Geochemical characteristics of soil gas in the north margin fault of Huaizhuo basin，Hebei Province

Two surveying lines were carried out in Haojiapo and Xiaoshuiyu of Huaizhuo basin, Hebei Province, and four periods of Rn, CO₂ and Hg concentrations were measured. This paper studies the spatial differences of soil gas geochemical characteristics in the southwest and northeast segments of the northern margin fault of the basin and its possible relationship with fault activity. The results show that the average concentrations of soil gas Rn, CO₂ and soil Hg in Haojiapo profile are 8 371.16 Bq/m³, 0.85% and 14.82 ng/m³, respectively, and those in Xiaoshuiyu profile are 2 813.18 Bq/m³, 0.42% and 13.08 ng/m³ respectively, suggesting that the former are higher than the latter. The spatial difference of concentration distribution characteristics may be caused by different fault activity and fault fragmentation degree. The comparative analyses of regional soil Hg （total mercury, Hereinafter THg） concentration and soil gas measurement concentratio show that the two contents are consistent in spatial distribution, and the high-value abnormal points are distributed near the fault zone. Combined with the regional fixed-point precursory observation data and seismicity analyses, it is considered that the current activity of the fault is relatively weak. Therefore it is feasible to detect the shallow position of concealed faults in this area by using the distribution of soil gas concentration. At the same time, continuous observation of soil gas concentration has certain indicating significance for judging the fault activity in this area.