Baikal Rift Research Articles

The Curie Surface and Lithospheric Thermal Structure in Mongolia‐Baikal Region

AbstractThe Mongolia‐Baikal region, located between the Siberia, North China, and Tarim cratons, exhibits the most pronounced lithospheric accretion and extension since the Phanerozoic. Despite its distance from plate boundaries, the region's notable tectonic activity underscores its importance for studying intraplate lithospheric deformation. However, its lithospheric thermal structure, particularly at high resolution, remains inadequately characterized. By integrating magnetic anomaly constraints with heat flow measurements, we have obtained a high‐resolution lithospheric thermal structure for the Mongolia‐Baikal region, revealing lithospheric thicknesses ranging from 60 to 180 km. The Curie depth and mantle heat flow also show diverse variations across different regions. Our results suggest that elevated mantle heat flow in the Baikal Rift Zone, coupled with a uniform Curie depth and minimal lithospheric thinning, likely reflects lithospheric weakening from mantle upwelling, facilitating rifting in the early stages of the rift's formation. Additionally, some localized lithospheric thermal variations identified in our results may be closely linked to the formation of the Hangai Plateau and the low magnetic anomalies observed in the Mongolia‐Okhotsk suture.

Seismic moment of local earthquakes in the central part of the Baikal rift by the coda envelope inversion

The article presents an estimation of scalar seismic moment by the coda envelope inversion of local earthquakes recordings in the central part of the Baikal rift zone. In addition to the earthquake source parameters, the method allows us to simultaneously estimate seismic energy loss and site amplification factors for frequency bands from 0.53 to 34 Hz. Because of the compromise between spectral source energy and site responses, we estimated amplification factors for used stations relative to the "UlanUde" (UUDB) reference site located in crystalline rock. In order to estimate intrinsic and scattering attenuation we used events recorded at three broadband seismic stations and epicentral distances between 40 and 120 km. The result suggests that intrinsic absorption is dominant over scattering attenuation in the central part of the Baikal rift zone for most frequency bands, but the seismic albedo B0, expressing the contribution of scattering to the total attenuation, showed variations from 0.23 to 0.6 for frequencies below 1 Hz, with mean B0 value 0.33. Attenuation and siteamplification factors were used to estimate the seismic moment and moment magnitudes Mw of local earthquakes. The resultant moment magnitudes exhibit a good agreement with routinely reported local magnitude (ML) estimates for the study area. The coda inversion estimates of seismic moment provide stable, unbiased moment magnitudes for events that are too small to be seen at teleseismic distances.

Chemical hydrogeodynamics of the Kultuk groundwater reservoir vs. paragenetically related large earthquakes in the central Baikal Rift System, Siberia

Chemical hydrogeodynamics of the Kultuk groundwater reservoir vs. paragenetically related large earthquakes in the central Baikal Rift System, Siberia

Quasilinear Earthquake Chains in Groups of Seismic Events Baikal Rift System

Identification of a large number of quasi-linear chains of earthquakes in the epicentral seismicity field of the Baikal region and their study showed that among these chains there may be not only chains of “migration” earthquakes, but also chains formed during a random spatio-temporal distribution of earthquakes. In this work, using a statistical analysis of the distribution of distances between epicenters, the possibility of forming chains of “migrations” within groups of seismic events is shown and their distribution is studied. It is noted that chains in groups of earthquakes are distinguished not only by the distribution of distances, but also by the time between them. The formation of chains of earthquakes was established in the areas of the following groups of earthquakes: Busiyingol and 1976 and 1991. in the same area, the South Baikal, Kyakhtinsky, Kichersky earthquakes of the Tompudinsky series, Oldongsinsky and Charudinsky groups. It is shown that these chains were formed during the implementation of these groups. In the identified areas of concentration of chains of grouping seismicity, a connection can be traced between the location and direction of the chains with the strike of fault zones, near-fault cracks and the orientation of nodal planes in earthquake sources.

