Significant hydrogeochemical changes may occur prior- and post-earthquakes. The Xiaojiang fault zone (XJF), situated in a highly deformed area of the southeastern margin of the Tibetan Plateau, is one of the active seismic areas. In this study, major and trace elements, and hydrogen and oxygen isotopes of 28 sites in hot springs along the XJF were investigated from June 2015 to April 2019. The meteoric water acts as the primary water source of the hot spring in the XJF and recharged elevations ranged from 1.8 to 4.5 km. Most of the hot spring water in the study area was immature water and the water–rock reaction degree was weak. The temperature range was inferred from an equation based on the SiO₂ concentration and chemical geothermal modeling: 24.3~96.0 °C. The circulation depth for the springs was estimated from 0.45 to 4.04 km. We speculated the meteoric water firstly infiltrated underground and became heated by heat sources, and later circulated to the earth’s surface along the fault and fracture and finally constituted hot spring recharge. Additionally, a continuous monitoring was conducted every three days in the Xundian hot spring since April 2019, and in Panxi and Qujiang hot springs since June 2019. There were short-term (4–35 d) seismic precursor anomalies of the hydrochemical compositions prior to the Xundian M_L4.2, Dongchuan M_L4.2, and Shuangbai M_L5.1 earthquakes. The epicentral distance of anomalous sites ranged from 19.1 to 192.8 km. The anomalous amplitudes were all over 2 times the anomaly threshold. The concentrations of Na⁺, Cl⁻, and SO₄²⁻ are sensitive to the increase of stress in the XJF. Modeling on hydrology cycles of hot springs can provide a plausible physicochemical basis to explain geochemical anomalies in water and the hydrogeochemical anomaly may be useful in future earthquake prediction research of the study area.

Hydrogeochemical changes in association with earthquakes are considered as a potential means of identifying earthquake precursors. The Xianshuihe fault zone (XSHF) is considered one of the most active seismic fault zones in China; 43 hot springs were sampled and analysed in the laboratory for major elements, silica, stable isotopes (δD and δ¹⁸O) and strontium isotopes were investigated from 2008 to 2021. The meteoric water acted as the primary water source of the hot spring in the XSHF, and recharged elevations ranged from 1.9 to 4.8 km. The geothermometers method was used to estimate the region of thermal storage temperature and its temperature between 8 and 142 °C. And the circulation depth ranged from 0.1 to 6.9 km. Most of the hot spring water was immature water with a weak degree of water-rock reaction. However, the degree of water-rock reaction and the depth of hot spring water circulation were high in part of the Kangding and Daufu segments, which also had the highest reservoir temperature and the most frequent strong earthquakes. Temporal variations of hydrogeochemical showed that Na⁺, Cl⁻ and SO₄²⁻ decreased obviously following the 12 May 2008 Wenchuan Ms8.0 and existed abnormal value fluctuations from the 20 April 2013 Lushan Ms7.0 to 22 November 2014 Kangding Ms6.3 occurred and after 20 July 2017 returned to the normal levels. And the ion concentrations in hot springs increased by 5% to 35% three months before 22 November 2014 Kangding Ms6.3 with the obvious precursor anomaly. Hydrogeochemical anomalies could be useful for predicting an earthquake in the study area.

