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Hydrogeochemistry of 32 hot springs in the western Sichuan Province after the Wenchuan MS 8.0 earthquake was investigated by analyzing the concentrations of cation and anion and the isotopic compositions of hydrogen and oxygen. The water samples of the hot springs were collected four times from June 2008 to April 2010. Hydrogeochemical data indicated the water samples can be classified into 9 chemical types. Values of δ D and δ(18)O indicated that the spring waters were mainly derived from meteoric precipitation and affected by water-rock interaction and mixture of deep fluids. Concentrations of K(+)and SO4(-) of the samples from the Kangding district exhibited evident increases before the Wenchuan earthquake, indicating more supplement of deep fluids under the increase of tectonic stress. The chemical and isotopic variations of the water samples from the area closer to the epicenter area can be attributed to variation of regional stress field when the aftershock activities became weak.

This study explores the potential of machine learning algorithms for earthquake prediction, utilizing fluid chemical anomaly data from hot springs. Six hot springs, located within an active fault zone along the southeastern coast of China, were carefully chosen as hydrochemical monitoring sites for an extended period of two and a half years. Using this data, a prediction model integrating six algorithms was developed to forecast M ≥ 5 earthquakes in Taiwan. The model's performance was validated against recorded earthquake events, and the factors influencing its predictive capability were analyzed. Our comprehensive analysis conclusively demonstrates the superiority of machine learning algorithms over traditional statistical methods for earthquake prediction. Additionally, including sampling time in the data sets significantly improves the model's predictive performance. However, it is important to note that the model's predictive performance varies across different hot spring and indicators type, highlighting the importance of identifying optimal indicators for specific scenarios. The model parameters, including the anomaly detection rate (P) and earthquake response time threshold (M), significantly impact the model's predictive capabilities. Therefore, adjustments are needed to optimize the model's performance for practical use. Despite limitations such as the inability to differentiate pre‐earthquake anomalies from post‐earthquake anomalies and pinpoint the precise location of earthquakes, this study successfully showcases the potential of machine learning algorithms in earthquake prediction, paving the way for further research and improved prediction methods. Plain Language Summary: This study explores the potential of utilizing machine learning algorithms for earthquake prediction based on hydrothermal fluid chemical anomaly data. An earthquake prediction model integrating six machine learning algorithms was developed using continuous hot spring hydrochemical monitoring data. The model's prediction performance was validated using actual earthquake event data, and various factors influencing its predictive capability were analyzed. The results demonstrate that the machine learning algorithm's prediction performance exceeds that of traditional methods, and the selection of hot springs, indicator types, and model parameters significantly impacts the model's prediction performance. This study successfully showcased the potential of machine learning algorithms in earthquake prediction, while also indicating new avenues for further research and improvement of prediction methods. Key Points: An earthquake prediction model based on machine learning using hydrochemical data from hot springs is proposed with sufficient accuracyThe earthquake prediction accuracy of the local outlier factor algorithm outperformed the traditional statistical methodThe predictive performance of the model varies across different hot springs, indicators type, and model parameters [ABSTRACT FROM AUTHOR]

Trace elements in hot-spring waters are indicators for tracing hydrochemical processes, such as the deep circulation of geothermal water, the degree of water–rock interaction, redox reactions, the contamination of shallow water by thermal water, etc. In this study, 74 hot springs in the Xianshuihe–Xiaojiang Fault Zone (XSHF-XJF) were chosen for an investigation of the geochemical characteristics of trace elements using classic geochemical tools and multivariate statistical analysis. The results indicated (1) the hot-spring waters were mainly derived from atmospheric precipitation as indicated by δD and δ18O values that generally increased with decreasing elevations; (2) the high concentrations of B, As, Fe, and Mn in the waters, as well as the values of the Water Quality Index (WQI), indicated that the water quality was spatially heterogeneous and the hot-spring waters are not entirely suitable for drinking; and (3) B/Cl ratios showed that hot springs have different reservoir sources in the XSHF and XJF, respectively. The leaching of surrounding rock during water circulation contributed predominantly to the trace elements of the study's waters. Diversity of lithology was the main factor affecting their concentrations. In addition, deep circulation controlled by the fault could influence trace-element enrichment. Our results offer a guide for the exploration and use of geothermal resources. [ABSTRACT FROM AUTHOR]

