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.

In this study, we calculated seismic intensity for the Baikal-Olekma section of the Baikal-Amur railway based on the data on seismicity at the northeastern flank of the Baikal rift zone in 1985–2021. During the analysis of earthquakes with intensity at the epicenter higher than 4 (1 484 events), we selected from the total number of the recorded seismic events (more than 150 thousands of earthquakes with M ≥ 1.0) 270 earthquakes with MS = 2.1–6.6 whose calculated intensity exceeded 4 directly on the railway track. The highest intensity values (8–9) were obtained for the large earthquakes close to the Baikal-Olekma section of the Baikal-Amur railway (∆ < 10 km, MS = 6.1–6.2). Along the considered section of the railway, a recurrence interval of such seismic events is 20 to 60 years. The data obtained through calculations are in good agreement with the available macroseismic data which confirms the correctness of the results. At the same time, macroseismic effects from strong (МS ≥ 6.6) but remote earthquakes (∆ = 360–1000 km) were found to have no significant impact on the Baikal-Olekma section of the Baikal-Amur railway. The calculated values of intensity and recurrence intervals of earthquakes must be taken into account when designing, constructing and operating industrial and civil facilities within the study area.

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.

The Severomuysky region should be considered as one of the key areas of the northeastern flank of the Baikal rift zone. The most important infrastructure facilities of the Baikal-Amur Mainline, in particular the Severomuysky tunnel, require an objective assessment of the seismic hazard of this territory. The work on converting a unique set of seismological data obtained during the period of operation of the analog local network of seismic stations (1978-1993) into digital format was carried out at the BB GS RAS in order to ensure the safety of all primary seismological observation materials and comprehensive analysis. This information was presented in printed form only. A database has been developed. It provides a relational approach to storing and managing large volumes of data. The database contains complete information on 15,832 earthquakes with KP=4–13 (station information, catalogues, bulletins, etc.). A client application has been created for convenient work with the database. The article illustrates examples of working with a database when solving typical seismological problems. It is obvious that extensive seismological information presented in digital form is of great importance for assessment of seismic hazard of the study under region.

This paper considers two earthquakes which occurred on October 18, 2017 and October 25, 2017 in western Transbaikalia. In spite of the moderate energy level of both events, they can be treated as significant for the study area, because such earthquakes have been recorded there relatively rarely compared with the adjacent high-seismicity areas in the Baikal Rift Zone. The mechanisms of both earthquakes based on surface wave amplitude spectra showed that these events occurred under a dominating east–west near-horizontal compression and an inclined or nearly vertical NW‒SE tension, which is typical for western Transbaikalia. For both earthquakes, we also computed source parameters: scalar seismic moment M0 = 5.0 × 1015 N m, moment magnitude Mw = 4.4, and source depth h = 7 km for the October 18, 2017 event; M0 = 3.5 × 1015 N m, Mw = 4.3, and h = 29 km for the October 25, 2017 event. These earthquakes have caused noticeable macroseismic effects in the near-field; the maximum observed shaking intensity was IV–V (MSK-64) during the October 18, 2017 earthquake and V during the October 25, 2017 earthquake. These data were the basis for our analysis of present-day activity of faults in the study area. The results may be helpful for more accurate assessment of earthquake hazard and seismic risk in western Transbaikalia.

The article presents analysis of macroseismic data on the September 21, 2020 (Mw = 5.6) Bystraya earthquake, which occurred in the eastern part of the Tunka basins system on the southwestern flank of the Baikal rift zone. Macroseismic data were collected mainly through an Internet questionnaire posted on the website of the Baikal Branch of the Geophysical Survey, Russian Academy Sciences. A total of 3013 eyewitness responses were collected, which is currently an unprecedented number in the entire history of macroseismic observations in the Baikal region. In total, we collected data for 263 Intensity Data Points. The maximal shaking intensity (VI–VII MSK-64) was observed in the Bystraya village and the Kultuk settlement. The shaking intensity V MSK-64 was noted at a distance of up to 180 km; intensity IV MSK-64 was recorded at a distance of up to 550 km. Analysis of data on the Bystraya earthquake revealed significantly lower attenuation compared to that expected from the regional macroseismic equation. Due to the large volume of macroseismic data collected, as well as the high efficiency of the data collection method used, the Bystraya earthquake can be considered an important milestone in macroseismic research in East Siberia.

