Yuxuan Chen ^1* Guiping Liu ^1* Fuqiong HUANG ^2* Zhiguo Wang ^1* Leying Hu ^1* Mingbo Yang ^1* Xiaoru Sun ^1* Peixue Hua ^1* Shijun Zhu ^1* Yanan Zhang ^1* Xiaodong Wu ^1* Zhihui Wang ^1* Lvqing Xu ^1* Kongyan Han ^1* Bowen Cui ^1* Hongyan Dong ^1* Yonggang Zhou ^3*

• ¹ Beijing earthquake agency, Beijing, China
• ² China Earthquake Networks Center, Beijing, China
• ³ The Bureau of seismology of Yanqing District of Beijing, Beijing, China

Hydrological changes in groundwater coupled to earthquakes had been documented in the previous studies, while few reports investigate multiple geochemical parameters respond to earthquakes, particularly in trace elements received less attention, whereas they were suggested to be more sensitive to small earthquakes. Beijing is located in the Zhangjiakou-Bohai (Zhang-Bo) Seismic Belt of North China, to find effective precursors is the significant for Operational Earthquake Forcasting of Beijing area, we carried out one-year test research project through weekly collecting groundwater samples during June 2021 to June 2022 from seismic monitoring well of Wuliying in northwest of Beijing. The 41 trace chemical composition were analyzed for each sample. During the project ongoing period, the biggest earthquake with a magnitude of ML3.3 occurred in Chaoyang district of Beijing on February 3th, 2022. The content changes of these trace elements were systematically monitored before and after the earthquake. Through retrospective research, it was found that a few sensitive trace elements are were anomalous to be coupled to the earthquake, including Li, Sc, Rb, Mo, Cs, Ba, W, U, Sr, Mn, Ni, Zn. Besides trace elements, we examined stable isotopes of hydrogen and oxygen and existing hydrological data of 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 to the earthquakes, while no shifts of groundwater level. Such characteristics of multiple geochemical parameters indicate that trace elements are likely to be more sensitive to crustal strain than groundwater level and major ions. We supposed a most likely mechanism of combination result of mixing and water-rock interactions to explain the phenomenon. The probably scenario was that minor stresses caused by the earthquakes might create micro-cracks in bedrocks, thereby leading to small volume of chemically distinct water mixing with the original water of aquifer, finally, the earthquake-induced rock fractures enhanced the water-rock interactions, resulting in post-seismic recovery of trace elements and δ 18 O value migration to the GWML. More testing works to find other sensitive sites to investigate multiple geochemical characteristics is continuing.