Studying the hydrogeochemical characteristics of hot springs provides essential geochemical information for monitoring earthquake precursors and understanding the relationship between fluids, fractures, and earthquakes. This paper investigates the hydrogeochemical characteristics of hot springs along the Tingri–Nyima Rift (TNR) in southern Tibet, a seismically active zone at the collision front of the Indian and Asian-European plates. The major elements, hydrogen, and oxygen isotopes of seven thermal springs were analyzed from July 2019 to September 2021. The findings indicate that Mount Everest's meteoric water, which has a recharge elevation of roughly 7.5–8.4 km, is the main source of recharge for the hot springs. The water samples have two main hydrochemical types: HCO3-Na and Cl-Na. The temperature of the geothermal reservoir is between 46.5 and 225.4 °C, while the circulation depth is between 1.2 and 5.0 km based on silica-enthalpy mixing models and traditional geothermometers. Furthermore, continuous measurements of major anions and cations at the Yundong Spring (T06) near Mount Everest reveal short-term (8 days) seismic precursor anomalies of hydrochemical compositions before an ML4.7 earthquake 64.36 km away from T06. Our study suggests that seismicity in the northern section of the TNR is controlled by both hydrothermal activity and tectonic activity, while seismicity in the southern section is mainly influenced by tectonic activity. In addition to magnitude and distance from the epicenter, geological forces from deep, large fissures also affect how hot springs react to seismic occurrences. A fluid circulation model is established in order to explain the process of groundwater circulation migration. The continuous hydrochemical monitoring of hot springs near Everest is critical for studying the coupling between hot springs, fractures, and earthquakes, as well as monitoring information on earthquake precursory anomalies near Everest. [ABSTRACT FROM AUTHOR]

Hydrogeochemical characterization studies are regarded as an important method for determining the origin of hot springs. The major elements, trace elements, and stable isotopes of four groups of hot spring water samples and two groups of gas samples collected from the intersection of the Altyn Tagh fault zone and the East Kunlun fault belt were investigated in this study. The hot spring water temperature ranged between 6 °C and 14 °C. The water chemistry types of the hot springs were Na·Mg-Cl, Mg·Na-Cl·HCO3, Na-Cl·SO4, and Na-Cl·HCO3. The δD values ranged from −50.00% to −68.60%, while the δ18O values ranged from −6.90% to −8.60%. The hot spring water was recharged mainly by infiltrating precipitation, with a recharge elevation of 3390~3676 m. The heat storage temperature ranged from 66.7 to 164.9 °C. The circulation depth was estimated to range between 1043 and 2679 m. The strontium isotopic composition of the water samples in response to the main weathering sources comprised carbonate and sulfate. CO2 was the main component in the hot spring gas in the study region, and its content was over 95%. The 3He/4He-R/Ra relationship diagram revealed that the mantle-sourced helium from the Yitunbulake spring was 3.06%. In comparison, that from the Aiken spring was 7.38%, which indicated an intrusion of mantle-source material mixed into the hot springs in the study region. The crustal marine limestone contributed significantly to the carbon inventory of the hot spring gas samples (>75%). The dissolution of the marl aquifer resulted in the release of CO2. Carbon was primarily obtained through metamorphism and hydrothermal reactions in the basement lithologies. The Yitunbulake and Aiken hot springs are found near the intersection of the Arjin and East Kunlun fractures, where the water–rock response is relatively strong and the depth of circulation and thermal storage temperature are both high. This causes relatively high ambient pressure to be released from the deep fluid, resulting in microseismic activity in this region. The continuous observation of Aiken spring water chemistry allows for the monitoring of fracture activity in the region. The results of the study could serve as a foundation for further exploration of the relationship between geothermal water and deep faults, shallow geological formations, hydrogeological conditions, and geothermal resource development in the region. [ABSTRACT FROM AUTHOR]

