Abstract:

The geochemical characteristics of soil gas in fault zone are closely related to regional tectonic evolution and seismic activity. He is a noble gas element with high chemical inertness, diffusivity and mobility, which can migrate from the deep underground to the surface along fractures. Its escape level in the crust is generally controlled by crustal stress. Therefore, soil gas He is an effective tracer, which can be used to study the process before the occurrence of dangerous earthquakes. Monitoring the He in a seismically active area may provide evidence of stress field changes caused by tectonic activity, and better understand the seismic genesis process within the region. The eastern Liupanshan fault zone is located between the Ordos block and the northeastern margin of the Qinghai-Tibet Plateau and is an important part of the northern section of the North-South Seismic Belt. The medium-strong earthquakes in this zone are frequent, and there have been many strong earthquakes above 7 in history.
In order to explore the spatial distribution characteristics of soil gas He concentrations in the eastern Liupanshan fault zone and its relationship with tectonic activities, eight cross-fault soil gas measurement profiles were deployed in the eastern Liupanshan Fault, along which soil gas He concentrations were measured. In the meantime, in order to carry out comparative studies, the Xiaoguanshan Fault located 14km away from the eastern Liupanshan Fault was selected, and three cross-fault soil gas measurement profiles were also set to measure the soil gas He concentration along the survey line. The measurement results showed that the concentration of soil gas He in the eastern Liupanshan Fault is 3.786~7.472ppm, and the average He concentration of eight measurement profiles is between 4.983~6.335ppm. The He concentration range in Xiaoguanshan Fault is 2.168~7.043ppm, and the average He concentration of the three measurement profiles is between 4.784~5.235ppm. The soil gas He concentration in the eastern Liupanshan Fault has a decreasing trend from north to south. The soil gas He concentration in the Xiaoguanshan Fault is the highest in the north of the fault and smaller in the south-central part of the fault. The spatial distribution of He concentration in the north parts of the two faults is higher than that in the middle and south parts of the fault, which is closely related to the difference of regional activity of the faults. In combination with previous research results on the age of fault activity, locking degree, slip rate and tectonic stress in the eastern Liupanshan Fault, it is considered that the activity of the northern segment of the fault is more intense than that of the middle and southern segments of the fault. The comparative analysis shows that the tectonic evolution process of the eastern Liupanshan Fault is similar to that of the Xiaoguanshan Fault, and both faults are thrust type. Therefore, the spatial distribution characteristics of soil gas He concentration in the two faults are consistent. Combined with previous studies, it was found that the spatial variation of soil gas He concentration measured in the eastern Liupanshan Fault was consistent with the spatial variation of soil gas Rn and CO₂ flux. The He concentration of soil gas in the eastern Liupanshan fault zone is close to that in Tangshan area, lower than that in Yanhuai Basin, the middle and southern section of Xiadian Fault and the rupture zone of Wenchuan earthquake, and higher than that in Xinbaoan-Shacheng Fault.

Key words: soil gas, He, geochemistry, tectonic activity, the eastern Liupanshan fault zone

摘要：

断裂带土壤气体地球化学特征与区域构造演化密切相关。文中为探讨六盘山东麓断裂带土壤气He浓度的空间分布特征及其与构造活动之间的关系, 在六盘山东麓断裂跨断层布设了8条土壤气测量剖面, 沿测线开展土壤气He浓度的测量; 同时, 为进行对比分析, 选择位于六盘山东麓断裂以东14km的小关山断裂布设3条跨断层土壤气测量剖面, 沿测线开展土壤气He浓度的测量。测量结果显示, 六盘山东麓断裂各测量剖面的He浓度平均值为4.983~6.335ppm; 小关山断裂各测量剖面的He浓度平均值为4.784~5.235ppm。2条断裂的He浓度在空间分布上均呈现出断裂北部高于断裂中南段的特征, 这与断裂的区域活动性差异密切相关。结合前人对六盘山东麓断裂活动时代、 闭锁程度、 滑移速率、 构造应力的研究成果, 分析表明六盘山东麓断裂北段的活动性强于断裂中南段。经对比分析认为, 六盘山东麓断裂和小关山断裂的构造演化过程相似, 且断裂性质均为逆冲型, 因此2条断裂土壤气He浓度的空间分布特征一致。

关键词: 土壤气, 氦, 地球化学, 构造活动, 六盘山东麓断裂带