Groundwater monitoring has been performed in a well of the Kultuk area on the western shore of Lake Baikal since 2013. Compression and extension of the near-surface crust are defined through measurements of an AR4/8 (234U/238U activity ratio) and an A4 (234U activity) in groundwater from the Kultuk reservoir. Its thermal state is estimated by determining thermophilic macrocomponents Si, Na, and microcomponent Li. The recorded change in the groundwater reservoir and coeval seismogenic processes, which resulted in earthquakes of the central Baikal Rift System, are considered paragenetically related near-surface and deeper processes of the crust, respectively. It is inferred that compression of the Kultuk area, accompanied by the Goloustnoe earthquake in 2015, was changed by its extension during the strong Baikal–Khubsugul seismic reactivation in 2020–2023. Under compression of the crust, groundwater ascended from a shallow part of the reservoir of 0.5–0.9 km episodically heated up to 116°C by friction in a fault plane. Afterward, a deeper hydrogeodynamic center was generated with its final localization at a depth of about 1.2 km in 2019–2020; during the subsequent Baikal–Khubsugul seismic reactivation, groundwater mainly upraised from the hydrogeodynamic center with frictional heating in a fault plane up to 99°C. Episodic penetration of groundwater portions from depth up to 1.6 km accompanied a slight upward enlargement of an active part of the reservoir to 1 km. The further monitoring of chemical hydrogeodynamics of the Kultuk reservoir may provide a forecast of seismic hazards in the central Baikal Rift System.

Field observations were performed on the Ust-Zhom-Bolok lava cover of the Oka basin in the southeastern part of Eastern Sayans. In the Sailag and Nomto-Gol sections, loess-like deposits, underlying and overlying the lava cover, were identified. It is inferred that the lava cover erupted in conditions of a dry cold climate at the end of the Pleistocene and was dissected by the Oka River in conditions of a warmer climate and abundant water content of the territory in the Holocene.

Real-time groundwater monitoring on a shore of Lake Baikal reveals peculiar effects in the form of quasi-periodic negative ORP pulses with varying time intervals and a decrease in ORP relative to the background during the week of February 2024. We perform an express analysis of the observed effects and pay attention to their occurrence during increasing air temperature. At the beginning of the observation interval, we determine the regime of initial ORP pulses, which occur in conditions of a slightly increasing air temperature, then the middle one in a conventionally-optimal regime in conditions of significant warming, and the final regime of retrograde evolution in cooling conditions. The revealed regularities in changes of pauses between pulses and amplitudes of ORP reductions relative to the background form the basis for the analysis of ongoing effects.

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.

The introduction to the special issue of the journal provides a brief description of the organization of geological education in Irkutsk, closely connected with the development of geological science in the south of Eastern Siberia.

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.

Study of components in groundwater from springs, wells, and river waters in the Orlik settlement and adjacent areas is performed for arrangement of coseismic monitoring in the western part of the Baikal rift system. The first groundwater data series obtained from 17 November 2023 to 16 April 2024 show contrasting coseismic U variations, indicating the high sensitivity of the selected monitoring sites.

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.

Real-time monitoring of coastal groundwater of Lake Baikal in a well of the Kultuk village from February 7 to March 7, 2024 shows negative ORP pulses. The following intervals are distinguished: I – February 07–13 (pulses 1–26), IIa – February 13–18 (pulses 27–43), IIb – February 18–22 (pulses 43–54), IIIа – February 22–26 (pulses 55–71), IIIb – February 26 – March 4 (pulses 71–80), VIa – March 4–6 (pulses 80–82), and March 6–7 (pulses 82–83). These intervals demonstrate a distinct development of the pulse-generating process from its origin (interval I) through successive intensification (intervals IIa, IIb, and IIIa) to degeneration (intervals IIIb and IV). It is assumed that the 30-day activity of the pulse-generating process is regulated by neap and spring lunar-solar tides.

The results of monitoring concentrations of the thermophilic element Si in fresh subthermal and cold groundwaters from the Kultuk polygon with a temperature range at the output from ca. 0 to 20 °C are presented. A stepwise zonal increase in the Si concentration is recognized in groundwaters of the polygon centered at station 40. In this center of the Kultuk hydrothermal reservoir, the minimum temperature of 25 °С on September 17, 2014 (before the earthquakes of 2014–2015) and the maximum temperature of 60 °С on January 23, 2021 (11 days after the strongest Khubsugul earthquake Mw=6.8) is obtained using the chalcedony geothermometer. It is proposed that groundwater came from the deep Kultuk reservoir with elevated temperature during the strong Kultuk seismic reactivation (August 27, 2008 – January 04, 2011) and changed to those with a lower temperature during the weak Tolbazikha one (June 24, 2011 – October 11, 2012) reaching a temperature minimum by 2014. Then the inflow of groundwaters with elevated temperature was revived again during the preparation and implementation of the Baikal-Khubsugul seismic reactivation that was marked by strong earthquakes in 2020–2022.