Characteristic Ddepth distribution of crustal earthquakes in Southern Siberia

The key parameters of an earthquake are magnitude, epicenter coordinates, and depth. Depth often has a decisive influence on the macroseismic effect of certain earthquakes. In this regard, for example, to assess seismic risk, one should know information on the statistics of earthquake occurrence at certain depths. In this work, catalogs of continental crustal earthquakes in the Southern Siberia are analyzed. The distributions of earthquake depths are approximated by various functions. The Weibull distribution, with a maximum at 8 km, is shown to describe the depth distribution of these crustal earthquakes most accurately. When the western (Altai–Sayan) and eastern (Baikal Rift Zone) parts of the region are considered separately, the Weibull distribution is also preferable. The maximum of the distribution is found to be at 9 km for the Baikal rift zone and at 7 km for the Altai–Sayan zone.

TECTONOPHYSICAL ZONING OF ACTIVE FAULTS OF THE BAIKAL RIFT SYSTEM

Tectonophysical zoning of active faults of the Baikal rift system (BRS) was performed based on the degree of hazard caused by the generation of earthquakes of magnitude 7.0 and higher. The first basis for this procedure was the results of the natural stress state reconstruction, earlier performed from seismological indicators of rupture deformations (earthquake focal mechanisms). The second important element of fault zoning was electronic maps of active faults of Eurasia hosted on the GIN RAS sever. Within the algorithmic framework for Rebetsky’s method of cataclastic analysis, both of these datasets allowed calculating the Coulomb stresses for the segments of the BRS faults. During the study, a development of the cataclastic method has been carried out in using a diagram of brittle fracture as the Mohr–Coulomb model, considering a decrease in the range of positive Coulomb stress values with an increase in effective normal stress levels. Such an approach provides a more reliable identification of the fault segments with the maximum Coulomb stress levels. The performed calculations showed that in the BRS crust there are several up to 50 km long fault segments having critically high (80–100 % of the maximum) and high (40–80 % of the maximum) Coulomb stress levels. It is these corebased hazardous zones that are considered as places where seismogenic ruptures of future М>7.0 earthquakes may start. There have been distinguished three zones that present such hazard: 1) in the western segment of the BRS in the western part of the Tunka Valley in the Tunka, Khamardaban-Mondy and Baikal-Mondy fault systems; 2) in the Selenga River delta in the Proval, Delta, Ust-Selenga and Sakhalin-Enkhauk fault systems; 3) within the northeastern flank of the BRS on the fault system of the Muyakan basin (along the North Muya ridge). It is proposed to perform tectonophysical monitoring of changes in the stress state of these three zones and make observations of their surface motions using remote sensing methods.

MIDDLE PLEISTOCENE NEOTECTONIC ACTIVATION OF THE MOUNTAIN BORDERING THE MUYA-KUANDA RIFT VALLEY (BAIKAL RIFT ZONE)

Geological and geomorphological studies of the northeastern branch of the Baikal rift zone have revealed structural features of sedimentary strata, as well as the features of lithology and interaction of heterochronous loose sedimentary complexes, block structure of rift valley bottoms and their mountain border, fault network, and activated Cenozoic faults. The Middle Pleistocene neotectonic uplift was identified on the southern slopes of the North Muya and the Kodar ranges and in the mountains bordering the Muya-Kuanda and Parama rift valleys. The authors have studied the parameters of this uplift, determined neotectonic movement amplitudes in the uplift areas, traced the marginal disjunctives and the kinematics of their movements, and identified homochromous sedimentary strata and determined their formation age.The activation of tectonic processes, during which the neotectonic uplift was formed, occurred in the early Middle Pleistocene, before the onset of glaciation. The formation of a dammed lake in the system of Muya-Kuanda rift valleys is associated with the tectonic damming of the Vitim River. During the Middle Pleistocene glaciation, the Kodar Range glaciers have twice formed the ice dams in the area of Lake Oron, which led to the emergence of dammed lakes in the Vitim valley connected with lakes in the Muya-Kuanda troughs. The shoreline of these lakes is preserved as a high rock terrace with an absolute height of 860±10 m along the perimeter of the rift valleys.In the Late Pleistocene, the terrace complex in the Muya-Kuanda rift valleys was mainly formed under the influence of climatic factor.

