14 survey lines, with a total of 167 measuring points, were laid out in the northern section of the Red River Fault, the Longpan-Qiaohou Fault, the Heqing-Eryuan Fault, the eastern piedmont fault of Yulong Mountains, and the Lijiang-Jianchuan Fault in the northwest of Yunnan Province, China. Cross-fault soil gas radon, hydrogen, and carbon dioxide have been measured on the above-mentioned faults. The concentration intensity and distribution characteristics of soil gas in the study area were calculated and analyzed. The results show that:
(1)The concentrations and distribution patterns of soil gas radon and hydrogen vary greatly in different faults. The concentrations of radon vary from 6.18Bq/L to 168.32Bq/L, while that of hydrogen are between 7.72ppm to 429ppm, and carbon dioxide are from 0.73% to 4.04%.
(2)The average results of soil gas measurement show that the concentrations of radon are higher than 40kBq/m³ in the sampling sites of Yinjie, Niujie, Gantangzi, while the concentrations of radon in En’nu and Tiger Leaping Gorge measuring lines are smaller; The concentrations of hydrogen are higher than 60ppm in the sampling sites of Yangwang village, Houqing, Dawa, Yangcaoqing and Tiger Leaping Gorge, while the concentrations in Gantangzi and Niujie measuring lines are smaller.
(3)The spatial distribution characteristics of soil gas concentration in faults in northwest Yunnan are obvious, and the intensity of radon and hydrogen concentrations in different active fault zones vary greatly. The intensities of radon and hydrogen concentration are higher and have good consistency in Yinjie and Yangwang village measuring lines located in the northern section of the Red River Fault, the Houqing survey line located in Longpan-Qiaohou Fault, Dawa survey line in the Lijiang-Jianchuan Fault and Yangcaoqing in the south of Chenghai Fault. The soil gas concentration in such sample sites is high and the degassing ability is strong, indicating the different activity characteristics of different segments of the above faults to some extent.
Under the action of tectonic stress, the fault will slip and the rock properties and material structure of the fault will change, thus causing changes of underground material, gas transport channel and transport mode, which is characterized by the change of the concentration and distribution characteristics of escaped soil gas. Combined with the active characteristics of faults, slip rate and geomorphological features, the characteristics of concentration and spatial distribution of two soil gases(radon and hydrogen)are discussed, and the following conclusions are obtained.
(1)There is a large difference in the concentration of escaped soil gas from different faults in the study area, indicating that the content of soil gas is controlled by regional geochemical background values, and there are certain differences in the gas concentration of different sections of the same fault, indicating that the local concentration/flux change is greatly affected by the transport.
(2)The concentration of fault soil gas is related to fault activity, and for different faults, the higher the degree of fault activity is, the higher the concentration of fault gas will be. From the point of fault gas concentration characteristics, the concentrations in the survey lines in the northern section of the Red River Fault and Heqing-Eryuan Fault in the study area vary greatly, suggesting that the fault segmentation is obvious. Compared with other faults, the northern section of the Red River Fault has a higher concentration of soil gas, indicating that the fault is more active. However, there is no simple linear relationship between the soil gas concentration and the fault slip rate, and it may also depend on its material source, transport channel structure, etc.
(3)The concentration of soil hydrogen at the outcrop of faults(especially normal faults and strike-slip faults)is generally higher, which shows that hydrogen has better indicative significance in revealing the location of fault rupture, and the distribution pattern of soil gas radon concentration is a good indicator for analyzing the characteristics of fault movement.

The migrating and enriching of fault gas during dynamic load-unload process are important indexes to evaluate the stress state and tectonic activity of underground medium. The Hutubi underground gas storage provides a natural experiment site for the analysis of the relationship between the gas geochemistry and the stress-strain status. In this paper, the soil gas concentrations of Rn, CO₂, Hg and H₂ during the gas injection in the Hutubi underground gas storage were analyzed. The results show that the soil gas contents and changing trend are close to the background value in the non-reservoir area and fault zone, which may reveal the weak activity of the fault. Significantly higher concentrations of soil gas H₂ and Hg are observed in the gas storage area, where H₂ maximum reaches 5.551×10^-4 and Hg maximum reaches 53ng/m³. Moreover, the abnormal soil gas H₂ and Hg measurement locations are more consistent. The variation trends of soil gas Hg, H₂, Rn, and CO₂may be related to the different gas generation and response mechanisms. The concentrations of soil gas H₂ and Hg are sensitive to the variation of pressure and the development of cracks in the underground gas storage, and they can reveal gas injection's effect on fault activity. This study provides a new basis for analyzing the influence of gas injection and withdrawal in Hutubi underground gas storage on fault activity.