The coupling relationship between regional seismic activity and the hydrogeochemical field provides an important theoretical basis for regional earthquake precursor exploration. The intersection area of the Red River fault zone (RRF) and the Xiaojiang fault zone (XJF) in southeast Yunnan province has become the focus area of earthquake monitoring and prediction because of its special tectonic position in China. There were 20 hot springs that were sampled and analyzed in the laboratory for major elements, including trace elements, silica, stable isotopes (δ18O and δD), and strontium isotopes, from the years 2015 to 2019. (1) The meteoric water is the main source of recharge for thermal springs in the study area, and recharged elevations ranged from 1.1 to 2 km; (2) the geothermometer method was used to estimate the region of thermal storage temperature, and its temperature ranged between 64.3 to 162.7 °C, whereas the circulation depth ranged from 1.1 to 7.2 km. Hydrochemical types were mainly controlled by aquifer lithology, in which sodium bicarbonate and sulphuric acid water gathered mainly in the RRF, while calcium bicarbonate water gathered mainly in the XJF. According to the silicon–enthalpy equation method, the temperature range and cold water mixing ratio were 97–268 °C and 61–97%, respectively; (3) the circulation depth of the RRF was deeper than that of the XJF, and it was mainly concentrated in the second segment and the fourth segment on the RRF. Most of the hot spring water was immature with a weak water–rock reaction; (4) the hot water intersections of RRF and XJF were obviously controlled by the fault and the cutting depth of granite; (5) the relationship discussed between geothermal anomaly and earthquake activity had a good correspondence with regional seismicity. The intensity of the reaction between underground hot water and the surrounding rock may lead to the change of pore pressure, and the weakening effect of groundwater on fracture may change accordingly, followed by the change in the adjustment of tectonic stress. Eventually, the difference in seismic activity was shown, implying that deep fluid has an important control action on the regional seismicity. [ABSTRACT FROM AUTHOR]

Continental regions are essential for the outgassing of deeply‐sourced helium in response to volcanic and tectonic processes. However, the helium fluxes remain largely unknown for continental collision settings such as the Tibetan Plateau. Here, we focus on hydrothermal helium degassing from the Simao block, Southeast Tibetan Plateau margin, and report flux estimates of (0.03 – 32) × 105 atoms m−2 s−1 for 3He and (3.2 – 32) × 1010 atoms m−2 s−1 for 4He, with mantle fractions of helium fluxes up to 2−3 orders of magnitude greater than those of stable continents. Geologically recent magma recharge beneath Quaternary volcanoes is proposed to account for the high mantle helium fluxes and 3He/4He up to 7.24 Ra. Active tectonics driven by the India‐Asia continental collision possibly maintained efficient release of crustal helium over geological timescales. These findings present the first flux estimates for hydrothermal helium degassing controlled by volcanic and tectonic processes in continental collision settings. Plain Language Summary: Helium (including 3He and 4He isotopes) is an element mainly outgassing from Earth's interior to the atmosphere. The fluxes and isotopic compositions of helium are critical parameters for monitoring the hazardous volcanic eruptions and earthquakes, estimating the fluxes of other volatile species (e.g., CO2 and N2) from the solid Earth, and exploring the helium resources and geothermal energy. We conducted a quantitative study on the magnitude of helium outgassing from hot springs in the Simao block of the Southeast Tibetan Plateau margin, which is located in a region influenced by the India‐Asia continental collision. The results show that the mantle and crustal helium fluxes of the Simao block are comparable to those estimated for the representative continental regions worldwide. This observation can be explained by the intrusion and degassing of mantle‐derived magmas beneath Quaternary volcanoes and efficient release of crustal helium in response to regional active tectonics. We suggest that India‐Asia continental collision zone is an important natural laboratory for studying helium degassing from solid Earth. Key Points: We report the first estimates of hydrothermal helium fluxes for continental collision settingsGeologically recent magma recharge is proposed to feed the mantle helium degassingActive tectonics driven by India‐Asia continental collision possibly maintained efficient release of crustal helium over geological timescales [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• Hydrothermal helium is mainly from radiogenic production in the crust with 7.52 % is mantle derived. • CO 2 is mainly constrained by both decarbonization of Tethyan marine carbonate rocks and mantle fluid. • The He-CO 2 system is constrained by the regional magmatic intrusions and degassing process. • The contrasting slab subduction style and the south boundary of EHS are constrained by helium isotopes. • Location of mantle suture in EHS is extended and the south boundary is also constrained by helium isotopes. While understanding the origins of volatiles has strong implications on heat and tectonic processes, the relationship has not well understood, especially in the Eastern Himalayan Syntaxis (EHS) in the Tibetan Plateau, where tectonic framework, petrologic properties, stress field, and geomorphic water system changes greatly due to the strongest continental collision. Based on samples from warm/hot springs in the EHS, geothermal fluid is sourced from local precipitation and simultaneously recharged by deeply-sourced mantle volatiles. Volatiles from hydrothermal systems are mixture of crustal and mantle origin, in which helium is mainly from radiogenic production in the crust with only up to 7.52% is mantle derived whereas CO 2 is mainly constrained by both decarbonization of Tethyan marine carbonate rocks and mantle volatiles. The helium fluxes are estimated to be almost 1–2 orders higher than some representative volcanically and tectonically active continental regions. The He-CO 2 system is dominantly constrained by the regional magmatic intrusions and hydrothermal degassing process. The boundary of the Indian plate underplating beneath Tibet is extended eastward to across the EHS by evidences from He isotope and this boundary is probably the south boundary of the EHS. Differences of helium isotopes between the EHS and its southeastern region indicate the contrasting slab subduction style in the deep structure. These results present some quantitative constraints for the interpretations of regional tectonic processes on transportation of deeply-sourced volatiles and its indications on continental collision style and present essential basis for understanding volatiles degassing and cycling in the continental collision settings. [ABSTRACT FROM AUTHOR]