The paper presents analysis of the seismicity and deep structure of the Trans-Baikal region in the section of the reference geophysical profile 1-SB. It was determined that the Earth’s crust and upper mantle has a complex heterogeneous structure. The thickness of the Earth’s crust varies from 40 km in the South-Eastern part of the profile and in the areas of intermountain depressions in the North-Western part, and up to 48 km in the areas of mountain ranges. The values of the boundary velocities along the M boundary also vary greatly, from higher values of 8.4‒8.5 km/s for P-waves and 4.9‒4.95 km/s for S-waves (especially in the South-Eastern part of the profile) to reduced values of 7.8‒8.0 km/s for P-waves and 4.6‒4.7 km/s for S-waves in the section of the Baikal rift zone in the North-Western part of the profile. A strong inhomogeneous structure of the medium in terms of elastic wave velocities, Vp/Vs velocity ratios, and the Poisson’s ratio is determined for the upper and the middle crust at depths of 8‒20 km. The authors determined that zones of increased seismicity are referred to blocks of the Earth’s crust with inhomogeneous velocity structure according to data of differently polarized P- and S-waves. The area of the Baikal rift zone, in the immediate vicinity of the largest Muya earthquake of 1957 with M = 7.6, is characterized by elevated inhomogeneity in the upper part of the Earth’s crust according to the elastic wave velocities and secondary parameters of the medium (Vp/Vs ratio, K* = = Vp/(γ – 1), where γ = Vp/Vs, Poisson’s ratio (σ)). A number of other inhomogeneous deep zones have also been identified in the profile based on anomalies of P- and S-waves velocities and secondary parameters of the medium, which correlate to varying degrees with seismically active sites according to long-term instrumental observations. The established unambiguous connection of large inhomogeneous zones of the upper crust of the Trans-Baikal region with the accumulation of stresses and their discharge in the form of strong earthquakes allows us to make a reasonable medium-term forecast of catastrophic events.

This study is concerned with an analysis of seismicity and deep structure in the Transbaikalia along the 1-SB reference geophysical traverse. We have found a complex inhomogeneous structure of the crust and upper mantle. The crustal thickness varies between 40 km in the southeastern part of the traverse and in intermontane troughs in its northwestern part on the one hand and 48 km in mountain ranges. Strong variation also affects boundary velocities at the Moho, ranging from 8.4‒8.5 km/s for compressional waves and 4.9‒4.95 km/s for shear waves (especially in the southeastern part of the traverse) to lower values of 7.8‒8.0 km/s for compressional waves and 4.6‒4.7 km/s for shear waves in the area of the Baikal Rift Zone in the northwestern part of the traverse. A strongly inhomogeneous earth structure based on elastic wave velocities, Vp/Vs velocity ratios, and Poisson’s ratio was found for upper and middle crust at depths of 8‒20 km. It was also found that zones of higher seismicity tend to coincide with crustal blocks with inhomogeneous velocity structure based on differently polarized compressional and shear waves. Higher inhomogeneity in upper crust as inferred from elastic wave velocities and secondary earth parameters (Vp/Vs velocity ratios, the parameter K* = Vp/(γ ‒ 1), where γ = Vp/Vs, and Poisson’s ratio (σ)) characterize the area of the Baikal Rift Zone in an immediate vicinity of the great Muya earthquake of 1957 with М = 7.6. As well, several other deep zones of inhomogeneity have been identified along the traverse line based on anomalies of Р and S velocities and secondary earth parameters that correlate to varying degrees with seismically active areas based on multiyear instrumental observations. We have identified an unambiguous relationship of large inhomogeneous zones in the Transbaikalia crust with stress buildup and stress release in the shape of large earthquakes, thus substantiating the intermediate-term prediction of catastrophic events.

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.

