Geochemical characteristics of soil gases on the Huangzhuang-Gaoliying fault in Beijing and their indications for seismic activity

Based on the field flow observation of soil gas （CO₂, Rn and Hg） in May of 2017 and continuous monitoring of soil gas Rn from September 1, 2020 to June 30, 2021, the residual signal and seismic index K_S are used to study the activity difference between the south and north segments of Huangzhuang-Gaoliying fault and their regional seismicity. The results indicate that this fault is a relatively active fault compared with other faults in the capital circle region of China. The concentrations and fluxes of the soil gases in the north segment of the fault are higher than those in the south segment. And the CO₂ and Rn concentrations in the hanging wall of the northern segment are significantly higher than those in the footwall. These indicate that the northern segment of the fault has stronger activity. Analyses on the continuous monitoring station data of soil gas Rn show there is no significant correlation between Rn concentration and air temperature, soil temperature, and air pressure. The Rn concentration at the monitoring station, six days prior to the Shunyi M_L3.0 earthquake on March 25, 2021, was recorded to significantly increase, which doubled and persisted for a month after the earthquake. The anomalous variations of Rn concentration before and after the Shunyi M_L3.0 earthquake indicate that the soil gas Rn concentration at the monitoring station exhibits a strong response to earthquakes with K_S＞0.1.