Important fault zone processes can be discerned from the hydrothermal fluid circulation associated with active seismic sources. This study focused on the geochemical and isotopic features of the thermal waters along the Ganzi-Yushu fault (GZYSF) located in the eastern Tibetan Plateau. 22 spring samples were collected to characterize the chemical compositions and calculate the geothermal reservoir temperatures. The hydrothermal waters are mainly recharged by meteoric water based on the δ18O and δ2H values. Immature waters with a Na + -HCO 3 - or Ca2+(Mg2+)-HCO 3 - composition were recognized in the study area, combined with the poor correlation between K+, Na+, Mg2+, HCO 3 − and Cl− suggesting a shallow hydrothermal fluid source. The 87Sr/86Sr ratios and high concentrations of B (from 20.3 to 9445 μg/L) and Li (from 3.04 to 2380 μg/L) show a dissolution of silicate minerals containing calcium and magnesium during fluid circulation. The geothermometry in the silicon-enthalpy system suggested equilibrium temperatures up to 146 °C, whereas most are low temperatures. The shallow geothermal fields along GZYSF show that most large earthquakes (M＞5) were distributed in the transitional zone between high-value area and low-value area of the shallow geothermal field, suggesting a potential genetic link between geothermal field structure and earthquake nucleation along active faults. In conclusion, we proposed a conceptual model to portray the fluid circulation process. Such work can provide insights into the potential geothermal resource in Tibetan Plateau, and also investigate the processes controlling the fluid chemistry and the correlation with occurrence of earthquakes in the region. • Characterize the chemical compositions, analyze the isotopic features and calculate the geothermal reservoir temperatures. • Most earthquakes (M > 5) were distributed in the transitional zone of the shallow geothermal field. • Put forward a genetic link between geothermal field structure and earthquake nucleation along active faults. • Provide insights into the potential geothermal resource in Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Located in the western part of the India-Asia continental collision zone, the Tian Shan-Pamir range is the currently most active and largest intracontinental orogenic belt worldwide. The uplifting process of the Pamir Plateau and deep structures below the Tian Shan orogen has attracted a lot of attention. The behavior of He in the geothermal systems holds great potential for addressing questions about local deep crustal processes and the Plateau uplifting mechanism. However, there is still a lack of study on volatiles in the Tian Shan-Pamir region, especially, helium, which is a very powerful geochemistry tool to reflect deep structures and processes. Through constraining the fluxes of 3He, we recognized different transportation modes for deep volatiles between Tian Shan and Pamir. The mantle-derived He along the Tian Shan is connected to the main faults at the mountain foot of the North Tian Shan and the South Tian Shan, and the rapid ascent of fluid from the mantle is implied. The relatively high levels of 3He/4He on the Pamir Plateau are indicative of crustal magmatism that is younger than ∼11 Ma, indicating a contribution of mantle-derived melts to the crustal thickening and uplifting of the Pamir Plateau since the middle Miocene. • The mantle-derived He in springs along the main faults on both sides of Tian Shan implied a rapid ascent of fluid from the mantle. • Recent magmatism is proposed to feed the helium degassing on the Pamir Plateau. • Mantle-derived melts shall contribute to the crustal thickening of the Pamir Plateau since the middle Miocene. [ABSTRACT FROM AUTHOR]

