-Earthquake focal mechanisms that are atypical for the South Baikal basin, which is under the extension of the Earth's crust in the NW-SE direction, are analyzed. Atypical mechanisms are understood as focal solutions of strike-slip and reverse fault types, as well as solutions with normal fault movements along NW-trending planes transverse to the main structures of the basin. Whereas normal faults along NE-trending planes dominate, 29% of solutions from the sample of focal mechanisms are of non-normal fault type, of which 18% account for strike-slip faults and their combinations with other types of displacements (with a normal or reverse component) and reverse faults (with a strike-slip component) - 11%. Such displacements occur predominantly along NW-trending planes, as well as along submeridional and sublatitudinal ones, and strike-slip movements are characterized by right-lateral displacement along NW and submeridional planes, and, accordingly, left-lateral displacement along sublatitudinal and some NE planes. Earthquakes with atypical mechanisms are distributed almost throughout the entire basin, but it is necessary to note an increase in their number on its southwestern termination (the Kultuk segment) and on the eastern side of the Central Basin. In the current field of crustal extension, transverse shears play the role of transfer faults, accommodating differences in the rates and vectors of deformation of local blocks within the basin, and on a regional scale between neighboring rift basins.

Knowledge of the regional crustal deformation and stress field is fundamental to understanding and constrain the ongoing evolution of Hovsgol basin, northwest Mongolia. The 2021 Mw 6.7 Turt earthquake provides an unprecedented opportunity to probe the local tectonic stress field and upper crust deformation. We investigate the coseismic surface displacements and invert fault slip models using Interferometric Synthetic Aperture Radar observations and teleseismic data. The mainshock occurred as a result of normal faulting with a right-lateral strike-slip component on an NW striking plane, which is consistent with the transtensive local stress field inverted from regional focal mechanisms. It is surrounded by transpressive and strike-slip stress fields proposed by previous studies indicating regional stress heterogeneity probably controlled by the distinct structure geometries in and around Hovsgol basin. Our results also suggest that the current deformation of the Hovsgol basin is dominated by half-graben forming. Seven historical strong earthquakes (M 7) may advance the 2021 Turt earthquake by-12% recurrence interval, based on the assumption of equal stress release from recurrence earthquakes, meanwhile, the 2021 Turt earthquake may increase the potential seismic hazard on the neighbor Mondy and South Hovsgol faults, which deserves more attention.

Earthquake depth frequency distribution is often used as a constraining factor in assessing lithospheric strength. The accuracy of hypocenter locations is therefore a key factor in these types of analyses. This paper presents a summary of earthquake depth distributions in the Baikal rift system reported in various previous studies. Datasets on both background seismicity and aftershock sequences were analyzed by taking into account hypocenter depth uncertainties. The results show that the most seismically active part of the Earth's crust is in a depth range of 10–25 km. The lower cutoff depth varies for different datasets from 20 to 40 km. The latter value corresponds to the Kichera sequence and coincides with the Moho depth, but other datasets show that the lower 8–18 km of the crust is aseismic. The South Baikal basin is distinguished by the shallower cutoff depth (20–24 km) compared to the datasets outside of it. The peak depths of the earthquake depth frequency distributions vary from 8 to 23 km. Given the depth uncertainties, only the Kumora sequence appears to have double peaks at 9–10 km and 16–17 km. Variations in the lower limit of earthquake distribution and depth of the main peak may imply a spatial change in the depth of the brittle–ductile transition and consequently in the thermomechanical properties of the crust.

This article reports the results of detailed seismological observations in the Central Baikal region conducted by the local network of seismological stations of the Buryat Division of the Geophysical Survey of the Russian Academy of Sciences. The local network was created in the 1990s. A crucial feature of the network is the combination of seismic monitoring both in the passive mode (the study of natural seismicity) and in the active mode, with a controlled vibration source of seismic waves. The study area covers the Lake Baikal region and adjacent territories characterized by high seismic activity. Here occurred several catastrophic earthquakes including the strongest one during the period of instrumental observations – the Middle Baikal’1959 earthquake. Recently here occurred the Kudarinsky earthquake on December 9, 2020 with mb=5.4. For more than twenty years the network of observations has been expanding, the equipment has been upgrading. A significant amount of seismo-logical material has been accumulated. Broadband data was processed by the receiver function method. The Moho and the lithosphere-asthenosphere boundaries for stations of the network are determined. Shear seismic wave attenuation characteristics are obtained and the possibility of energy classification of Baikal earthquakes by coda-waves total oscillations is shown.

The paper presents previously unpublished earthquake focal mechanism solutions for earthquakes which occurred in South Pribaikalye, Transbaikalia, Tuva and Northern Mongolia derived from the first-motion polarities of P-waves recorded by the Baikal, Buryat, Altai-Sayan networks of the Geophysical Survey RAS and Mongolian national network.

Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Geophysical Research Letters. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Coseismic Fault Slip and Transtensional Stress Field in the Hovsgol Basin Revealed by the 2021 Mw 6.7 Turt, Mongolia EarthquakeAuthorsXiaogeLiuiDWenbinXuiDNataliaRadziminovichiDNanFangLeiXieSee all authors Xiaoge LiuiDCentral South UniversityiDhttps://orcid.org/0000-0002-5762-5218view email addressThe email was not providedcopy email addressWenbin XuiDCorresponding Author• Submitting AuthorCentral South UniversityiDhttps://orcid.org/0000-0001-7294-8229view email addressThe email was not providedcopy email addressNatalia RadziminovichiDInstitute of the Earth`s crust, Siberian Branch of the Russian Academy of SciencesiDhttps://orcid.org/0000-0002-0577-0675view email addressThe email was not providedcopy email addressNan FangCentral South Universityview email addressThe email was not providedcopy email addressLei XieThe Hong Kong Polytechnic Universityview email addressThe email was not providedcopy email address

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 results of studying the Bystrinskii earthquake of September 21, 2020, in the southern Baikal region are presented. Its geodynamic position, focal mechanism, and basic parameters are indicated. The deformation monitoring data showing six days of anomalous growth of rock deformations preceding the earthquake are provided. It is shown that the scenario of the deformation process before the earthquake is similar to the scenario of preparing dynamic slip during the experimental simulation of the intermittent “stick–slip” process. On the basis of similarity in the implementation of deformation processes in nature and in the model, it is stated that the deformation anomaly, which appeared just before the Bystrinskii earthquake, is a regular phenomenon and can be considered as its short-term precursor.

––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.

—The paper presents results of a seismogeological study based on analysis of seismic data and historical facts about the seismic activity of the Khambinskii fault zone. According to the data obtained, a genetic type of dislocations on conjugate faults (Gusinoe Ozero and Orongoi paleoseismogenic structures) is related to reverse faults with a strike-slip component. Geophysical studies of the Gusinoe Ozero structure have determined the dip of the fault plane toward the mountain framing of the depression and its outcrop at the bottom of the seismic scarp. The significant seismic potential of the Khambinskii fault is responsible for the maximum intensity of shocks in the nearby cities and settlements of southeastern Transbaikalia. The seismic fault activity has been confirmed by the historical earthquakes of 1856 and 1885, the M = 5 earthquake that occurred on 2 October 1980, and at least two prehistoric earthquakes. The latest of the latter occurred no earlier than ~4 ka and had M = 7.0–7.3, while the earliest was even more intense and took place in the first half of the Holocene, no later than ~6 ka.

Magnitude of completeness, b-value, and correlation dimension of spatial epicenter distribution were estimated for the South Baikal basin using earthquake catalogue for the period of digital observations. All estimated parameters were mapped with a one-for-all approach that allowed revealing their spatial variability. The value of the completeness magnitude, which was evaluated on the frequency-magnitude distribution, was found to range from Mc 1.0 to 1.6 over the basin depending on network configuration change, with the average value of 1.1 +/- 0.1. Then, b-value in the Gutenberg-Richter relation was estimated by maximum likelihood method based on the catalogue with M >= 1.1 earthquakes. The lower b-value (0.81 +/- 0.03) was revealed for the southern part of the basin which is under transtension stress regime in contrast to pure extension characteristic for the rest of the territory. The higher b-value (1.01 +/- 0.01) was obtained for the area to the north-east of the Selenga delta, where two of the largest Baikal earthquakes occurred in the last 150 years, and which is characterized by more heterogeneous crust structure. Correlation dimension in the southern part of the basin was found to be closed to 1 that is interpreted as a tendency of epicenters to be distributed along a line; whereas the central part is characterized by a higher value that corresponds to a tendency of epicenters to be distributed over a plane rather than along lineaments. A comparison of the Southern and Central basins, which make up the tectonically single South Baikal basin, shows that seismicity of the Central basin is characterized by higher b-values and correlation dimension that may imply different stress level and/or faulting type.

The 2011 M(w)9.0 Tohoku earthquake and the 2012 M(w)8.6 and 2016 M(w)7.8 Sumatra earthquakes caused water level oscillations in Lake Baikal which were recorded by a water pressure tensor transducer with high sampling rate. Periods of water oscillations were about 100 s, and maximum peak-to-peak amplitudes were as large as 0.15 m for the 2011 Tohoku earthquake and 0.24 m for the 2012 Sumatra earthquake, although the Tohoku earthquake was closer and stronger. The difference in the amplitude of the level oscillations for these earthquakes was probably caused by their focal mechanisms, namely thrust and strike-slip, as well as the direction of the wave propagation. CTD (conductivity-temperature-depth) measurements conducted after these earthquakes at the regularly tested stations showed a temperature increase in the near-bottom water layer after the Tohoku earthquake in March 2011, and a decrease in temperature and electrical conductivity after the Sumatra earthquake in March 2016. These observations cannot be explained by the known processes of deep water renewal, so based on the gas hydrate presence in the Baikal sediments we assume that seismic wave passing could change permeability in the sedimentary layer (at least for the 2011 Tohoku case study) and promote methane flux from the base of the hydrate stability zone and formation of gas hydrates with heat release. As to the 2016 Sumatra study case, we suppose to explain it by gas hydrate dissociation in the subsurface sediments. We present rough estimates of the volume of the formed/dissociated gas hydrates.