Analyzing the hydrochemical composition in thermal springs is an advantageous method for studying the coupling mechanism of the deep and shallow fluids in active fault zones. Here we conducted sampling in 30 thermal springs near fault zones in Fujian Province, and the major elements, trace elements, silica, stable isotopes (δD and δ¹⁸O) and strontium isotopes were tested in the laboratory. The results show that (1) the thermal springs in the study area can be divided into six types according to the content of the major elements: HCO₃-Na, HCO₃·SO₄-Na, Cl·HCO₃-Na, Cl-Na, Cl-Na·Ca and HCO₃·SO₄-Ca; (2) hydrogen and oxygen isotopes indicate that precipitation is the main source of recharge for thermal springs in the study area, and the recharge height is between 258 m and 1859 m; (3) the content of SiO₂ in the thermal spring varies from 18.1 mg/L to 59.3 mg/L. The geothermal reservoir temperature calculated is 90~226 °C, and the circulation depth is 2.9~5.4 km, except for the W10 thermal spring, whose circulation depth is 8.4 km; and (4) the ⁸⁷Sr/⁸⁶Sr of the thermal springs in southwestern Fujian and eastern Fujian has obviously different characteristics, indicating the influence of different rock formations on the groundwater cycle process. Additionally, a continuous measurement of the main anions and cations was performed in five thermal springs every three days since January 2020. There were obvious abnormal changes in the hydrochemical compositions, chlorine in four of the five springs, sodium at three springs, and four ions at one spring, which all showed abnormal high-value changes by 15% to 80%, and which occurred 85~168 days prior to the M6.1 earthquake in Hualien, Taiwan. An inspiration could be provided for obtaining effective earthquake precursor anomalies by monitoring the change in ion concentration in thermal springs.

Based on the observation of the geochemical characteristics of 19 hot springs in the Litang Fault Zone (LFZ) from 2010 to 2019, the major elements, trace elements, and stable isotopes were investigated, and a conceptual model of ground fluid circulation in the LFZ was established. The main hydrochemical type of hot spring water samples is HCO₃⁻-Na⁺. The δ²H values range from −157.6‰ to −123.4‰ and δ¹⁸O values range from −24.5‰ to −15.4‰. The hot spring water in the Litang fault zone is mainly recharged by infiltrating precipitation, with a recharge elevation of 4062~6018 m. Hydrochemical types of Litang hot springs are mainly controlled by the circulation of groundwater in a deep fault system, and are related to the rock lithology of thermal reservoir and water–rock reaction areas. Hot springs in the Litang fault zone attribute to three different heat sources, belonging to three geothermal systems. The flow direction of groundwater in the LFZ is roughly from northwest to southeast along the Litang fault. The deeper the circulation depth of hot spring water on the fault, the higher the thermal reservoir temperature and the stronger the seismic activity of the segment, which is closely related to the increase in pore fluid pressure, rock weakening, and deep fluid upwelling. This study is helpful for further study on regional hydrogeological environments and provides a scientific basis for revealing geothermal fluid movement in fault zones.

The Weixi–Qiaohou Fault (WQF) is considered an important zone of the western boundary of the Sichuan–Yunnan block, and its seismicity has attracted much attention after a series of moderate–strong earthquakes, especially the Yangbi Ms6.4 earthquake that occurred on 21 May 2021. In the present research, we investigate major and trace elements, as well as hydrogen and oxygen isotopes, of 10 hot springs sites located along the WQF, which are recharged by infiltrated precipitation from 1.9 to 3.1 km. The hydrochemical types of most analyzed geothermal waters are HCO₃SO₄-Na, SO₄Cl-NaCa, and SO₄-Ca, proving that they are composed of immature water and thus are characterized by weak water–rock reactions. The heat storage temperature range was from 44.1 °C to 101.1 °C; the circulation depth was estimated to range between 1.4 and 4.3 km. The results of annual data analysis showed that Na⁺, Cl⁻, and SO₄²⁻ in hot springs decreased by 11.20% to 23.80% north of the Yangbi Ms5.1 earthquake, which occurred on 27 March 2017, but increased by 5.0% to 28.45% to the south; this might be correlated with the difference in seismicity within the fault zone. The results of continuous measurements of NJ (H1) and EYXX (H2) showed irregular variation anomalies 20 days before the Yangbi Ms6.4 earthquake. In addition, Cl⁻ concentration is more sensitive to near-field seismicity with respect to Na⁺ and SO₄²⁻. We finally obtained a conceptual model on the origin of groundwater and the hydrogeochemical cycling process in the WQF. The results suggest that anomalies in the water chemistry of hot spring water can be used as a valid indicator of earthquake precursors.