# Hydrogeochemistry of Fault-Related Hot Springs in the Qaidam Basin, China
## 4 Discussion
### Water Chemistry Anomalies and Regional Earthquake Response  
Fluids are an important medium connecting the lithosphere, atmosphere, and biosphere. Fluids also play important roles in information transfer and material transfer in the geological evolution of Earth's interior [93; 94]. Groundwater acts in the Earth's crust with a wide range and intensity, and its geochemical characteristics and genesis are closely associated with the geological environment and hydrogeological conditions of the region [95; 96]. The chemical fraction of groundwater is governed mainly by the nature of the rocks it flows through, fracture tectonic activity, and deep-fluid supply, and factors such as seismic events might alter the chemical fraction of groundwater in a region [97; 98; 99]. According to studies, there is a close relationship between subsurface fluids and seismic activity. A series of fluid anomalies, such as water temperature, water level, and ion concentration, occur before and during moderate-to-strong earthquakes. During the earthquake precursor phase, hydrogeological and geochemical changes in wells or springs are observed, and subsurface fluid anomalies are clearly visible [100; 101]. For example, K\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\), and Cl\({}^{-}\) concentrations increased significantly before the \(M_{\rm S}\)7.2 earthquake that struck Van, Turkey, on 23 October 2011. The maximum values were reached 12 d before the earthquake, and the increase was nearly twofold [102]. Significant water chemistry anomalies related to Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Cl\({}^{-}\), and SO\({}_{4}\)\({}^{2-}\) were observed in the Beitianshan mud volcano and Shawan spring before and after the Xinyuan-Hejing \(M_{\rm S}\)6.6 earthquake that occurred on 30 June 2012 [103]. Among the hot springs in the present study region, the hot spring that is most active in terms of external appearance is the AK spring, also referred to as the \"Eye of the Earth\" and the \"Devil's Eye\". The AK spring stretches to a diameter of over 10 m, and the  
Figure 9: Conceptual model of the hot spring circulation in the study area. spring water is similar to boiling water, constantly tumbling upward. The sulfur minerals around the spring precipitated long ago (and have remained so to this day) and are colorful (Figure 10). The water chemical ion analysis results for the four hot springs in the study region revealed that the AK spring has a higher degree of trace element enrichment and a thermal storage temperature of 164.9 \({}^{\circ}\)C, which is typical of a marl thermal reservoir. The circulation depth reaches 2679 m, which is unusually high in comparison to other hot springs. Meanwhile, mantle-source material intrusion was discovered using gas isotope analysis. Therefore, when regional tectonic activity occurs, the AK spring generates more pore-fluid pressure during water circulation. The water-rock response is more intense, i.e., it responds better to regional tectonic activity and stress changes. An \(M_{5}5.5\) earthquake occurred in the study region on 19 October 2022, close to the East Kunlun fault belt and spatially positioned 150 km southeast of the AK spring. Since May 2022, the project team has been conducting continuous measurements of the water chemistry ions at AK springs to continuously observe and collect data for the analysis of seismic efficiency. Water testing samples are collected at sampling points every 3 days and subjected to water chemistry measurements and mean-variance calculations. Any information pertaining to observed anomalies is documented. An anomaly is defined as a change in amplitude greater than \(\pm 2\) times the mean variance. A positive high anomaly is defined as an increase in ion concentration that exceeds +2 times the mean-variance value. In contrast, a negative high anomaly is defined as a decrease that exceeds \(-2\) times the mean-variance value.