Throughout the history of the Baikal seismic network (since 1901), great efforts have been made to preserve the most detailed information about recorded seismic events. The article considers a method for detailed summary processing of earthquakes in the Baikal and Transbaikalia regions used in the BB GS RAS. The level and quality of regional processing are shown on the example of 2019. 7273 earthquakes were registered during the year, most of them are (62%) weak earthquakes (KР=6), for which the error of determining the coordinates of the epicenters was on average less than 5 km. Localization accuracy of epicenters for earthquakes with a KР≥9 is less than 2 km. The results of some studies using data from the Kultuk and Muyakan local temporary networks are presented. A comparison of the results and volumes of earthquake processing with other branches of the GS RAS is shown. Thus, the number of earthquakes in the region of the Baikal and Transbaikalia (N=63081) exceeds any of the other seismically active regions of Russia by at least 2.4 times for the period 2014–2017 (Altai and Sayan – N=26458; Kamchatka and Commander Islands – N=26301). According to the number of seismic stations which are used in the processing of one earthquake (K≥9) and the number of phases of seismic waves, regions of the Baikal, Transbaikalia and of the North Caucasus are leading and close to each other. There are 29 and 33 stations respectively, 87 and 68 phases. In other words, the volume of earthquake processing in the Baikal branch is much larger than in any of the branches of the GS RAS. It was concluded that currently the processing of earthquakes in the BB GS RAS is carried out in an optimal way in view to the size of the region, the number of seismic stations, the quality of communication and the number of recorded earthquakes. Reforming the system of processing seismic events in the BB GS RAS is expedient after a significant increase in the observation points in the region.

In this paper we consider four moderate earthquakes that occurred on November 9, 2011; September 5, 2015; March 16, 2018; and October 10, 2019, in the southern basin of Baikal Lake near the Goloustnaya River delta. Seismotectonically, the earthquake sources are confined to the southwestern part of the Olkhon graben. The earthquake focal mechanisms allow us to suggest the activity of local submeridional faults, feathering the structures of the general northeast strike. For two seismic events, the following parameters were determined: scalar seismic moment M0 = 4.0 × 1015 N m, moment magnitude Mw = 4.4, and source depth h = 12 km for the September 5, 2015 earthquake and M0 = 2.2 × 1015 N m, Mw = 4.2, and h = 7 km for the March 16, 2018 earthquake. None of the seismic events was followed by noticeable aftershock activity. In 2011–2019, these earthquakes caused a noticeable macroseismic effect within the Irkutsk agglomeration, as well as a rather wide public response. Macroseismic data were collected mainly using an online survey system; the number of responses received within the first day after an earthquake varies from 127 to 341. The maximum intensity, IV–V, was observed for the November 9, 2011 earthquake in the settlement of Bolshoye Goloustnoye. The results can be used for assessing the seismic hazard in the Southern Baikal region.

This paper presents the results of a detailed study of two relatively strong seismic events, occurred on November, 22, 2016 (МW = 5.0) and April 3, 2017 (МW = 4.8) at the northeastern flank of the Baikal rift zone in the areas of the South Muya and Kalar ranges. Both the events were followed by weak aftershocks (N = 178–539, Кр ≤ 11.0), whose epicenters were densely distributed over an area. The seismic moment tensors and hypocentral depths of the mainshocks were calculated from surface wave amplitude spectra. It has been shown that their sources were formed under the influence of the subhorizontal SE–NW extension and the subhorizontal or inclined NE–SW compression. Both the earthquakes were followed by noticeable macroseismic effects at epicentral distances up to 500 km. In the area of the Baikal−Amur Railway, their shaking intensity was IV–V MSK-64. The results obtained can be used in continuous seismic monitoring at the northeastern flank of the Baikal rift zone and can contribute to an objective seismic hazard assessment of the study area.

— We study in detail the source parameters, seismotectonic position, and macroseismic effects of the February 3, 2016, earthquake that occurred in the Middle Baikal region. This earthquake is one of the relatively few instrumentally recorded seismic events in the northern part of the South Baikal basin that has occurred in a previously low-active region. The main source parameters of the earthquake are calculated from the joint inversion of the amplitude spectra of surface waves and polarities of the first arrivals of P -waves: scalar seismic moment ( M 0 = 1.60 × 10 16 N m), moment magnitude ( M w = 4.8), source depth ( h = 22 km), and a focal mechanism (which is an almost pure normal fault). The obtained focal mechanism corresponds to the general geodynamic situation in the region of the Middle Baikal. The earthquake caused perceptible shaking over a large area, while shaking intensity of IV MSK-64 was observed at distances up to 360 km. Macroseismic data were collected by mailing questionnaires to the administrations of settlements, as well as using an online survey of the population of the southern Baikal region. The results are of interest in carrying out work to clarify the seismic hazard of the territory of the southern Baikal region.

