Abstract: On 12 May 2008,a magnitude 8.0 earthquake occurred beneath the steep eastern margin of Tibetan plateau in Sichuan,China.Rupture occurred over a length of～240km along the central Longmenshan Fault(Beichuan-Yinxiu Fault)and ～72km along the Longmenshan piedmont fault(Guanxian-Jiangyou Fault).In order to know clearly the activity of the middle segment of Longmenshan Fault,we surveyed the earthquake rupture and excavated 5 trenches in the north part of the central Longmenshan Fault.Four of the five trenches have revealed the deformation of the Wenchuan earthquake and the characteristics of strong earthquake activity on this segment.The 4 trenches are briefly described as follows.The Fenghuang village trench locates on T₂ terrace or T₃ terrace of Pingtong River.The trench logs show that there is another earthquake event except the Wenchuan earthquake.As a tectonic deformation character,the thrust fault is exposed on the surface,the underground soils were thrust over the cultivated surface soil,forming thrust nappe and extrusion wedge.The bedrock near the fault has been compressed and fractured seriously,which is represented by overlap of some old scarps with new ones on the ground surface.The trench on T₁ terrace at Pingtong Panxuanlu records the zigzag deflections of marker bed.According to the recovery of marker bed,it is possible that there was an earlier earthquake event whose magnitude is equivalent to the present Wenchuan earthquake,because the footwall is higher than the hanging wall after the marker bed flattened,and the Wenchuan earthquake scarp on flood plain and T1 terrace is much lower than the scarp at the trench site.The scarp with a length of～20m and height of～2.7m is revealed by the trench excavated near the Da′ai School,but we can't see obvious signs of fault and faulting.Perhaps the fault displacement is represented by slightly folding of each deposit layer,which is one of the surface deformation models of thrust faulting.On T₁ terrace at Miaoziwan village,Nanba town,the trench displays that the vertical displacement of arc-deflection on marker bed is equal to the earthquake scarp height,indicating that this phenomenon is caused by the present earthquake event alone.Now,there are no results of dating samples,so we obtained the topographical age by comparing the adjacent surfaces.According to the dating results of terrace,the forming time of T₁ terrace is about 3000～5000a and T₂ terrace about 12000～20000a.It is revealed that the recurrence interval of strong earthquake on the northern segment of central Longmenshan Fault is more than 3000a.

Key words: Wenchuan M_S8.0 earthquake, trench information, earthquake events, tectonic deformation, Longmenshan thrust belt

摘要： 汶川8.0级地震在龙门山中央断裂(北川-映秀断裂)上形成了长度约240km的地震地表破裂带,同时在前山断裂(灌县-江油断裂)上形成了长约72km的地震地表破裂带。我们在中央断裂北段(北川以北)的地震陡坎上开挖探槽,揭露了本次地震的构造变形特征,同时通过对探槽内所揭露地层的相互关系的讨论,以及邻近区域内地貌面的对比,认为该段断裂在本次汶川8.0级地震之前可能还存在一次震级相当的地震事件,其发生时间至少早于该区域内T1阶地形成的最新年龄3000 a。

关键词: 汶川8.0级地震, 探槽, 地震事件, 地震变形, 龙门山构造带