Abstract: Xizhang trench is located at 10 kilometers northwest of the Taiyuan City,Shanxi Province,and it was excavated at the front of a NNW-trending scarp of 4.6m height on the northern segment of the Jiaocheng Fault zone.The dimensions of the trench are 108m in length,8m in width,and 10m in depth.There are 18 strata revealed in the trench.The upper stratum is sandy loam;the upper stratum of the downthrown block of the fault is gravel,the lower stratum is interbed of brown loam and sandy loam.The strata on the upthrown block of the fault are sandy loam containing gravel.The trench shows 3 fault planes,and the upper fault point on the fault plane is 1.5m deep from the ground surface,the newest dislocated stratum is(3.74±0.06)ka BP.The trench reveals a lot of deformation traces,such as fault plane,dislocated strata,colluvial wedge and the dipping of formation.The relation between the strata and the faults in the trench shows that 3 paleoearthquake events have occurred at the Jiaocheng Fault zone since Early Holocene,they are about(3.74±0.06)～(3.06±0.26)ka BP,(8.35±0.09)～(3.74±0.06)ka BP,and(10.66±0.85)～(8.35±0.09)ka BP,respectively.The average interval of the events is2.6～3.6ka.The minimum coseismic vertical displacements of the 3 events are 3.0m,2.5m,and 3.2m,respectively.The significance of Xizhang trench is that the Jiaocheng Fault used to be active thousands years ago,though there wasn't M≥7 earthquake recorded in historical words.The evidence of newest activities of the Jiaocheng Fault zone during Holocene is important for earthquake safety assessment of Taiyuan City in the future.

Key words: paleoearthquake, Holocene, Xizhang trench, Jiaocheng Fault, Shanxi

摘要： 西张探槽位于山西太原市西北10km,沿交城断裂带北段NNW方向高4.6m的低陡坎前缘开挖。探槽长108m,宽8m,深10m,揭露18层地层。探槽上部地层为亚砂土,探槽内断层下降盘的上部地层为一套砾石层,下部地层为一套褐色垆土与亚砂土互层,断层的上升盘地层以亚砂土为主夹砾石层。探槽揭露出3条断面,断面的上断点距地表1.5m,断错的最新地层距今(3.74±0.06)ka。探槽揭示出断面、地层断错、崩积楔、地层倾斜等现象。由探槽揭露出的地层与断层的关系可知早全新世以来交城断裂带曾发生3次断层活动事件,其时间分别为距今(3.74±0.06)～(3.06±0.26)ka、(8.35±0.09)～(3.74±0.06)ka与(10.66±0.85)～(8.35±0.09)ka,具有2.6～3.6ka的强震重复间隔;3次同事件最小垂直位移分别为3.0m、2.5m及3.2m。西张探槽的意义在于揭示了历史上未有强震记载的晋中盆地几千年前有过多次强震活动。西张探槽获得的交城断裂带全新世古地震活动的依据,对评价太原市未来的强震危险性具有重要意义。

关键词: 古地震, 全新世, 西张探槽, 交城断裂带, 山西