阳原盆地北缘断裂带中段土壤气体地球化学特征与断裂活动性

Geochemical Characteristics of Soil Gas and Fracture Activity in the Middle Segment of the North Margin Fault of the Yangyuan Basin,Hebei Province

摘要:

有研究认为1989年和1991年阳原盆地西侧冀晋交界附近发生的2次中强地震震群导致盆地东侧应力随之调整,意味着阳原盆地开启了新的地震活跃周期。盆地东侧地震活动主要集中在盆地北缘断裂附近,其中段大蟒沟至台家庄为该北缘断裂最活动段,该断裂带的最新活动情况对于震情形势分析具有重要意义。本研究在河北阳原盆地北缘断裂带中段的大蟒沟、姚家庄、南口村、台家庄布设了4条测线开展2期Rn、CO₂及Hg浓度测量,在南口村、台家庄剖面开展一期土壤化学成分分析,以研究该盆地北缘断裂带中段土壤气体地球化学特征空间差异及其与断裂活动的关系。结果表明:气体浓度高值点分布在断层陡坎、破碎带上,利用断层气在该区域探测隐伏断裂的浅层位置具有可行性;南口村剖面土壤气体释放强度相对高于其他剖面,这种浓度空间分布差异性可能是由于地质构造结构与断裂活动性引起的断裂破碎程度不同所致;土壤中总汞(THg)含量与气体浓度空间分布上具有一定吻合性,基于土壤中总有机碳(TOC)能够吸附Hg元素的特征,THg浓度分布特征能反映一段时间内断层活动状态;结合断裂带周边定点前兆连续观测数据及地震活动性分析认为,该断裂目前活动水平相对较弱。

Abstract:

The previous study suggested that the two moderately strong earthquakes that occurred in 1989 and 1991 in the western part of the Yangyuan Basin near the junction of Hebei and Shanxi caused a consequent stress adjustment on the eastern side of the basin implying a new seismic activity cycle in the Yangyuan Basin. The seismic activity on the east side of the basin is mainly concentrated near the northern margin of the basin rupture,where the most active section is from Damanggou to Taijiazhuang,and the latest activity of this rupture zone is of great significance for the analysis of the seismic situation. In this paper,four survey lines were deployed in the middle section of the northern rim of the Yangyuan Basin in Hebei,China,to measure Rn,CO₂ and Hg concentrations in phase 2,and to analyze soil chemical composition in phase 1 in the profiles of Nankou and Taijiazhuang to study the spatial differences of soil gas geochemical characteristics in the middle section of the northern rim of the basin and their relationship with the rupture activity. The results show that the high value of gas concentration points are distributed in the faulted steep canyons and fracture zones,and it is feasible to use fault gas to detect the shallow location of hidden fractures in this area. The intensity of soil gas release in the Nankou section is relatively higher than that in other sections,and this difference in concentration spatial distribution may be due to the different degrees of fracture fragmentation caused by geological structure and fracture activity. The total mercury(THg)content in soil and the gas based on the feature that total organic carbon(TOC)in soil can adsorb Hg elements,and the THg concentration distribution characteristics can reflect the fault activity status over a period of time. Combined with the continuous observation data of fixed precursors around the fracture zone and seismic activity analysis,it is concluded that the current activity level of the fault is relatively weak.