The North Tianshan orogen consists of a series of thrust faults and is the most seismic region in China. In this paper, the characteristics of hot springs in the North Tianshan orogen are analyzed. Hydrochemical analysis of 13 hot springs and four mud volcanoes reveals that the cations of most hot springs are mainly Na+, while the anions are mainly Cl−.Atmospheric precipitation is the main recharge source for the hot springs in the study area, with recharge heights ranging from −9 to 2344 m. Heat storage analysis and trace element results showed deep fluid characteristics, which is consistent with deep circulation processes and regional extrusion pressure background. In addition, continuous measurements of six hot springs were conducted once every three days. The results showed that Cl− and SO 4 2− concentration anomalies occurred before several earthquakes of M L 4.0 in the North Tianshan orogen. The hot springs SWQ, HTB, and WS, which have deeper circulation depths, show better responsiveness than X10, which has a shallow circulation depth. The deep circulation of hot springs increases fault sliding friction, which contributes to energy accumulation and strong seismogenesis, and participates in the seismogenesis process. Therefore, continuous monitoring of ion concentrations of deep-circulation hot springs can help identify effective precursor anomalies in the North Tianshan orogen. • A conceptual model of cycling processes of the hot springs in the North Tianshan orogen was developed. • The depth of circulation is one of the factors affecting the seismic anomaly sensitivity. • The concentration changes of Na+, Cl−, and SO 4 2− are easier to capture Co-Seismic Anomalies. • Hot springs create both the conditions for and are participants in the occurrence of earthquakes. [ABSTRACT FROM AUTHOR]

• As, Cr, B and F were the trace elements with high concentrations in the Gulu-Yadong rift. • Water quality throughout the whole rift are heterogeneous. • Subduction-related magmatic processes, leaching and water-rock interaction are the controlling factors of trace elements in the thermal water. • Trace elements in geothermal water is a non-negligible environmental and health issue in the Tibetan Plateau. Thermal springs are enriched in many trace elements that are closely related to environmental and human health. The occurrence of trace elements in groundwater systems is one of the most significant environmental and public health concerns. The northward subduction of the Indian continental lithosphere beneath the Asian continent has produced an ongoing continental subduction zone. The Tibetan Plateau serves as the 'Water Tower of Asia', supplying drinking water to more than one billion residents in Asia. In this study, eighteen geothermal springs in the Gulu-Yadong Rrift (GYR) were selected for investigation of the geochemical characteristics of their trace elements. The geological process–related geochemical characteristics of trace elements in geothermal springs in the GYR were quantitatively assessed. The results indicated that there were high concentrations of Cr, As, B, and F in the geothermal waters in the GYR. The concentrations of Cr, As and B greatly exceeded the World Health Organization (WHO) guidelines for drinking water. The water quality throughout the whole rift was spatially heterogeneous. The concentrations of trace elements in only six geothermal springs were within the guidelines of the WHO, while the concentrations of trace elements in eleven geothermal springs considerably exceeded the WHO guidelines. Continental subduction–related magmatic processes, leaching and water-rock interactions may be the main factors controlling trace elements in the geothermal water. The deep circulation of the thermal water facilitated the addition of upwelling hot materials and magmatic fluids into the shallow aquifers, as well as enhancing the water-rock interactions. The results of this study indicate that geological processes have a large influence on the geochemical characteristics of trace elements in geothermal springs in the Tibetan Plateau. It is a non-negligible environmental and health issue in the plateau. [ABSTRACT FROM AUTHOR]

The southeast Tibetan Plateau is a region with high level seismic activity and strong hydrothermal activity. Several large (7.5 > M > 7) historical earthquakes have occurred in the Litang fault zone (LFZ), eastern Tibetan Plateau since 1700. Litang Ms 5.1 earthquake occurred On Sept 23, 2016, indicating the reactivation of the LFZ. This study was undertaken to elucidate spatial-temporal variations of the hot spring gas geochemistry along the LFZ from Jun 2010 to April 2016. The chemical components, He, Ne and C isotropic ratios of bubbling gas samples taken from 18 hot springs along LFZ were investigated. Helium isotope ratios ( 3 He/ 4 He) measured in hot springs varied from 0.06 to 0.93 Ra (Ra = air 3 He/ 4 He = 1.39 × 10 −6 ), with mantle-derivd He up to 11.1% in the LFZ (assuming R/Ra = 8 for mantle) indicated the fault was a crustal-scale feature that acts as a conduit for deep fluid from the mantle. CO 2 concentrations of the majority of hot spring gas samples were ≥80 vol%, CO 2 / 3 He ratios varied from 1.4 to 929.5 × 10 10 , and δ 13 C CO2 values varied from −19.2‰ to −2.3‰ (vs. PDB). The proportions of mantle-derived CO 2 varied from 0 to 1.8%. Crustal marine limestone was the major contributor (>75%) to the carbon inventory of the majority of hot spring gas samples. Before Litang Ms 5.1 earthquake, the 3 He/ 4 He ratios obviously increased in the Heni spring from May 2013 to Apr 2016. The geographical distribution of the mantle-derivd He decreased from east to west along 30°N in the southeast Tibetan Plateau relative to a corresponding increase in the radiogenic component. The gas geochemical data suggested that the upwelling mantle fluids into the crust play an important role in seismic activity in the strike-slip faults along 30°N in the southeast Tibetan Plateau. [ABSTRACT FROM AUTHOR]

