地震地质 ›› 2007, Vol. 29 ›› Issue (1): 63-76.

• 研究论文 • 上一篇    下一篇

2004年西藏懂错MS 5.6地震的宏观烈度调查与控震构造分析

吴中海, 吴珍汉, 胡道功, 彭华, 叶培盛   

  1. 中国地质科学院地质力学研究所,北京 100081
  • 收稿日期:2006-03-06 修回日期:2006-11-01 出版日期:2007-03-03 发布日期:2012-02-14
  • 作者简介:吴中海,男,1974年出生,1997年毕业于长春地质学院地质系,2004年在中国地质科学院获构造地质专业博士学位,副研究员,主要从事第四纪地质和活动构造研究,电话:010-68486987,E-mail:wzhly@sina.com.
  • 基金资助:
    中国地质调查局"青藏铁路沿线地质灾害调查与监测"项目(1212010541403);国家自然科学基金(40501006);科技部国际合作项目(2001CB711001)共同资助

THE INTENSITY AND EPICENTER OF THE 2004 MS 5.6 DUNG CO EARTHQUAKE AND THE RELATION WITH ACTIVE FAULTING IN CENTRAL TIBET

WU Zhong-hai, WU Zhen-han, HU Dao-gong, PENG Hua, YE Pei-sheng   

  1. Institute of Geomechanics, Chinese Academy of Geological Science, Beijing 100081, China
  • Received:2006-03-06 Revised:2006-11-01 Online:2007-03-03 Published:2012-02-14

摘要: 地表调查结果表明,发生在西藏中部的2004年懂错MS5.6地震的极震区位于懂错东侧的贡巴淌—怕尔淌之间,最大烈度为Ⅶ度,宏观震中的地理坐标:31.70°N,91.26°E。此次地震是懂错盆地东缘边界断裂活动的结果。该断裂带是一条长40km左右、NNE走向的全新世活动正断层,在断裂带的北段发育可能形成于全新世晚期的古地震地表破裂带。地表的晚第四纪断裂活动和近期的地震活动特征显示,蓬错-懂错-错那-安多地堑系构成了西藏中部一个重要的长约120km的NE向地震活动带,其北段和中南段是其中应变积累时间更长的地段。

关键词: 西藏, 懂错-安多地堑系, 地震烈度, SN向正断层, 震源机制解

Abstract: The MS 5.6 earthquake occurred in Amdo county of central Tibet on March 7,2004.The epicenter is 31.68°N,91.31°E and 31.64°N,91.24°E from CENC(China Earthquake Netwosks Center)and NEIC(National Earthquake Information Center of America)respectively(Fig.1).It is the maximum magnitude earthquake near Golmud-Lhasa railway after the 2001 MS 8.1 Kunlun earthquake.Based on field intensity susveying,the iso-seismal contours map of the earthquake was drawn.The map shows that iso-seismal lines for the earthquake are NNE-trending and the area of the same intensity is larger to north and west than to south and east(Fig.2).Its maximum intensity is Ⅶ and lies between Gongbatang and Jianong at the east of Dung Co Lake(Photo 1,2),so this earthquake is called the 2004 MS 5.6 Dung Co earthquake.The intensity survey results suggest that the macroscopic epicenter is 31.70°N,91.26°E,which approximately coincides with the epicenter located based on instrument monitoring.The distribution of intensity indicates the earthquake probably results from faulting of NNE-trending fault zone along the eastern margin of the Dung Co basin.The results of active faults survey confirm that the fault zone controlling the earthquake is a main boundary normal fault zone of the Dung Co basin and called the eastern boundary fault zone of the NNE-trending Dung Co basin,which is a half graben about 10~15km wide and 40km long,connecting with the NNE-trending Peng Co basin to the south and adjacent to the NE-trending Tsona-Amdo graben to the north.The boundary fault zone of Dung Co graben is NNE-trending,about 40km long and composed of three minor normal faults,all dipping to west and forming a right-stepped en echelon array indicating the left lateral strike-slip component of the fault zone(Fig.3).Surface observation along the faults shows that the fault underwent prominent normal faulting during late Quaternary.The severe faulting is expressed by the prominent northwest facing triangular facets along mountain front and multiple generations of fault scarps in late Quaternary sediments(Photo 5~7).The results of in-suit measurement with level instrument and tape show that there are three sets of fluvial terrace surfaces and a piedmont slope that were vertically offset by 1~1.2m(terrace 0~1 and 0~2),2~3m(terrace 1)and 13~19m(or piedmont slope)along the northern segment of the fault zone,respectively;and there are three sets of fluvial terrace surfaces which were vertically offset by 0.5~0.7m(terrace 1),2~3m(terrace 2)and 12~15m(terrace 3)along the central segment of the fault zone,respectively.The offset of terraces all took place during late Pleistocene and Holocene,implying that the normal faulting has continued and the average rate of vertical displacement is estimated as 0.2±0.1mm/a and about 0.06mm/a on the northern and central segment respectively since late Pleistocene based on the relationship between the terraces evolution and climate change during late Quaternary and comparing the distributions of similar terraces in the adjacent areas of Dung Co.Along the northern segment of fault zone,a ~6km long surface rupture,which offset the terrace 0~1 and terrace 0~2 of late Holocene,was discovered,indicating the latest paleo-earthquake event since late Holocene.The vertical displacement is 1.0~1.2m across the surface rupture,which represents the minimum offset during the last paleo-earthquake event on the northern segment(Photo 6).In space,the central segment of the eastern margin fault zone of Dung Co graben coincides with the maximum intensity area of the 2004 MS 5.6 Dung Co earthquake.It indicates that the earthquake results from the normal faulting along the central section of the fault zone.The results of field survey are in agreement with the fault plane solution from CENC,but are inconsistent with the focal mechanism solution from HRV.It suggests the results from CENC are reasonable for explaining the 2004 MS 5.6 Dung Co earthquake.Studies on regional tectonics and distribution features of earthquake suggest that the NNE-trending Peng Co basin,Dung Co graben,and Amdo-Tsona basin make up apparently of a regular left-lateral en echelon distributed rift system trending N35°~45°E from south to north,about 120 long(Fig.1).This rift zone constitutes a NNE-trending seismic zone called Dung Co-Amdo earthquake zone.The magnitude of late Quaternary faulting varies remarkably along different segments of Dung Co-Amdo seismic zone.Based on field survey,the vertical slip rates on the northern segment of Dung Co fault zone and the western segment of northern boundary fault zone of Amdo-Tsona graben are much faster than that of the central-southern segment of Dung Co fault and eastern segment of Amdo-Tsona northern boundary fault,and there are records about paleo-earthquakes of late Holocene along the two fault segments,but recent earthquakes(MS≥3)since 1971 are almost distributed around the central-southern segment of Dung Co basin and eastern segment of Amdo-Tsona graben.The prominent differences indicate that the southern segment of Dung Co fault zone and the eastern segment of northern boundary fault zone of Amdo-Tsona graben are the sites where future earthquake will possibly occur because the strain accumulating time is longer at these areas.

Key words: Tibet, Dung Co-Amdo rift system, earthquake intensity, North-south trending normal fault, focal mechanism solution

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