Hydrological changes in groundwater coupled with earthquakes had been documented in previous studies by global researchers. Although few reports investigate multiple geochemical parameters that respond to earthquakes, trace elements received less attention, whereas they were suggested to be more sensitive to small earthquakes than the commonly used geochemical parameters. Beijing is located in the Zhangjiakou-Bohai (Zhang-Bo) seismic belt of North China, and although the occurrence of small earthquakes is frequent, the great historic earthquake in the Sanhe-Pinggu area M8 in 1679 in the adjoining southeast of Beijing gained widespread public attention. To find effective precursors that are significant for operational earthquake forecasting of the Beijing area, we carried out a one year test research project through weekly collection of groundwater samples during June 2021 to June 2022 from the seismic monitoring well of Wuliying in northwest Beijing. The 41 trace elements chemical compositions were analyzed for each sample. During the project ongoing period, the biggest earthquake with a magnitude of ML3.3 occurred in the Chaoyang District of Beijing on 3 February 2022. The content changes in these trace elements were systematically monitored before and after the earthquake. Through retrospective research, it was found that a few sensitive trace elements were anomalous to be coupled to the earthquake, including Li, Sc, Rb, Mo, Cs, Ba, W, U, Sr, Mn, Ni, and Zn. In addition to trace elements, we examined stable isotopes of hydrogen and oxygen and the existing hydrological data on groundwater level, temperature, major ions, and gases to assess the validity of geochemistry as a monitoring and predictive tool. We only found that F- (fluorine) ions and He (helium) gas had apparent shifts related to the earthquakes, while no shifts in the groundwater level were observed. Such characteristics of multiple geochemical parameters indicate that trace elements are likely to be more sensitive to crustal strain than the groundwater level and major ions. We assumed a most likely mechanism of the combination of mixing and water–rock interactions to explain the phenomenon. The probable scenario was that minor stresses caused by the earthquakes might create micro-cracks in bedrocks, thereby leading to a small volume of chemically distinct water mixing with the original water of the aquifer, and finally, the earthquake-induced rock fractures enhance the water–rock interactions, resulting in the post-seismic recovery of trace elements and δ¹⁸O value migration to the GWML. More testing works to find other sensitive sites to investigate multiple geochemical characteristics aiming at long-term to short-term earthquake prediction in the Beijing area and Zhang-Bo seismic belt are in progress.

In comparison with conventional hydrological parameters such as water levels and temperatures, geochemical changes induced by earthquakes have become increasingly important. It should be noted that hydrogen (δ²H) and oxygen isotopes (δ¹⁸O) offer the greatest potential as precursor proxies of earthquakes. Here, we conducted high-resolution sampling (weekly, 59 samples), measuring consecutive δ²H and δ¹⁸O levels at the two sites of the WLY well and SS spring in the Yan-Huai Basin of Beijing from June 2021 to June 2022. During the period of this sampling, several small earthquakes of ML > 1.6 occurred in Beijing. We used statistical methods (analysis of variance) to test the significant differences, used Self-Organizing Maps (SOMs) for data clustering, and then used Bayesian Mixing Models (MixSIAR) to calculate the proportions of the source contributions. We found significant four-stage patterns of change processes in δ²H and δ¹⁸O at both sites. The WLY well exhibited a distinct four-stage variation pattern: initial stable development (WT1) followed by a rapid rise (WT2) and sudden fall (WT3) before the small earthquakes, and finally gradual stabilization after earthquakes (WT4). In contrast, the SS spring displayed an inverse pattern, beginning with stable development (ST1), then undergoing a rapid falling (ST2) and sudden rising (ST3) before the small earthquakes, and finally stabilizing through stepwise reduction after the earthquakes (ST4). The most likely mechanisms were differences in the time of rupture between the carbonate in WLY and granite in SS under sustained stress. The stress induced source mixing of fluid from the surface or deeper groundwater-source reservoirs. The hypothesis was supported by the MixSIAR model, calculating the variational proportion of source contributions in the four stages. This work permitted the use of high-resolution isotopic data for statistical confirmation of concomitant shifts during the earthquakes, provided the mechanisms behind them, and highlighted the potential for the consecutive monitoring of hydrogen and oxygen isotopes indicators in earthquake-prediction studies.