• The geothermal waters of in the LXF zone are contaminated by Na-Cl type fluid obviously. • The lower crust flows through the LXF from west to east, causes internal deformation of the LXF zone and occur earthquakes. • Monitoring a proper geothermal waters and gases will help us to study the pre-seismic hydrochemical precursors. Geothermal fluid is considered to be one of the important methods of earthquake forecasting, and some achievements have been made in previous studies. However, there is still a lot of work to be done before geothermal fluid can actually be used for earthquake forecasting. Lijiang-Xiaojinhe fault (LXF) which is a quake-prone region that lies on the border of Tibetan Plateau and Yangtze block is a good natural experimental field for earthquake research. In this contribution, we performed a systematic hydrogeochemistry and isotopic analysis of the geothermal waters and gases in the LXF zone. High Cl- (0.69–529.44 mg/L) concentrations in hot springs indicate that they are contaminated by deep fluid obviously. Further continuous observation (2020–2022) indicates that there are at least four times correspondence between ion concentration in two hot springs (Jinhe (94) and Xiaoshuiping (331)). Two earthquakes (M L 5.2 earthquake on January 02, 2022 and M L 4.6 earthquake on April 16, 2022) in Ninglang are only 50 km away from 331 hot spring, and obvious abnormal concentrations of Cl-, Na+ and SO 4 2- were observed before the earthquakes. Combined with geological background and previous studies, we propose that lower crustal flow could be the reason why earthquake-prone in LXF. The lower crust flow through the LXF from northwest to southeast, causes internal deformation of the LXF zone. Then, a large amount of groundwater infiltrates deep underground along the fault, strengthens the pore fluid pressure, promotes the water–rock reaction, weakens fault zone, and trigger earthquakes. Ion concentrations in hot springs are sensitive to seismic activity in LXF. Unusually high or low ion concentrations in hot springs could represent the process of earthquake development, monitoring proper geothermal waters and gases in LXF will help us to study the pre-seismic hydrochemical precursors, and have a potential to provide important bases for earthquake forecasting. [ABSTRACT FROM AUTHOR]

The outpouring hot spring water is deep circulated groundwater and its chemical constituents act as information carriers of the hydrological setting, including the recharge source, circulation path, and reservoir properties. However, the temporal variation in hydrochemical components and their inducing mechanisms remain to be elucidated. In this study, hot springs along the Anninghe-Zemuhe Fault, a controlling fault on the eastern edge of the Tibetan Plateau, were investigated. According to the spatial chemical variations of the sixteen hot springs and the two-year hydrochemical monitoring results of four hot springs, the aquifers in such fault-controlled regions exhibit heterogeneous anisotropy and elastic properties. Earthquake activities were found to play a significant role in both spatial and temporal changes in chemical compositions. We found that the waters with the highest reservoir temperatures (∼100 °C) and deepest circulation depth (∼3 km) outcropped in the north of the fault, where earthquakes occur more intensively, deeply, and stronger than those in other parts of the fault. During the stress accumulating and releasing processes in the earthquake sequence, the fractured aquifer systems are elastic and subject to impulsive mixing with subordinate fracture waters which induced hydrochemical changes at different levels. The chemical constituents responded to seismic energy densities as low as 6.5 × 10−4 J/m3, showing superiority over the groundwater monitoring wells. However, earthquakes that occurred in a tectonically independent area or with an inappropriate stress loading-unloading direction could fail to trigger a distinguishable mixing process with apparent chemical fluctuations. These findings provide insights into the hydrochemical response to earthquakes in orogenic areas and further draw attention to hydrochemical changes in hot spring waters in seismic studies. • Hot spring waters circulate deeply where earthquakes occurred intensively. • Fractured aquifers in orogenic regions are elastic, heterogeneous, and anisotropic. • 3D analysis of the temporal chemical change uncovers the impulsive mixing process. • Mixing with subordinate fracture waters is triggered during the earthquake cycle. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 ML4.2, Dongchuan ML4.2, and Shuangbai ML5.1 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. [ABSTRACT FROM AUTHOR]

