Abstract: The Huai’an Basin is located at the intersection of the Zhangjiakou-Bohai seismic belt and the Shanxi seismic belt. In this study, two survey lines were set up in the southern margin fault zone of the Huai’an Basin in Hebei Province, specifically in Sigou Village and Guanzhuang Village. Three periods of Rn, CO₂ and Hg concentration measurements were carried out to obtain regional background and analyze the spatial variations in the geochemical characteristics, as well as their relationship with fault activity. The results show that the average concentrations of Rn, CO₂ and Hg at the Sigou Village survey line were 2623.82 Bq/m³, 0.27%, and 10.88 ng/m³, respectively, while those at the Guanzhuang Village survey line were 4195.33 Bq/m³, 0.49%, and 13.43 ng/m³, respectively. The release intensities of Rn, CO₂ and Hg were 1.42, 1.04, and 1.41, respectively, at the Sigou Village survey line, and 2.16, 2.92, and 1.34, respectively, at the Guanzhuang Village survey line. The average concentrations of Rn, CO₂, and Hg in the Guanzhuang area were all higher than those in the Sigou area, possibly related to differences in fault activities and the degrees of fault fragmentation. The correlation coefficient between Rn and CO₂ was the highest, followed by that between Rn and Hg, with the lowest correlation coefficient between CO₂ and Hg, which may be related to CO₂ acting as a carrier of Rn. Based on the continuous CO₂ monitoring data from fixed observation points around the area, and the M-t and frequency analyses of earthquakes, it is inferred that the southern margin fault of the Huai’an Basin has experienced strong activities in the past but is currently in a quiet period with relatively weak tectonic activity.

Key words: Southern margin fault zone of the Huai’an Basin, Soil gas, Gas concentration, Fault activity