Abstract:

Seven measuring areas were selected and named Caowa(CW), Caiyuan(CY), Wanjia(WJ), Shuiquan(SQ), Santang(ST), Songshan(SS) and Anyuan(AY) respectively, in which soil gas was measured and geochemical composition in groundwater was detected. Characteristics of tectonic activity in Haiyuan fault zone was analyzed according to the soil gas concentration, chemical composition and isotopic composition in different area. Analysis results show that (1) those areas with higher soil gas concentration were ST and SQ along the fault strike；along the vertical direction of the fault, the concentration of Rn in soil gas was lower in the central position of fault than both sides; helium concentration and isotopic testing results show that mantlederived gas was mixed in the soil gas in ST, SQ and CY areas; that (2) according to results of the chemical composition and hydrogen isotope in spring water, we thought that there were more interaction and higher equilibrium state between groundwater and rock, and the depth of groundwater circulation were deeper in the areas of SQ and ST, compared to other areas in Haiyuan fault zone; that (3) the result of spring tritium activity showed that the groundwater recharge area in SQ area was far away, the water cycle period is longer, and the exchange action between old and new water is slow. Above all, the central and western areas of Haiyuan fault (SQST), which is close to the rupture zone of the 8.5 earthquake in 1920, has lower fault blocking degree and deeper connectivity with deep crust. So, different from other areas, those areas in central and western of Haiyuan fault has more conducive to subsurface fluid to diffuse towards the surface.

Key words: concentration of soil gas, water chemical composition, isotopic composition, Haiyuan fault zone