Earthquake-related hydrochemical changes in thermal springs have been widely observed, yet the temporal dynamics and controlling mechanisms of pre-and post-seismic hydrochemical changes remains poorly constrained. By employing unsupervised machine learning (SOM-KM), this study investigates earthquake-induced changes of major elements, trace elements, and hydrogen-oxygen isotopes by high-frequency monitoring data in two thermal springs (XXX and LTG) along the Xianshuihe fault zone during the 2021-2022 Ms >= 6.0 earthquake sequence. The result shows that: the XXX spring (Moxi segment) exhibited consistent co-seismic dilution of major ions (Na+, K+, Cl-, HCO3-) and trace elements, attributed to shallow fracture opening under low normal stress (200-400 MPa), thereby facilitating meteoric water recharge (81-89 % mixing ratio). Conversely, the LTG spring (Kangding segment) showed enrichment of deep-sourced constituents of pre-seismic major elements (Na+, K+, Cl-, HCO3-) surges during the 2021 Luxian Ms6.0 earthquake, and post-seismic trace element fluctuation in 2022 Ms >= 6.0 earthquake sequence, driven by high stress (400-600 MPa) enhancing deep fracture dilation, fluid ascent, and water-rock interaction (obvious 518O drift). Cross-analysis of hydrogeochemical signatures with geological and geophysical evidence demonstrates that earthquake-induced hyrochemical anomalies are governed by (1) regional tectonics stress; (2) fault-segment heterogeneity; (3) fluid pathway reorganization and water-rock interaction dynamics. This work advances the understanding of fault-zone fluid response mechanisms to seismicity and offers novel insights into crustal fluid-tectonic interactions during earthquake cycles.

Groundwater discharge to rivers is crucial to river quality and aqueous ecosystem, and has increasingly drawn researchers' attention. However, accurately identifying the locations of groundwater discharge in a curved river is still a challenge in hydrology and hydrogeology because the concave and convex banks cause the river -groundwater interaction complicated. This study applied spatial radon isotope (Rn-222) continuous-monitoring and spatial high-precision measurements of multiple parameters (temperature, pH, and dissolved oxygen) to identify the groundwater discharge location in a similar to 10 km curved reach in the lower Ganjiang River (LGJR). The river water influenced by groundwater discharge in the LGJR contained high Rn-222 activity, low pH and high temperature. Six sections with groundwater discharge then were identified in the LGJR. Groundwater discharge is more significant on the east bank than on the west. On the concave and convex banks, the groundwater discharge generally occurs in the front neck, which is contrary to the results of most previous studies considering the hyporheic exchange. We inferred that the groundwater discharge in the LGJR is dominated by heterogeneity of regional groundwater flow. This work provides a new insight on the groundwater discharge in the curved river (a common natural morphology of rivers).

Although many mechanisms of earthquake-induced hydrological response have been proposed in recent decades, the origins of these responses remain enigmatic, and a quantitative understanding of them is lacking. In this study, we quantitatively analyze the mechanism of coseismic response in water level and flow rate from an artesian well in southwestern China before and after multiple earthquakes and reveal the origin of the earthquake-induced hydrological response based on the monitoring data of water temperature. Water level and temperature always show coseismic step-like increases following earthquakes, which are independent of the earthquakes' epicentral distances and magnitudes. Tidal analysis finds changes in aquifer and aquitard permeability following these earthquakes, which corresponds to the post-seismic total discharge of 85–273 m3 in 20 d after earthquakes. Furthermore, we couple the flow rate and temperature data to model the mixing processes that occurred following each earthquake. The results indicate that coseismic temperature changes are the result of the mixing of different volumes of water from shallow and deep aquifers, with the mixing ratio varying according to each earthquake.

Radon concentration in groundwater is affected by meteorological and seismo-tectonic factors and is sensitive indicators of crustal stress. While previous studies have focused on the mechanisms of periodic and precursory radon anomalies, few have quantitatively assessed their influences, and compared their rates of change under normal and earthquake conditions. This study constructs four models to analyze the mechanisms of radon variation under natural and seismic conditions using the Extreme Gradient Boosting method: 1) the multi-year dynamic variation model, 2) the maximal radon concentration model, 3) the minimum radon concentration model, and 4) the precursory anomaly model. The feature relative importance of external factors influencing radon concentration in groundwater are estimated. The optimized extreme gradient boosting model estimated the feature importance of spring discharge (SD), water temperature (WT), precipitation (P), barometric pressure (BP), and antecedent radon (AR) to be 23.79%, 22.16%, 9.81%, 18.8% and 25.44%, respectively. Thus, SD is the most important influence on radon variation under normal conditions. For radon variation during the earthquake preparation period, the feature relative importance indicates significant changes in WT (increasing from 22.16% to 27.70%) and SD (decreasing from 23.79% to 17.99%). Although WT was the most important predictor in the precursory anomaly model, its effect on radon solubility is insufficient to explain the radon anomalies prior to the Lijiang Mw 7.0 earthquake. Analysis of the precursory mechanisms of these radon anomalies in terms of SD, WT, P, BP, and radon emanation found that radon anomalies are most likely caused by increases in radon emanation due to the earthquake-induced formation of microfractures in rock.

