TRENCH LOGS OF EARTHQUAKE SCARP OF THE M_S 8.0 WENCHUAN EARTHQUAKE IN THE SEGMENT NORTH OF BEICHUAN
TRENCH LOGS OF EARTHQUAKE SCARP OF THE M_S 8.0 WENCHUAN EARTHQUAKE IN THE SEGMENT NORTH OF BEICHUAN
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汶川M_S 8.0级地震北川北段地震崖海沟测井记录
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
Wei Zhan
中科院分区：
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作者：
Wei Zhan
2008年5月12日，中国四川青藏高原陡峭东缘下方发生8.0级地震。断裂沿龙门山中段断裂(北川-银秀断裂)长约240公里，沿龙门山山前断裂(冠县-江油断裂)长约72公里。为了解龙门山断裂中段的活动情况，对龙门山断裂带中部北段的地震破裂情况进行了调查，发掘了5条沟。其中4条沟揭示了汶川地震的形变和该段强震活动的特征。4条沟概述如下：凤凰村沟位于平通河T2阶地或T3阶地。沟槽测井显示，除汶川地震外，还有另一次地震事件。地表暴露的逆冲断层作为一种构造变形特征，地下土壤被推覆在耕作表土上，形成逆冲推覆和挤压楔形。断层附近的基岩受到严重的压缩和破裂，平通盘玄路T1阶地上的沟槽记录了标志层的曲折变形。根据标志层的恢复情况，可能发生了一次较早的地震事件，其震级相当于现在的汶川地震，因为标志层被夷为平地后，下盘高于悬壁，而泛滥平原和T1阶地的汶川地震陡坎远低于沟槽遗址的陡坎。大爱学校附近挖掘的沟槽揭示了长约20m，高约2.7m的陡坎但我们看不到明显的断层和断层迹象。断层位移可能表现为各沉积层的轻微褶皱，这是逆冲断裂的地表变形模式之一。在南坝镇庙子湾村T1阶地上，海沟显示标志层弧形偏转的垂直位移与地震陡坎高度相等，表明这一现象是由当前地震事件单独造成的。目前还没有测年样品的结果，所以我们通过对比相邻地表获得了地形年龄。根据阶地测年结果，T1阶地的形成时间约为3000~5000A，T2阶地的形成时间约为12000~20000。结果表明，龙门山断裂中段北段强震复发间隔大于3000A。
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.