Variations of U components and Si–Na/Li temperatures in groundwater reservoirs on the southern flank of the South Baikal Basin and Tunka Valley in 2012–2024: relationship between axial and flank seismogenic deformations in the Baikal Rift System

Coordinated variations of U components and Si–Na/Li temperatures in groundwater reservoirs on the southern flank of the South Baikal Basin and the Tunka Valley were recorded that emphasized the unity of a neotectonic structure of the Khamar-Daban range, passing from the southern flank of the Tunka Valley to the one of the South Baikal Basin. On the southern shoulder of the Tunka Valley, in an area of junction between the Khamar-Daban and Urgudey ridges, the Tory-Urgudey-Nilovka boundary belt of groundwater with a high activity ratio of 234U/238U (AR4/8 = 2–3), limiting from the east, south, and west the core of the Tunka geothermal anomaly with low OA4/8 (less than 1.8), was traced. In the time interval from 2012 to 2024, hydrogeochemical signals of groundwater were obtained at the BRS monitoring stations, indicating a widespread transition from the Kultuk seismic reactivation to the Baikal-Khubsugul one in 2014–2015 and the lack of hydrogeochemical changes in some areas during strong earthquakes of the latter reactivation in late 2020 – early 2021. It is suggested that in 2014–2015, comprehensive seismogenic deformations of the BRS earth's crust developed in both its axial part and on the flanks, while the localization of strong deformations in the axial seismogenic structures entailed strong earthquakes of the Baikal-Khubsugul seismic reactivation of 2020–2023. Both flank and distal expansion of the BRS seismicity in 2023–2025 may reflect a seismogenic entry of the earth's crust into a state similar to the one of 2014–2015. The key significance of choosing a monitoring point for assessing the state of the earth's crust is emphasized. For the BRS, complete information is obtained at the axial Kultuk polygon, while on the flanks, the most important pre-seismogenic state of the earth's crust for strong earthquakes is not recorded, and observations are useless.