The aseismic structure identified by the analysis of seismicity in the region of Olkhon Island which is associated with the central part of the Shebarta complex, composed of metamorphic rocks. The effect of subsidence of the seismically active layer under Olkhon Island is observed, and it rises to the center of the Baikal depression.

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.

The article discusses the advisability of conducting detailed macroseismic surveys within large cities and urban agglomerations. A retrospective analysis of information about earthquakes that occurred in the past decades and were felt in Irkutsk with an intensity of I = V or higher revealed the problem of preserving and availability of primary data on earthquake effects. Processing of the macroseismic data collected using internet-based questionnaires for the Irkutsk area after the September 21, 2020 Bystraya earthquake was carried out. The usage of online questionnaires has demonstrated high efficiency and information content, and also opened up certain possibilities such as improving the method with respect to the particular conditions of East Siberia. A large number of responses from earthquake eyewitnesses makes it possible to assess the shaking intensity separately in every administrative unit of Irkutsk, which in turn contributes to an increase in the detail of documenting the earthquake macroseismic field. The results allow us to consider assessment of the shaking intensity within certain parts of Irkutsk city as more rational versus assessment for the entire territory of the city.

The paper considers three relatively strong earthquakes that occurred in 2015 in the northern Lake Baikal region: July 7 Upper Akuli earthquake (Mw=4.6) with the epicenter at the headwaters of the Akuli River, and September 25 Gulonga-I (Mw=4.7) and December 13 Gulonga-II earthquakes (Mw=4.6) with the epicenters near the mountain lakes Gulonga. Instrumental and macroseismic data on these seismic events are reported. A seismic moment tensor, calculated from surface wave records, shows normal fault focal mechanisms for Upper Akuli and Gulonga-II earthquakes and strike-slip movements in the source of the Gulonga-I seismic event. The results obtained could be used in further studies of seismic zoning and seismic hazard assessment in the northern Lake Baikal region.

Байкальская рифтовая зона – это уникальный регион мира, который отличается высокой сейсмической активностью. Одним из примеров сильных землетрясений является Цаганское 12.01.1862 г. с эпицентром в северо-восточной части дельты р. Селенги. Интенсивность сотрясений в эпицентре достигала I0=10 баллов. В результате землетрясения под воду ушла территория площадью около 230 км2 , образовался новый залив – Провал. Необходимость регулярных сейсмических наблюдений в Восточной Сибири стала ясна еще в конце 19 века. Александр Петрович Орлов, создатель первых каталогов землетрясений России, в 1868 году начинает деятельность по организации метеорологической службы и инструментальных наблюдений за землетрясениями в Иркутске. По его инициативе Постоянной центральной сейсмической комиссией Академии наук было принято решение об организации сейсмической станции при Иркутской обсерватории. Организацией сети сейсмических станций занимался Аркадий Викторович Вознесенский, директор Иркутской магнитно-метеорологической обсерватории. Станция «Иркутск», которая стала третьей в Российской империи и первой в Сибири, приступила к регулярным наблюдениям 2 декабря 1901 г. [1, 5].

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.

We calculated seismic moment tensors in a double-couple approximation (focal mechanisms, scalar seismic moments, and moment magnitudes) and hypocentral depths for twenty earthquakes with Mw≥4.2 that occurred in the Baikal region and Transbaikalia in 2015. The initial data were amplitude spectra of Rayleigh and Love waves obtained from their records at the broadband seismic stations of the IRIS and the DK networks and first-motion polarities of body waves recorded at regional distances. A combination of the normal fault and strike-slip movements dominate in the sources of the major part of the study earthquakes. For the strongest of the considered seismic events (Mw≥4.6), the subvertical compression and subhorizontal tension in the SE-NW direction prevail, i.e. the tension is perpendicular to the main structures of the Baikal rift zone. The seismic events with Mw<4.6 are characterized by a more scattered orientation of compression and tension axis that could be caused, for instance, by stress redistribution in small-scale crustal blocks after stronger earthquakes. The obtained results are of great value for issues concerned with seismic hazard assessment and the development of geodynamical models of the lithosphere evolution of the study region.