海原断裂带土壤气与地下水地球化学特征研究

Geochemical characteristics of emergent gas and groundwater in Haiyuan fault zone

选取海原断裂带曹洼(CW)、菜园(CY)、万家水(WJ)、水泉(SQ)、三塘(ST)、松山(SS)和安远(AY)7个测区进行土壤气测量与地下水地球化学检测,依据海原断裂带不同构造部位的土壤气浓度、水化学组分和同位素组成等,分析了海原断裂带的构造活动性分段特征。结果显示:研究区内土壤气Rn释放量较高的区域主要集中于ST、SQ测区,且断裂带中部区域气氡浓度相对较低;氦浓度及其同位素检测结果表明ST、SQ和CY测区土壤逸出气中含有幔源气体;泉水化学组分及氢氧同位素分析结果显示SQ和ST测区地下水与围岩介质之间的水-岩相互作用程度较高、水-岩反应平衡状态较高、地下水循环深度较大;泉水氚活度分析结果显示水泉(SQ)测区地下水补给区较远、水循环周期较长、新老水体的交换作用缓慢。综合分析认为,相比其他区段,海原断裂中西段(SQ-ST)接近1920年8.5级地震破裂段,其断裂带闭锁程度较弱、与地壳深部的连通性较好,更有利于地下流体的扩散与运移。

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.