Groundwater level responses to solid tides in confined wells provide a means for investigating aquifer parameter variations and earthquake-related effects. Using hourly water-level observations from eight confined wells of the Beijing seismic monitoring well network (January 2017–December 2025), this study applied the Baytap-G program to extract the M2 tidal constituent contribution, amplitude, tidal factor, and phase lag, and analyzed their temporal variations and responses to earthquakes. The eight wells are classified into five groups: Group 1 (BQ, CP), stable during non-interference intervals but with permanent instrument-induced jumps; Group 2 (MF, XJ), stable background references; Group 3 (LX), reflecting genuine hydrological processes; Group 4 (DHC, XXZ), signals too weak for reliable analysis; and Group 5 (WLY), dominated by persistent instrumental disturbances. Using mean ±3σ thresholds derived from seismically quiet periods, nine persistent ≥3σ anomaly segments were identified; none coincides in time with the 14 Ms ≥ 4.0 earthquakes or the 41 coseismic water-level response events, and all can be attributed to documented instrument events. The hypothesis that earthquakes alter M2 parameters before or after their occurrence is not supported, nor is the hypothesis that coseismic water-level responses produce short-term M2 parameter changes; the hypothesis that instrument operational conditions and long-term hydrological changes dominate the observed variations is supported. The absence of earthquake-triggered M2 parameter responses is attributed to basin attenuation, limited strain conversion efficiency, response mechanism decoupling, and observational resolution. These results provide a background reference for applying well water-level tidal parameters to earthquake precursor monitoring in the Beijing area, showing that independent baselines must be established for each well and that instrument operational records are necessary for anomaly identification.

Establishing the groundwater geochemical background (GGBC) is fundamental for distinguishing seismic precursors from non-tectonic environmental influences. This study characterizes the spatiotemporal evolution of the GGBC in Beijing using 60 groundwater samples collected from 17 seismic monitoring wells between 2018 and 2025. Major ion concentrations, stable water isotopes (δ²H and δ¹⁸O), Self-Organizing Map (SOM) clustering, Principal Component Analysis (PCA), ionic ratios, and mineral saturation indices were integrated to analyze the hydrochemical framework for establishing the GGBC baseline. Piper diagram results reveal three main hydrochemical types: Na·HCO₃ type, Ca·Mg·HCO₃ type, and sulfate type (subdivided into Na·SO₄ and Ca·Mg·SO₄/HCO₃ subtypes). Hydrogen and oxygen isotope data from 2024 indicate that recharge sources range from deep bedrock fissure water to shallow mixtures of meteoric and surface water. Gibbs diagrams, SOM, and PCA consistently demonstrate that water–rock interaction (including carbonate, silicate, and evaporite dissolution, as well as cation exchange) dominates the GGBC. Ionic ratios and saturation indices further confirm that silicate weathering and forward cation exchange are persistently widespread, while carbonate and evaporite dissolution are locally important. Notably, 12 wells of the 17 monitoring wells (71%) retained the same hydrochemical type across all campaigns in which they were sampled. This stability shows no detectable temporal correspondence with either the V-shaped pattern of seismic activity (strong–weak–strong, 2018–2025) or the regional groundwater level rise caused by water diversion projects at the available annual-to-multi-year sampling resolution. Instead, the high stability of the GGBC reflects long-term water–rock equilibrium that is resilient to minor, transient perturbations. This study demonstrates that major ions are reliable indicators of the GGBC for assessing the sensitivity of the Beijing Plain to short-term seismic or hydrological disturbances. However, the limited temporal resolution of the sampling and incomplete datasets preclude definitive conclusions regarding earthquake-related geochemical signals. To strengthen regional earthquake monitoring and forecasting capabilities, we recommend establishing a continuous, long-term, high-frequency, multi-parameter monitoring network—including stable isotopes, trace elements, and dissolved and exsolved/free gases—with high data completeness in the Beijing area.