The geochemistry of the fluids is spatially closely related to faults and earthquakes that have been detected in the northern Sichuan-Yunnan (SC-YN) block. However, the processes and factors that control hydrothermal fluid circulation on a regional scale remain challenging. The geochemical circulation of the hot springs integrated with geological and geophysical data revealed the processes and controlling factors of the relationship between fluid circulation and tectonic activity. Our results showed that the hydrochemical type of the hot springs at the intersection of the Yushu-Ganzi fault (YS-GZF), Xianshuihe fault (XSHF) and Yunongxi fault (YNXF), Jinshajiang fault (JSJF) and Batang fault (BTF) as well as Ganzi-Litang fault (GZ-LTF) is composed of Na-HCO 3 and Na-HCO 3 ·Cl. It demonstrated higher reservoir temperature and circulation depth (5 − 7 km). The helium isotope (0.22 − 2.36 Ra) of the gas indicated that the mantle helium composition was up to 5 − 30 %. These hot springs may undergo water–rock-gas exchange reactions with deep fluids. Considering geological and geophysical background as well as previous studies, the geochemical characteristics of the hot spring at the intersections of faults in the north of the SC-YN block are thought to be controlled by the deep-driven upwelling lower crustal flows, mixing with shallow hot spring water and the temporal and spatial heterogeneity of the high strain fields, slip rates, and permeability of the fault intersections. Whereas, the transport of fluids in hot springs of fault may increase pore fluid pressure, and reduce the effective stress at the intersection of the fault zones, which can promote fault sliding and provoke seismic events as well as fault movements. It could result in the enhancement of regional tectonic activity. This study provides an important basis for seismic prediction with hot spring hydrochemistry stations. [ABSTRACT FROM AUTHOR]

The lithospheric structure of the Tibetan Plateau plays a key role in explaining the underlying plateau formation mechanism(s) and growth history. Although many studies have focused on the Moho discontinuity, lithosphere boundary, and lithospheric structure of central and eastern Tibet, there are few corresponding constraints for western Tibet. In this contribution, we systematically investigated the lithosphere in western Tibet by analyzing the He and C isotopes of seven hydrothermal spring systems along the southeastern segment of the Karakorum fault. We found a high Rc/Ra ratio (1.298) for springs along the Karakorum fault, suggesting that the fault plays a key role as a lithospheric-scale conduit for fluid uplift after mantle degassing. In addition, we identified two different origins of the gases at thermal springs, which may reflect two distinct mantle sources. We infer that the locations of these springs reflect the boundary between the Indian and Asian mantles, which is consistent with the geophysics-based interpretations of a "mantle suture" along the Karakorum fault. Our work confirms the boundary between the Indian and Asian mantle, which provides geochemical support to the model of the subduction of Indian lithosphere. [ABSTRACT FROM AUTHOR]

• The time series of stable isotopes were observed simultaneously in three hot springs. • The isotopes of all three hot springs changed before and after a major earthquake. • The amplitudes of isotopic variations are related to the seismic energy density. Earthquake-related hydrogeochemical changes are considered a possible means of identifying earthquake precursors. This work investigated the time series of hydrogen and oxygen stable isotopes at three hot springs in Kangding with different temperatures, namely, the Erdaoqiao, Longtougou, and Guanding hot springs, located in the Xianshuihe fault zone, one of the most active seismic fault zones in China. The hot springs were observed with a sampling frequency of 3 or 4 days for one year. The results showed that the three hot springs were recharged by meteoric water, and there was a slight water–rock interaction at the Longtougou hot spring. The different isotopic compositions of the three hot springs were related to the recharge and groundwater circulation environment. Temporal anomalies in the stable isotope values at the three hot springs occurred synchronously; these anomalies were closely related to the 2019 M w 5.8 Changning earthquake, indicating possible earthquake-related changes. Moreover, the amplitudes of stable isotope variations at the three hot springs were closely related to the seismic energy density, and the background isotopic changes were related to the water temperature and flow. These findings reflect the hydrological cycle processes of geothermal water in the Kangding geothermal area on the eastern margin of the Tibetan Plateau. Simultaneously observed temporal variations in hydrogen and oxygen stable isotopes among multiple hot springs may have relevance for distinguishing earthquake precursors. [ABSTRACT FROM AUTHOR]

