Abstract: The activities of fault zones may be monitored well by hydrogeochemical methods. From July 2015 to November 2019, concentrations of dissolved species including the major, trace elements and stable isotopes in 194 water samples collected from 12 thermal springs of Nantinghe fault zone were used to characterise their hydrogeochemical feature and to reveal the relationships between water chemistry and regional tectonic activity. And Xingfu hot spring was continuously monitored from February 2019 and water samples were collected every three days. The results show that ① The stable isotope data (δD, δ18O) suggesting that the water of hot springs are rechanged by circulation of meteoric waters, where the east and west branch fault recharge elevation ranges are 1.6~2.1 km and 1.0~1.9 km; ② There are four main types of water chemistry in the study area: HCO₃-Na、 HCO₃·SO₄-Na、 SO₄-Ca、 HCO₃-Ca. The east branch fault zone is HCO₃-Na type water. There are HCO₃·CO₄-Na and HCO₃-Na in the northeastern segment of the west branch, HCO₃-Na water in the middle section, and HCO₃-Ca water in the southwestern segment; ③ SiO₂ in the region is 11.1~101.0 mg/L, and aqueous geothermometry suggests that thermal storage temperature is 42.7℃~137.8℃. This data, along with estimated temperature, gave an estimated circulation depth for the spring waters of 0.6~2.5 km. Maolan and Xiaodingxi in the northeastern segment of the west branch are partially balanced water, while the eastern branch and other hot springs are immature water with low salinity. The comprehensive enrichment factors and contents of trace elements reflect that the water-rock reaction degree is weak in the internal circulation of the fault; ④ There is a certain correlation between chemical changes of hot spring water and seismic activity. Five days before the Yongde M_S4.4 earthquake, the Cl^- content increased by about 40.6% compared with the background value and the epicentral distance is 77 km. one day before Lincang M_L3.3 earthquake, the SO^2-₄ content increased by about 27.2% compared with the background value and the epicentral distance is 13 km. The dissolved Cl^- and SO^2- ₄in Xingfu hot spring were obviously abnormal to the seismic precursor of near-field earthquake. In a word, it is a great significance to judge and recognize the short-term and impending precursory anomalies of earthquakes with the study of the hydrogeochemical characteristics of the hot spring in NTHF (Nantinghe Fault).

Key words: Hot spring, Isotope, Hydrogeochemistry, Earthquake, Nantinghe fault