied the source zone of the Darkhad earthquake swarm which was formed in 2022‒2023 as several sequences of events, with the larger ones having magnitudes ML > 5. Simultaneously with the activation of the Hubsugul earthquake source zone, we observed high seismic activity in the source zones of past large earthuuakes: 1991 Busingol, 2011‒2012 Tuva events, and 2008 Belin-Bii-Kham. The action of the Hubsugul earthquake on the seismicity in boundary blocks of the Altai–Sayan and Baikal zones was different from that of the 2003 Chuya earthquake on Altai seismicity.

The paper presents the studies of the Khuvsgul earthquake on January 11, 2021 at 21:32 UTC (January 12, 2021 at 05:32 local time), M W = 6.7, M L = 6.9, and the seismicity structure in the aftershock period for the Altai-Sayan mountain region and the Baikal rift zone, where the epicenter of this earthquake was located. Two faults are seismically activated, diverging from the southern end of the aftershock area at an acute angle: one in the northeast and one in the northwest direction, as well as transverse faults between them. According to the epicenter position and studies of the source area by other authors, the main event corresponds to the northeastern fault, and large aftershocks occurred at the junction of the northwestern fault with transverse faults feathering from the east. The main event was immediately followed by a series of large aftershocks, the strongest of which occurred on March 31, 2021 with M L = 6.2 and on May 3, 2021 with M L = 6.4. Spatial changes in the seismic regime of the aftershock region led to the predominant activity of its southern end. The junction area of the collisional structures of the Altai–Sayan folded zone and the rift structures of the Baikal depressions system is distinguished in seismicity as a block structure with increased seismicity near the block boundaries. First of all, these are the Tuva-Mongolian block and the eastern part of the Sayano-Tuva block. After the Khuvsgul earthquake of 2021, a block structure with the activation of the epicentral zones of the 1991 Busingol earthquake, the 2011–2012 Tuva earthquakes, and other structures seismically active until 2021 has increased seismic activity. It is proved that the 2014 Khuvsgul earthquake occurred under the basin of the same name and is associated with other faults than the 2021 earthquake and is not a direct precursor of the 2021–2022 activation.

We considered a character of the seismic process in the Baikal region and Transbaikalia in 2018–2019. During this time period, 14545 earthquakes with КR≥5.6 were registered. More than 94 % of the seismic events were localized in the Baikal rift zone. Within it, relatively strong earthquakes occurred in Khubsugul-Tunka area (March 29, 2019, Mw=4.8) and in the junction zone of the northeastern flank of the Baikal rift and the Olekma-Stanovoy orogenic system (September 28, 2019, Mw=5.1). 49 earthquakes caused felt macroseismic effects which most often manifested themselves in the settlements of the region with intensity of 3–4 and 4. Intensity of 5–6 was observed in few cases. Earthquake source parameters were determined for 81 seismic events with КR≥9.1. Most of them occurred in the seismotectonic regime of subhorizontal extension. 61 focal mechanisms were determined from P-wave first-motion polarities. Source parameters for 13 earthquakes were estimated using a joint inversion of P-wave first-motion polarities and surface wave amplitude spectra. In general, 2018–2019 are characterized by a moderate level of seismic activity.

n Erratum to this paper has been published: https://doi.org/10.1134/S1069351322100019

We consider a character of the seismic process in the Baikal region and Transbaikalia in 2016–2017. During the considered period, 17869 earthquakes with KR≥5.6 were registered; 95 % of these seismic events were localized in the Baikal rift zone. The largest earthquakes occurred at the north-eastern flank of the Baikal rift on November 22, 2016 (KR=13.9, Mw=5.0) and April 3, 2017 (KR=13.5, Mw=4.8). Felt effects, not exceeding 5, were observed for 47 regional earthquakes. Focal mechanisms were determined from P-wave first-motion polarities for 73 seismic events (KR≥9.4) and seismic moment tensors (scalar seismic moments, moment magnitudes, focal mechanisms) were calculated from surface wave amplitude spectra and regional P-wave first-motion polarities for 12 seismic events (KR≥11.9, Mw≥4.2). It was found that normal faults and combined types of motions were realized in sources of 94 % of the earthquakes with the obtained focal mechanisms. In general, moderate seismic activity was observed in 2016–2017.

Abstract—The MW = 6.7, ML = 6.9 Khuvsgul (Khubsugul, Khövsgöl) earthquake occurred on January 12, 2021 in the Northern Mongolia close to the border of Russia. The earthquake caused ground shaking which reached intensity IX at the epicenter and was perceptible in the cities and villages of the Eastern and Western Siberia. The earthquake occurred in the region of the same-name fault and caused intense aftershock process on a segment between two fault’s bends. The fault segment encompassed by the aftershocks goes from the lake to the northwest. Seismic activation has involved a segment of the boundary of the ancient Tuva–Mongolian microcontinent hosting the parallel Khuvsugul and Darkhat rift depressions and the Busingol rift depression consisting of three linearly elongated troughs. Until the present, the region of the Busingol depression was distinguished by high seismicity, whereas the regions of the Darkhat and Khuvsgul depressions were characterized by moderate and low seismicity, respectively. The earthquake of 2021 is the largest earthquake that occurred on the Khuvsgul fault over the entire history. In less than two months, the level of the frequency–magnitude graph for the aftershocks has exceeded the level of the annual frequency–magnitude graph of the earthquakes for the Altai–Sayan mountain region. The aftershock density is nonuniformly distributed along the fault; the structure of the distribution is correlated to the unilateral block structure of the Earth’s crust east of the activated fault segment. The seismic potential of the Khuvsgul fault was estimated at Mmax = 7–7.5, and this earthquake, as suggested by the geological data, is not the maximum possible event. Given that seismic activation after the 1991 Busingol earthquake has not yet ceased, we have two strong activations in the Tuva–Mongolian block which can significantly affect evolution of its seismicity.