• Abrupt high δ13C -CO2-DIC in the hot springs occurred before earthquakes. • The carbon isotopic spikes are caused by fractionation due to tectonic activity. • The δ13C -CO2-DIC of hot springs changed markedly before and after the earthquakes. • The elevated carbon isotopic levels may be a signal for the subsequent earthquakes. Geothermal springs are controlled by deep faults, which are the major causes of earthquakes. A series of geochemical indicators of hot springs, such as carbon dioxide content (CO 2), carbon isotope and noble gas isotope etc., have been used for earthquake monitoring and early warning at home and abroad. Given that previous studies have failed to establish an inevitable link between the carbon isotopic variations of hot springs and seismic activities, in this study, mass spectrometry techniques were used to analyze the carbon isotopes of gaseous CO 2 and noble gas isotope in hot springs, thermal water, and newly formed travertines from western Sichuan before and after an Ms 4.8 earthquake (i.e., 7.14 earthquake). The results showed that the carbon isotope level of the hot spring system increased significantly before the earthquake, i.e., δ13C CO2 = -2‰ to 3.3‰, δ13C DIC = 2.2‰ to 6.6‰ (DIC = dissolved inorganic carbon in geothermal water), R/R A = 0.90–1.83, and δ13C travertine = 1.76‰ to 5.75‰. These indicators decreased markedly 1 week after the earthquake, whereas the helium isotope ratio of R/R A did not exhibit a noticeable change (δ13C CO2 = -3.9‰ to 0.4‰, δ13C DIC = 1.6‰ to 4.1‰, R/R A = 0.68–1.87). Combined with previous thermodynamic calculation and experimental results on carbon isotope fractionation of the CO 2 –calcium carbonate system, we suggest that the high carbon isotope characteristics of hot springs are caused by carbon isotope fractionation following the release of CO 2 from fault carbonate decomposition owing to tectonic activity, not by a mixture of different carbon sources. A high carbon isotopic delivery path in geothermal water, from released CO 2 to DIC and then to travertine, was observed. The new mechanism proposed in this paper reveals the intrinsic relations between the changes of the hot spring geochemical properties and deep tectonic activities, and the variations of these geochemical parameters before and after earthquakes may assist in capturing seismic precursors. A post-peak earthquake warning model has thus been preliminarily proposed, however, long-term monitoring data are required to reduce some uncertainties. [ABSTRACT FROM AUTHOR]

Deep faults could provide fluent channels for geothermal water uplift so that it is one of the targets for high-temperature hydrothermal exploration in orogenic geothermal belt. In the eastern edge of the Qinghai Tibet Plateau, significant geothermal potential reflected by hot springs, fumaroles, and sinters, are indeed exposed along a series of lithospheric-scale faults, including the Jinshajiang, the Ganzi-Litang and the Xianshuihe faults. However, as the controlling fault of the eastern edge of the plateau, the Longmenshan Fault has few geothermal manifestations. In order to uncover the role of deep faults in the genesis of high-temperature geothermal systems, a comprehensive chemical and isotopic comparison of geothermal fluid between the Xianshuihe-Anninghe Fault and the Longmenshan-Minjiang Fault was investigated in this paper. According to FixAl modeling and cation geothermometric calculations, the reservoir temperatures of geothermal systems along the Longmenshan Fault are lower than 150 °C with circulation depth of geothermal water less than 4 km while those in the Xianshuihe Fault reaches up to 260 °C with geothermal water circulating as deep as 8 km. Compared to the Xianshuihe Fault, the low reservoir temperatures along the Longmenshan Fault are accompanied by two characteristics of geothermal fluid: (1) no distinctive oxygen shift occurs in stable isotopes of geothermal waters; and (2) little mantle-derived volatiles found in the gaseous components. We propose that, extensional fracture systems are locally formed in the strike-slip movement of the Xianshuihe and Anninghe faults, which not only act as conduits for deep-derived geothermal volatiles, such as metamorphic carbon dioxide and mantle helium, but also enhance the heat convection processes, resulting in the formation of high-temperature geothermal systems. In contrast, in the Longmenshan Thrust Fault, the shallow circulation of geothermal water in closed fracture systems accounts for its lower reservoir temperatures. Therefore, deep extensional fault is a crucial element in forming a high-temperature geothermal system in the eastern edge of the Qinghai Tibet Plateau. • The reservoir temperature in the Xianshuihe Strike-slip Fault reaches up to 260 °C. • The reservoir temperature in the Longmenshan Thrust Fault is lower than 150 °C. • Deep extensional faults act as convective conduits for both heat and fluid. • Deep extensional fault is critical in forming high-temperature geothermal systems. [ABSTRACT FROM AUTHOR]