Seismic Signals of the Wushi MS7.1 Earthquake of 23 January 2024, Viewed Through the Angle of Hydrogeochemical Characteristics

On 23 January 2024, a MS7.1 earthquake struck Wushi County, Xinjiang Uygur Autonomous Region, marking the largest seismic event in the Southern Tianshan (STS) region in the past century. This study investigates the relationship between hydrothermal fluid circulation and seismic activity by analyzing the chemical composition and origin of fluids in natural hot springs along the Maidan Fracture (MDF). Results reveal two distinct hydrochemical water types (Ca-HCO3 and Ca-Mg-Cl). The δD and δ18O values indicating spring water are influenced by atmospheric precipitation input and altitude. Circulation depths (621–3492 m) and thermal reservoir temperatures (18–90 °C) were estimated. Notably, the high 3He/4He ratios (3.71 Ra) and mantle-derived 3He content reached 46.48%, confirming that complex gas–water–rock interactions occur at fracture intersections. Continuous monitoring at site S13 (144 km from the epicenter of the Wushi MS7.1 earthquake) captured pre-and post-seismic hydrogeochemical fingerprints linked to the Wushi MS7.1 earthquake. Stress accumulation along the MDF induced permeability changes, perturbing hydrogeochemical equilibrium. At 42 days pre-Wushi MS7.1 earthquake, δ13C DIC exceeded +2σ thresholds (−2.12‰), signaling deep fracture expansion and CO2 release. By 38 days pre-Wushi MS7.1 earthquake, Na+, SO42−, and δ18O surpassed 2σ levels, reflecting hydraulic connection between deep-seated and shallow fracture networks. Ion concentrations and isotope values showed dynamic shifts during the earthquake, which revealed episodic stress transfer along fault asperities. Post-Wushi MS7.1 earthquake, fracture closure reduced deep fluid input, causing δ13C DIC to drop to −4.89‰, with ion concentrations returning to baseline within 34 days. Trace elements such as Be and Sr exhibited anomalies 12 days before the Wushi MS7.1 earthquake, while elements like Li, B, and Rb showed anomalies 24 days after the Wushi MS7.1 earthquake. Hydrochemical monitoring of hot springs captures such critical stress-induced signals, offering vital insights for earthquake forecasting in tectonically active regions.