Soil Gas Geochemistry Characteristics In The Capital Area Of China

The Capital area of China includes Beijing,Tianjin and part of Shanxi and Hebei,which has significant status in the fields of politics,economic and cultures.Currently,the active fault tectonic activity in the Capital area is quite severe and the seismic risk is relatively high.So,it is particularly important to monitor earthquake in this area.The geochemical characteristics of soil gases have been proved to be an effective way to assess fault activity and the relative risk of seismic.Hence,in this study,soil gas concentrations and flux above 19 active faults have been measured in the north segment of Shanxi fault zone,Yanhuai basin,Huai’an basin,Sanhe area,Tangshan area and Tianjin area.The results will be analyzed to investigate the geochemical characteristics of soil gas concentrations and flux,the spatial distribution of soil gas and the relative influence factors.Furthermore,the relationship between geochemical characteristics of soil gas and fault structure and seismic activity will be discussed.These new data and results could be used as a significant reference for the earthquake monitoring in the Capital area of China.In total,soil gas Rn,CO₂ and Hg were measured at 1088 measuring points of 36 soil gas profiles above 19 active faults from April to June,2016.The concentration of soil gas Rn was measured by using the RTM2200 radon meter,and its flux was measured by RAD7 radon measuring instrument.GXH-3010 E portable infrared CO₂ analyzer was used for the measurement of soil gas CO₂ concentration and flux.The measurement of soil gas Hg concentration was carried out by using the RA-915+ Zeeman Effect instrument.According to the primary results,the average concentration variation range of soil gas Rn,CO₂ and Hg in the Capital area of China is 0.706²8.992 k Bq/m3、0.13¹.29%、5.8²0.2 ng/m3,respectively.The highest average concentration of soil gas Rn,CO₂ and Hg was respectively found in Dadongguan profile,Canfangying profile and Wuzhuangcun profile.The average flux variation range of soil gas Rn,CO₂ in the Capital area of China is respectively 12.95¹28.3 Bqm-2h-1、9.15⁹1.3 gm-2d-1 and the highest average flux is in Fennan profile and Hejiawa profile respectively.The spatial distribution characteristics of soil gas concentration and flux in the capital area were analyzed,and the factors affecting that were discussed.Current data reveal that both the concentration of soil gas Rn,CO₂ and Hg and the flux of soil gas Rn and CO₂ are high in the east part of this region and relatively low in the west part.A possible reason for this phenomenon is that owing to the nearly EW tectonic stress in the capital area,earthquake intensity of Zhangbo seismic zone is higher than that of the northern section of Shanxi seismic belt,and the overall stress level is higher than that of the northern section of Shanxi seismic belt.The influence of fault slip rate on the geochemical characteristics of the soil gases is mainly concentrated in the profiles of the same active fault.Another point needed to be noted here is that the soil composition and particle size nearly contribute nothing to the variation of the geochemical characteristics of soil gas in the capital region.The analysis of He isotope of spring gas suggests that the spatial distribution of 3He/4He value is characterized by higher in the west and lower in the east,which is opposite of the distribution of soil gas Rn、CO₂ and Hg.The main reason for the distinction is that the deep fracture connectivity may be better and the permeability is higher in the western region,therefore the mantle contribution rate of He is high in this region.By contrast,the source of soil gas Rn is probably not deep into mantle,but in some areas of crust,especially when considering the distribution of the element U in the interior of earth together.Similarly,the depth of the source of soil gas CO₂ and Hg may be similar to Rn,as their geochemical characteristics are similar to that of Rn.