Geochemical characteristics of peridotite xenoliths from the Vitim volcanic field: Insight to late Cenozoic mantle upwelling in SE Siberia

The Vitim volcanic field, comprising Cenozoic basaltic lava flows associated with the Baikal Rift volcanism, contains abundant mantle xenoliths that offer insights into the subcontinental lithospheric mantle (SCLM) beneath the region. This study investigates the mineralogical and geochemical compositions of these mantle xenoliths, entrained in Miocene and Pleistocene basalts. The xenoliths, predominantly garnet-, spinel-, and garnet-spinel-bearing lherzolites, exhibit high modal content of clinopyroxene and low fosterite content of olivine, suggesting a relatively fertile SCLM. Distinct variations in CaO and Al2O3 observed in spongy rims and cores of clinopyroxenes likely result from decompression-induced partial melting. Trace element patterns of clinopyroxene reveal three types: light rare-earth elements (LREE) depleted, LREE enriched, and transitional LREE types. Whole-rock trace element compositions of the four Vitim lherzolites showing flat to LREE and large ion lithophile elements enrichments are entirely consistent with those of their clinopyroxenes. The LREE depleted type indicates that these lherzolites were residual mantle after melt extraction, whereas the LREE enriched type suggests that they were metasomatized after residual mantle formed by partial melting. The transitional LREE type showing in-between features among the above two end-members could represent that those lherzolite underwent less or incomplete metasomatism, thus clinopyroxene cores still retain primary depleted LREE type feature of residual mantle. Most Vitim lherzolites were affected by cryptic metasomatism with less stealth metasomatism, whereas only those with secondary amphibole and apatite could be influenced by modal metasomatism. The lithospheric mantle beath the Vitim volcanic field representing by these lherzolites was metasomatized predominantly by hydrous fluids with minor silicate melts. Using the clinopyroxene melting model, it was found that most Vitim lherzolites have experienced <10% partial melting. Geothermal gradients estimated from mineral geothermobarometry indicate that lherzolites in the Pleistocene basalts equilibrated at shallower depths with higher temperatures compared to those in the Miocene basalts with deeper depths and lower temperatures. SrNd isotopic ratios, combined with previous results (Ionov et al., 2005), demonstrate that the Miocene lherzolites have a broader range from depleted to enriched components, whereas lherzolites in the Pleistocene basalts show lesser enrichment. It is proposed that the Pleistocene basalts captured shallower SCLM lherzolites experienced less degrees of melt metasomatism than those deeper SCLM lherzolites hosted by Miocene basalts. Considering the deeper SCLM is more vulnerable to metasomatism by ascending melts, such temporal and geochemical variation further emphasizes progressive asthenosphere upwelling beneath the Vitim region from the Miocene to Pleistocene time.

Spatial and statistical analysis of seismic energy distribution in the Olekmo-Stanovoy seismotectonic zone (South Yakutia)

The seismic activity in the Republic of Sakha (Yakutia) is monitored across a vast territory of 1.5 million km² and is characterized by two major seismic zones of varying rank: the Olekmo-Stanovoy seismotectonic zone in the south andthe Arctic-Asian seismic belt in the north. This article explores the Olekmo-Stanovoy seismotectonic zone, which forms the eastern flank of the Baikal-Stanovoy seismic belt. For statistical analysis, we used a geological and geophysical model of the Southern Yakutia territory. According to this model, the Olekmo-Stanovoy seismotectonic zone is segmented into five structural-tectonic blocks. For the study area, we compiled a regional catalog of earthquakes, supplemented by data from the International Seismological Bulletin (ISC). Based on the seismotectonic features and geodynamic position of the region, a spatial-statistical analysis of seismic energy released during earthquakes over a 58-year instrumental period of regional seismological observations was conducted. The main parameter for calculations was the parameter ΣE (total released seismic energy), which is most often used in the comprehensive quantitative assessment of seismic activity in a region. This study examines the major earthquakes that made the largest contribution to the calculations of the total energy released in each of the five blocks. Analysis of seismic-statistical data collected in the aisles of large structural-tectonic blocks of the first rank revealed the following correlation: in the Olekmo-Stanovoy seismotectonic zone in the direction from west to east, with distance from the Baikal rift zone, seismic activity decreases by 104 times. This thesis also suggests a slowdown in the intensity of seismotectonic and geodynamic processes in the same direction. The results of spatial-statistical analysis of the distribution of seismic energy in large regional blocks of the Olekmo-Stanovoy seismotectonic zone can be relevant for conducting small-scale research to assess the potential seismic activity of poorly studied areas and for medium-term earthquake forecasting as an independent indicator of impending seismic events.

