SEISMOLOGY AND GEOLOGY ›› 2008, Vol. 30 ›› Issue (3): 697-709.

• Brief Report • Previous Articles     Next Articles

TRENCH LOGS OF EARTHQUAKE SCARP OF THE MS8.0 WENCHUAN EARTHQUAKE IN THE SEGMENT NORTH OF BEICHUAN

ZHENG Wen-jun, LI Chuan-you, WANG Wei-tao, YIN Jin-hui, WEI Zhan-yu   

  1. State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China
  • Received:2008-07-15 Revised:2008-07-22 Online:2008-09-18 Published:2009-11-27

汶川8.0级地震陡坎(北川以北段)探槽的记录特征

郑文俊, 李传友, 王伟涛, 尹金辉, 魏占玉   

  1. 中国地震局地质研究所, 地震动力学国家重点实验室, 北京, 100029
  • 作者简介:郑文俊,男,1972年生,1997年毕业于西安工程学院(现长安大学)地质矿产勘查专业,2003年在中国地震局兰州地震研究所获构造地质学硕士学位,现在中国地震局地质研究所在职攻读构造地质学博士学位,副研究员,主要从事活动构造和新构造及构造地貌学方面的研究工作,电话:010-62009114,E-mail:gazhwj@163.com.
  • 基金资助:
    地震科学联合基金(B07007);中国地震局地质研究所基本科研业务专项(DFIGCEA0607122);地震动力学国家重点实验室基金(LED0606);中国地震局汶川8.0级地震应急科学考察项目共同资助

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 MS8.0 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

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