The article discusses the results of processing instrumental and macroseismic observations of the strongest earthquake in Central Baikal in the last 60 years, which occurred on December 9, 2020 (MW = 5.5), as well as its aftershocks. The event was named the Kudara earthquake, after the locality where the maximum intensity of shocks was recorded. The epicenter of the main shock is confined to Proval Bay (Lake Baikal), which formed as a result of the catastrophic Tsagan earthquake of 1862. The sufficiently high density of seismic stations in the region allowed us to obtain reliable estimates of the position of epicenters and source depths for the main shock and aftershocks. In total, more than 70 earthquakes were recorded, located within an area extending in the sublatitudinal direction. The strongest aftershock (KR = 12.6) occurred to the west of the main source on the first day after the main shock. To estimate the seismic moment and focal spectrum of the earthquake, the coda envelope inversion method was tested. The moment magnitudes for the main shock and three aftershocks have been obtained.

Хубсугульское землетрясение 12.01.2021 г. с ML = 6.9 и глубиной очага порядка 8 км произошло в Северной Монголии недалеко от границы с Российской Федерацией на восточной границе Тувино-Монгольского блока и вызвало интенсивный афтершоковый процесс. Данный блок испытывает поднятие и одновременно к нему приурочены рифтовые впадины [6, 7, 10]. Столь сильного землетрясения в районе западного фланга оз. Хубсугул до сих пор не происходило. Землетрясение произошло в районе одноименного разлома и вызвало интенсивный афтершоковый процесс на участке между двумя изломами. Сейсмический потенциал Хубсугульского разлома оценивался в MMax = 7-7.5 [5], и данное землетрясение в соответствии с геологическими данными не является максимально возможным. Охваченный афтершоками участок разлома уходит от озера в северо-западном направлении. Сейсмически активизирован участок границы древнего ТувиноМонгольского микроконтинента. Учитывая, что сейсмическая активизация после Бусингольского землетрясения 1991 года еще не завершилась, в Тувино-Монгольском блоке наблюдаются две сильные активизации, способные оказать значимое влияние на эволюцию его сейсмичности

We consider the September 21, 2020, Mw = 5.5 seismic event, which occurred in the eastern part of the Tunka rift basin system and was followed by a relatively strong mb = 4.6 aftershock. The earthquake epicenter is localized within the Bystraya basin in the zone of prolonged seismic quiescence and is possibly confined to the Main Sayan fault zone. The hypocentral depth, according to the solution of the Baikal Branch, United Geophysical Survey, Russian Academy of Sciences (UGS RAS), is estimated as h = 18 km. The focal mechanisms of the main shock and strong aftershock are characterized by almost pure strike-slip movements, which agrees well with modern ideas about the tectonics of the area. The maximum shaking intensity observed in the populated areas closest to the epicenter is estimated at VI–VII on the MSK-64 scale. A shaking intensity of V and IV was observed in populated areas at distances over 180 and 500 km, respectively. This earthquake made it possible to obtain new data on the dynamic ground motion parameters. The maximum peak ground acceleration (PGA) of 187 cm/s2 was recorded at the Talaya seismic station, which is the highest value for the territory of the Southern Baikal Region during the digital period of earthquake detection.

Summary Models of the deep structure, made by the authors by the method of longitudinal receiver function based on the data of two mutually intersecting profiles of seismic stations, indicate anisotropy of the deep structure in the area of the Klyuchevsky group of volcanoes. A denser layer in the depth interval of 15–20 km, revealed on the northeastern profile, can be an obstacle to the direct ascent of more heated masses and the reason for the periodicity of eruptions of western volcanoes.

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.

Summary The performed interpretation of seismic-gravimetric data in the North-Muysky area of the Baikal rift zone makes it possible to estimate the thickness, extent and direction of sinking of the sole of large geological bodies of acidic composition and to isolate in the earth’s crust large-amplitude thrusts with a length of hundreds of kilometers.

Deep velocity sections of the transition zone from the Siberian platform to the Central Asian mobile belt are constructed by teleseismic tomography and P-receiver function techniques. An array of the dense ancient Siberian craton is identified in the velocity sections with areas of high seismic velocity. In the SSW section MOBAL_2003, the surface boundary of the craton corresponds to the southern margin of the Siberian platform and is nearly vertical to a depth of 120 km. At larger depths, the craton slides almost horizontally underneath the Tunka rift area. At depths from 150 to 250 km, it is in contact with the area under the Khamar-Daban mountain range. In the southeast, according to the SE velocity section PASSCAL_1992 across the South Baikal basin and the Khamar-Daban mountain range, the Siberian craton thickness is reduced from 270 to 150 km at the contact of the Siberian platform with the Baikal folded area. In this contact zone, the upper part of the craton is wedge-shaped and has an angle of about 45° with the ground surface; it completely tapers off at a depth of 150 km to the east of Lake Baikal. The vertical configuration of the southern segment of the Siberian craton, which evolved with time, may determine the nature of the Baikal rifting in the Cenozoic.