Radon in groundwater has long been recognized as a sensitive indicator of crustal stress. Significant changes in groundwater radon concentration before earthquakes have been documented in many studies. However, the radon concentration in groundwater may be affected by many interference factors. The anomalies before seismic activities may not be large enough to be clearly identified by the conventional statistical method. Therefore, new methods are needed to identify the possible pre-seismic anomalies. In this study, we investigated 38 years' worth of radon time series data (1977–2015) in a hot spring to identify the possible precursor anomalies. We first identify the factors that may affect the radon fluctuations by wavelet coherence analysis, spring discharge, water temperature, rainfall and barometric pressure. All are found to be closely related to the radon fluctuation. The time series (1980–2008) were used for further decision tree analysis as a high correlation in the duration. Following this, we constructed the decision tree models based on these factors to model the "background" radon fluctuation and identify the anomalies by comparing the difference between the observed radon changes and the "background" fluctuations. The modeled "background" fluctuation is closely related to the observed data during the non-seismic activity period, with the correlation coefficient of 0.8. Following this, we compared the modeled "background" fluctuation of the radon time series with the observed one during the seismic activities period. The decision tree could identify 15 possible radon anomalies among the 24 chosen earthquakes. The identified anomalies are also supported by the anomaly changes in water temperature and spring discharge. Therefore, we believe that the decision tree method could be an efficient way to identify the possible precursor anomalies in future studies. Additionally, we explore the mechanism of radon anomalies. The plausible mechanism for the anomalous increase is that radon is continuously supplied from newly formed internal surfaces of the crack to the aquifer system. For the anomalous decrease, it might be related to radon partitioning into the gas phase and the change of mixing ratio of shallow and depth water.

Earthquake-induced groundwater-level changes have been widely studied, though the mechanisms causing coseismic responses are still debated. In this study, we employ several models to fit the coseismic groundwater-level changes caused by the 2008 Wenchuan earthquake in the MP well, located in the Three Gorges Dam. The fits for all models are about the same. By comparing the model results with the results from tidal response and baseflow recession analyses, we conclude that a transient permeability model can best describe the coseismic groundwater-level changes in the MP well. The discharge from the Changmutuo fault zone estimated from the one-dimensional groundwater flow model during the 20 days following the earthquake is about 310 ± 90 m3.

Land subsidence usually bears strong relationship to abstraction of underground fluid. Land subsidence occurs commonly in the North China Plain (NCP) and has become a major environmental factor hindering regional sustainable development. This paper focuses on issues associated with mechanism of land subsidence in the NCP. The analysis shows that multi-layer aquifer systems with deep confined aquifers and the thick normally consolidated or unconsolidated compressible clay layers are the key of geological and hydrogeological conditions favorable for the development of land subsidence in the NCP. Groundwater withdrawal results in an increase in the distribution of effective stress within the strata and the compression of the aquifers and the confining layers, and then triggers land subsidence. In the middle-east plain of the NCP, the land subsidence volume approximately represents the amount of water released from compression of deep aquifers and aquitards in that land subsidence is primarily caused by excessive groundwater withdrawal in deep aquifer system. The percentage of water released from compression of aquifers and aquitards in deep groundwater abstraction is significant but distinct in Cangzhou City and in the whole middle-east plain. This is due to the difference in local lithological structure and recharge and discharge conditions of deep groundwater system. The hysteresis of land subsidence is also discussed in typical areas and the results reveal that the time for completing the primary consolidation ranges from less than one year to tens of years, and that the rate of secondary compression tends to increase with the moisture content.

Ground water can facilitate earthquake development and respond physically and chemically to tectonism. Thus, an understanding of ground water circulation in seismically active regions is important for earthquake prediction. To investigate the roles of ground water in the development and prediction of earthquakes, geological and hydrogeological monitoring was conducted in a seismogenic area in the Yanhuai Basin, China. This study used isotopic and hydrogeochemical methods to characterize ground water samples from six hot springs and two cold springs. The hydrochemical data and associated geological and geophysical data were used to identify possible relations between ground water circulation and seismically active structural features. The data for δ 18 O, δD, tritium, and 14 C indicate ground water from hot springs is of meteoric origin with subsurface residence times of 50 to 30,320 years. The reservoir temperature and circulation depths of the hot ground water are 57°C to 160°C and 1600 to 5000 m, respectively, as estimated by quartz and chalcedony geothermometers and the geothermal gradient. Various possible origins of noble gases dissolved in the ground water also were evaluated, indicating mantle and deep crust sources consistent with tectonically active segments. A hard intercalated stratum, where small to moderate earthquakes frequently originate, is present between a deep (10 to 20 km), high–electrical conductivity layer and the zone of active ground water circulation. The ground water anomalies are closely related to the structural peculiarity of each monitoring point. These results could have implications for ground water and seismic studies in other seismogenic areas.