Abstract: In order to study the geochemical characteristics of soil gas and the tectono-geochemical background reflected by fault gas in the Tong-Dong fault zone, the field monitoring method was used to analyze the concentration distribution of soil H₂ and Rn in the two observation lines of Zhanghe village and Xinglizhuang village. The results show that the distributions of H₂ and Rn concentration in Zhanghe village ranged from 0.24 to 174.7×10^-6 and 13.3 to 69.8 kBq·m^-3, respectively. The background values of H₂ and Rn concentration are 14.26×10^-6 and 24.8 kBq·m^-3, respectively. The range of concentration distribution of H₂ and Rn in Xinglizhuang observation line ranged from 11.8 to 67.06×10^-6 and 43.6 to 72.6 kBq·m^-3 respectively, with background values of 37.13×10^-6 and 72.6 kBq·m^-3, respectively. In the measure line of Zhanghe village, strong anomaly of H₂ and Rn appeared at 90～105 m, but a higher anomaly occurred at 120～150 m, with anomalous values near the fault zone, which indicates the location of the fracture effectively. Integrated geological structure and physical geography, it is concluded that the high value anomaly of the gas composition in the survey line are mainly controlled by the tectonic conditions, and it is also affected by sediment properties and meteorological factors. Therefore, we should further track and monitor the gas composition in the soil layer of the target fault zone, and analyze the temporal and spatial variation of fault activity, and take gas isotope techniques to explore the source of the components, so as to enhance the important role of gas geochemical indicators for fault tectonics activity and to strengthen the efficiency of earthquake prevention.

Key words: Active fault zone, Soil gas, Construction conditions, Tangdong Fault