Lake Baikal Microseisms Based on Regional Seismic Network Data

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.

Deep Structure of the Baikal Rift Zone and Central Mongolia

The upper mantle and the transition zone of the Baikal rift zone (BRZ) are studied. The observations are analyzed using P-wave receiver functions. It is found that in the BRZ central and northeastern part, the P410s converted seismic phase is preceded by a precursory wave with negative polarity which is formed in the low S-wave velocity layer at a depth of 350–410 km. A similar precursory wave with low S-wave velocity and negative polarity is formed at a depth of 600–660 km. The low-velocity layers are interpreted as resulting from the hydration of wadsleyite and ringwoodite during the subduction of the Pacific lithosphere. A similar study of the mantle in Central Mongolia found no expected signs of hydration. Modeling of the lithosphere–asthenosphere system in Central Mongolia by joint inversion of the body wave receiver functions and surface wave dispersion curves reveals a very thin lithospheric lid beneath Khangai and a thick layered asthenosphere to a depth of 200 km with a lithospheric inclusion between low-velocity layers.

Determination of Earthquake Depths Using Data of Cross-Sectional and Areal Deep Seismic Studies in Siberia

Abstract ––Information on the distribution of earthquake hypocenters for many seismically active zones of Siberia remains insufficient, which is associated with sparse networks of seismological observations. The paper presents the results of determining the earthquake depths in several seismogenic areas in the Altai-Sayan region, Cisbaikalia, Transbaikalia, and Yakutia using the travel time of longitudinal refracted waves from the Mohorovičič discontinuity (Pn waves) from earthquakes and the recently obtained information about the deep structure of these regions. The depth determination algorithm is tested using data from aftershocks of large earthquakes occurring in Tuva in 2011 and 2012 (ML = 6.7 and 6.8), recorded both by the regional seismological network and by a local group of seismic stations. Different methods are applied to reveal that some aftershock depths have a close match, including those for the main shocks – the Tuva-1 and Tuva-2 earthquakes. Another good match of earthquake depths is obtained using Рn waves with materials from regional and detailed studies of the Baikal branch of the Geophysical Survey of the Russian Academy of Sciences in the Muyakan activation site in the Baikal rift zone. The resulting data complement the information on the hypocenter of the main shock and confirm the change of the Muyakan activation cluster from large depths to shallow ones since the onset of activation in 2014. New information on the earthquake depths using Рn waves is obtained in Yakutia along the border of the largest Eurasian and Okhotsk tectonic plates. It is revealed that they decrease to 6–12 km as compared to higher depths of 20–30 km in adjacent areas. The resulting series of new information on the distribution of earthquake hypocenters using Рn waves is extremely important mostly because it indicates the possibility of identifying and redefining earthquake hypocenters using previous seismological observations in seismically active zones of Siberia.

Sandy Sediments in the Tokko Basin of the Baikal Rift Zone: Sources and Provenance Areas

Sandy Sediments in the Tokko Basin of the Baikal Rift Zone: Sources and Provenance Areas

Quasilinear Chains of Earthquakes in Clusters of Seismic Events: The Baikal Rift System

Quasilinear Chains of Earthquakes in Clusters of Seismic Events: The Baikal Rift System