The results of prospecting studies of trace elements and activity ratio 234U/238U (АR4/8) in surface waters and groundwaters from the Listvyanka, Buguldeyka, and Olkhon-Priolkhonye polygons of integrated monitoring, organized since 2020, are presented. At the Listvyanka polygon, wide interval of АR4/8 from 1.67 to 2.5 is obtained for groundwater in the Bolshie Koty village, at the Buguldeyka and Olkhon-Priolkhonye polygons that are located in the zone of the Primorsky fault – an interval of AR4/8 from 1.25 to 2.70. These values is general characteristic of large active faults of the central Baikal Rift Zone. Water from the Buguldeyka River differs in terms of both trace element compositions and AR4/8 values from those of its tributary Kurtun due to location of these rivers basins in rocks of the exposed basement of the Siberian Platform and its sedimentary cover, including evaporate carbonate sediments. On the Olkhon island, determined is groundwater that differs in elemental composition from the deep Baikal water.

Automatic probes were installed in two wells in the area of Kultuk settlement to determine real-time variations of ORP, pH, and temperature in groundwater. The obtained data were interpreted and compared with seismic pulses on November 22-25 and December 16, 2023 in the central part of the Baikal Rift System.

A special issue of the journal is dedicated to researchers and the history of geological exploration in Eastern Siberia. The articles in the issue highlight the initial contributions to the study of geology, geography and mineral resources of the territory by A.V. Lvov, S.P. Peretolchin, and V.M. Senyukov, provide information about the unknown pages of the construction of the Circum-Baikal Railway, studies of the Botogol graphite deposit, the Markov oil and gas condensate field, the Baikal system of basins, and volcanism spatially associated with its development. An overview is given of the 50-year career of geologists who graduated from ISU in 1973.

Abstract. On the west coast of Lake Baikal, we defined long-term and short-term variations in Hg concentrations of groundwater in 2017 to 2019 and from 2019 to the present, respectively; the latter variations were associated with the 2020 2021 Baikal-Khubsugul seismic reactivation of active faults. We inferred that increasing Hg concentration in groundwater was due to crustal extension with the highest Hg enrichment related to aftershocks. We argue that a Hg flux depends on the nature of fault activities and elevated Hg concentrations in sedimentary layers may designate the release of Hggas and Hggw from seismically active faults in the past 14.4 Ka.

The results of an seismic variation experiment at the Babushkin test site and monitoring observations of the 234U/238Uactivity ratio in groundwater from a paleoseismogenic dislocation in a zone of the Main Sayan Fault are presented. Found is a similarity between effects obtained in the experiment and data at the monitoring station in 2014, but no similar effects during preparation and realization of the strong Bystraya earthquake in 2020. It is inferred that the monitoring observation of a single station is not enough for a successful prediction of a strong earthquake. It is necessary to support hydrogeochemical monitoring of several stations located on the test site in different structural conditions and provided different information on seismogenic deformations that change over time.

From 5-year monitoring of groundwater in the Kultuk area, long-period (about 2.5 years) and short-period (months) variations in mercury concentration have been established. Long-period variations are associated with the preparation and implementation of the Baikal-Khubsugul seismic reactivation; short-period ones – directly with earthquakes. Mercury concentrations was low in groundwater in 2015, when seismogenic deformations developed in a state of crustal compression, and generally increased to 2020–2021, when seismogenic deformations occurred under extention. The course of the seismic process was reflected in a successive change in mercury/redox potential rеlationship in groundwater. It is proposed that the anomalous behavior of mercury during seismic activity resulted in increasing its concentration in the sedimentary layers of the second half of the 18th century through the present.

In the western part of the South Baikal Basin, spatial-temporal distribution of earthquake epicenters is characterized by quasi-periodic seismic reactivations. The strongest earthquakes occurred in 1999 (South Baikal, Мw = 6.0), 2008 (Kultuk, Мw = 6.3) and 2020 (Kudara, Мw = 5.4). Since 2013, we have been monitoring the 234U/238U activity ratio (AR4/8) in groundwater as an indicator of crack open/closing that promotes/prevents water circulation in active faults of the basin. From monitoring results, we define the concept of a complete seismogeodynamic cycle as a change from crustal compression to extension occurred during 12 years with a successive increase in seismic hazard levels.

A series of hydrogeochemical data obtained in 2020–2021 at station 184 of the Kultuk polygon, is interpreted in connection with the displaying of three processes during the Baikal-Khubsugul seismic reactivation: raise of reduced–oxidized fluids, extension–compression of microcracks, and deformation effects propagating from the sources of seismic shocks and resulted from self-oscillatory processes at the polygon area. The found temporal relationships between 234U/238U, 234U, U, Hg, Li variations and earthquakes can be used for prediction of future strong seismic events in the central part of the Baikal seismic zone.