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

Abstract

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.

Research highlights

• 10-year groundwater monitoring in the Kultuk area of the western shore of Lake Baikal shows the temporal change of compression and extension of the near-surface crust through measurements of an AR4/8 (²³⁴U/²³⁸U activity ratio) and an A4 (²³⁴U activity). Compression was accompanied by the moderate Goloustnoe earthquake in 2015. Change to extension results in strong Baikal–Khubsugul seismic reactivation in 2020–2023.

• The thermal state in the Kultuk water reservoir is estimated by determining thermophilic macrocomponents Si, Na, and microcomponent Li. 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. During the Baikal–Khubsugul seismic reactivation, groundwater mainly upraised from the hydrogeodynamic center with frictional heating in a fault plane up to 99°C.

• The further monitoring of chemical hydrogeodynamics of the Kultuk reservoir may provide a forecast of seismic hazards in the central Baikal Rift System.