CORRELATION OF RADON AND SEISMIC ACTIVITY IN THE BAIKAL RIFT ZONE ACCORDING TO EMANATION MONITORING DATA

Long series of the emanation monitoring organized at Tyrgan (2018–2023), Buguldeika (2020–2023) and Maksimikha (2022–2023) stations in the central part of the Baikal Region (the Eastern Siberia) are studied to reveal the relationship of the subsurface radon volumetric activity (RVA) with the seismicity of the Baikal rift zone (BRZ). The comparison of the weekly averaged RVA values at different stations and the degree of dependence of this parameter on the logarithm of the total earthquakes energy (LgΣE) showed that during the periods of seismic activity the impact of tectonic forces makes significant corrections in the character of emanations, which in general is determined by non-tectonic factors. It has been established that seismic activity represented by the enough strong for the Baikal Region earthquakes with K≥13.3 correspond to synchronicity intervals of RVA at different monitoring stations, as well as coincidence of maximum values of the logarithm of the total earthquakes energy with local maximums of the correlation coefficient between RVA and LgΣE parameters. The area, including the earthquake epicenters that generated such an emanation response at monitoring stations, is in the form of an ellipse extended in the northeastern direction – along the main fault structures of the BRZ. The obtained results, besides confirming the previously proposed model of radon field formation in the Baikal Region under the influence of external and internal forces, provide the for the identification of further stable precursors of strong earthquakes based on a comprehensive analysis of data from a branched emanation monitoring network.

Taxonomic Diversity of the Microbial Community in the Kuchiger Thermal Spring (Baikal Rift Zone)

Taxonomic Diversity of the Microbial Community in the Kuchiger Thermal Spring (Baikal Rift Zone)

Мониторинг окислительно-восстановительного потенциала подземных вод в режиме реального времени на Култукском полигоне в конце 2023 – начале 2024 г.: сопоставление электрических эффектов с землетрясениями в центральной части Байкальской рифтовой системы

The results of real-time observations of the oxidation-reduction potential (ORP) in groundwater from two wells in the Kultuk area from December 2023 to March 2024 are presented. Variations of this parameter show reorganizations, accompanied by short episodes of single earthquakes and longer intervals of earthquake series in the central part of the Baikal Rift System. A change from low to high ORP values accompanies the general transition from strong to weak earthquakes. The uneven variations in ORP of the monitoring stations are suggested to associate with piezoelectric effects that arise when seismic waves act on quartz of the ordered tectonite structures in different active structures of the South Baikal basin: in its marginal Obruchev fault and in the axial Kultuk tectonic step.

Мониторинг урановых компонентов и Si – Na/Li температур в резервуаре подземных вод Аршана Тункинской долины в 2012–2024 гг.: отслеживание парагенетических соотношений гидрогеохимических, сейсмических и вулканических процессов в Байкальской рифтовой системе

Monitoring series of the 234U/238U activity ratio, concentrations of U, and Si – Na/Li temperatures in reservoir of groundwater from the Arshan sanatorium area in the Tunka Valley are obtained in 2012–2024. With taking into account published materials and existing ideas on the groundwater genesis in the Tunka Valley, it is proposed that the entry of uranium components into groundwater is controlled by uranium dissolution under the influence of gases – oxidizers and reducers. It is augured that the isotopic equilibrium between 234U and 238 in cold carbon dioxide mineral waters can serve as an indicator of the impact of oxidized fluids on their deep reservoir, and the disruption of the equilibrium can serve as an indicator of the replacement of the affecting oxidized fluids by reduced ones. The monitoring data series are used to trace paragenetic relationship between hydrogeochemical, seismic, and volcanic processes in the Baikal Rift System.

ОЦЕНКА ИНТЕНСИВНОСТИ СЕЙСМИЧЕСКИХ КОЛЕБАНИЙ НА БАЙКАЛО-ОЛЁКМИНСКОМ УЧАСТКЕ БАЙКАЛО-АМУРСКОЙ ЖЕЛЕЗНОДОРОЖНОЙ МАГИСТРАЛИ

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 (∆ &lt; 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.

Серия ОВП-импульсов подземных вод побережья Байкала 20–27 марта 2024 г.: вероятная связь с магнитными бурями

A short (8-day) series of ORP pulses begins with low-amplitude (about 1 mV) oscillations, partially smoothes out, and progresses to deep minima reaching 138 mV. The series differs from the pulses covering the full (30-day) tidal lunar-solar cycle in February–early March 2024. It is suggested that variations in ORP pulses are temporally related to magnetic storms. Also, for generation of powerful ORP pulses, the spring lunar-solar tide is considered as a favorable factor. The March series of pulses is accompanied by decreasing background ORP values. Similar ORP decrease, but without pulses, characterizes the January episodes of earthquakes in the central Baikal Rift System.