First results of the analysis of GPS measurement data obtained from 18 sites of two local networks in the vicinity of Ulaanbaatar (Mongolia) for the period 2010-2015 have been presented. Horizontal velocity vectors are consistent with each other in the ITRF2014 system and with the velocities of the IGS permanent station ULAB. The sites move in the E-SE direction at a rate of 25-30 mm/yr, with the displacement azimuth averaging 105 degrees. With respect to Eurasia, the vectors for most of the sites are slighly turned to the south, but they are still of SE orientation with the azimuth range of 130 degrees-150 degrees and magnitude values of 2-4 mm/yr. Relative horizontal velocities tend to decrease towards southeast that determines a zonal distribution of different type of relative strain patterns. The western part of the Ulaanbaatar network is characterized by the W-E oriented extension with the elongation rate epsilon(1) = 12-16 x 10(-8) yr(-1). The shortening NW-SE trending strain with calculated value epsilon(2) = 22.4 x 10(-8) yr(-1) characterizes the network's eastern part. The highest values of the maximum shear strains (epsilon(max) = 10-14 x 10(-8) yr(-1)) form an extended area in the center of the testing ground, which is elongated in the northeastern direction, conformably with the strike of the major geologic structures. The strain distribution pattern of the Emeelt network located within the eponymous seismogenic structures is characterized by the crustal elongation (5 x 10(-6) yr(-1)) trending SE-NW and less pronounced shortening in the SW-SE directions. The axial part of the fault crossing the network in the NW direction exhibits maximum deformations. (C) 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

We have compiled and analyzed earthquake focal solutions for the territory of Mongolia and its surroundings in order to reveal a spatial variability of stress orientation and stress regimes of the crust. According to the stress inversion results, the SHmax is turning from W-E in the eastern Mongolia to SW-NE in the Gobi Altay and the central Mongolia, and then to S-N in the western part of the region. Comparison with data derived from GPS measurements shows that directions of the strain axes revealed by the geodetic and seismological observations are generally consistent. A contradiction is found for the Bolnai zone where results of GPS estimation indicate the predominance of extension (in the SE-NW direction), whereas earthquake data for the longer period of seismic observations reveal compression. Compression in this zone is mainly due to the Tsetserleg-Bolnai earthquakes contribution; however, a part of the recent data on focal mechanisms fits an extensional stress field with the NNW orientated extension axis. These data are in accordance with some published works which suggest a transtensive field from some structural geology studies in the eastern part of the Bolnai zone.The paper is supplemented with a list of M≥4.5 earthquake fault plane solutions and unpublished focal mechanisms for some M≤4.5 earthquakes of the northern Mongolia and the southern Baikal region.

We have compiled and analyzed earthquake focal solutions for the territory of Mongolia and its surroundings in order to reveal a spatial variability of stress orientation and stress regimes of the crust. According to the stress inversion results, the SHmax is turning from W-E in the eastern Mongolia to SW-NE in the Gobi Altay and the central Mongolia, and then to S-N in the western part of the region. Comparison with data derived from GPS measurements shows that directions of the strain axes revealed by the geodetic and seismological observations are generally consistent. A contradiction is found for the Bolnai zone where results of GPS estimation indicate the predominance of extension (in the SE-NW direction), whereas earthquake data for the longer period of seismic observations reveal compression. Compression in this zone is mainly due to the Tsetserleg-Bolnai earthquakes contribution; however, a part of the recent data on focal mechanisms fits an extensional stress field with the NNW orientated extension axis. These data are in accordance with some published works which suggest a transtensive field from some structural geology studies in the eastern part of the Bolnai zone. The paper is supplemented with a list of M≥4.5 earthquake fault plane solutions and unpublished focal mechanisms for some M≤4.5 earthquakes of the northern Mongolia and the southern Baikal region.

In the paper we report the state-of-the-art of seismicity study in the Baikal rift system and the general results obtained. At present, the regional earthquake catalog for fifty years of the permanent instrumental observations consists of over 185,000 events. The spatial distribution of the epicenters, which either gather along well-delineated belts or in discrete swarms is considered in detail for different areas of the rift system. At the same time, the hypocenters are poorly constrained making it difficult to identify the fault geometry. Clustered events like aftershock sequences or earthquake swarms are typical patterns in the region; moreover, aftershocks of M ⩾ 4.7 earthquakes make up a quarter of the whole catalog. The maximum magnitude of earthquakes recorded instrumentally is MLH7.6 for a strike-slip event in the NE part of the Baikal rift system and MLH6.8 for a normal fault earthquake in the central part of the rift system (Lake Baikal basin). Predominant movement type is normal faulting on NE striking faults with a left lateral strike-slip component on W–E planes. In conclusion, some shortcomings of the seismic network and data processing are pointed out.