2004年西藏懂错M_S 5.6地震的宏观烈度调查与控震构造分析
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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。
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.
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.
引文
邓起东,刘百篪,张培震,等.1992a.活动断层工程安全评价和位错量的定量评估[A].见:国家地震局地质研究所编.活动断裂研究(2).北京:地震出版社.236—246.DENG Qi-dong,LIU Bai-chi,ZHANG Pei-zhen,et al.1992a.Research of active fault in evaluating engineering safe-ty and assessing amount of displacement[A].In:Institute of Geology,SSB(ed).Research on Active Fault(2).Seismological Press,Beijing.236—246(in Chinese).
邓起东,于贵华,叶文华.1992b.地震地表破裂参数与震级关系的研究[A].见:国家地震局地质研究所编.活动断裂研究(2).北京:地震出版社.247—264.DENG Qi-dong,YU Gui-hua,YE Wen-hua.1992b.Relationship between earthquake magnitude and parameters ofsurface ruptures associated with historical earthquakes[A].In:Institute of Geology,SSB(ed).Research onActive Fault(2).Seismological Press,Beijing.247—264(in Chinese).
汪一鹏.1998.青藏高原活动构造基本特征[A].见:国家地震局地质研究所编.活动断裂研究(6).北京:地震出版社.135—144.WANG Yi-peng.1998.Principal features of the active tectonics in Qinghai-Xizang plateau[A].In:Institute of Geol-ogy,CSB(ed).Research on Active Fault(6).Seismological Press,Beijing.135—144(in Chinese).
吴章明,曹忠权,申屠炳明,等.1992.西藏中部活动断层[M].北京:地震出版社.183—192.WUZhang-ming,CAO Zhong-quan,SHENTUBing-ming,et al.1992.Active fault in the central Tibet[M].Seismo-logical Press,Beijing.183—192(in Chinese).
吴章明,邓起东.1989.西藏崩错8级地震地表破裂的变形特征及其破裂机制[J].地震地质,11(1):15—25.WU Zhang-ming,DENG Qi-dong.1989.Deformation features and fracture mechanism of surface rupture of1951BengCo,TibetMS=8earthquake[J].Seismology and Geology,11(1):15—25(in Chinese).
吴中海,胡道功,吴珍汉.2004a.青藏铁路邻侧昆仑山2001年MS8.1级地震地表破裂特征分析[J].地球学报,25(4):411—414.WUZhong-hai,HUDao-gong,WUZhen-han.2004a.Characteristics of co-seismic surface rupture of the2001MS8.1central Kunlun earthquake in the area adjacent to the Qingzang railway[J].Acta Geoscientica Sinica,25(4):411—414(in Chinese).
吴中海,叶培盛,刘琦胜,等.2004b.青藏高原中部温泉盆地西缘的晚新生代正断层作用[J].地震地质,26(4):658—675.WUZhong-hai,YE Pei-sheng,LIU Qi-sheng,et al.2004b.The normal faulting at the west side of Wenquan grabensince late Cenozoic,in central Qinghai-Tibet Plateau[J].Seismology and Geology,26(4):658—675(in Chi-nese).
吴中海,吴珍汉,胡道功,等.2005a.青藏高原中部温泉盆地西侧晚第四纪正断层作用的地貌标志及断裂活动速率[J].地质通报,24(1):48—57.WU Zhong-hai,WU Zhen-han,HU Dao-gong.2005a.Geomorphologic indicators and rates of late Quaternary normalfaulting at the west side of the Wenquan graben,central Qinghai-Tibet Plateau[J].Geological Bulletin of China,24(1):48—57(in Chinese).
吴中海,赵希涛,吴珍汉,等.2005b.西藏安多-错那湖地堑的第四纪地质、断裂活动及其运动学特征分析[J].第四纪研究,25(4):490—502.WU Zhong-hai,ZHAO Xi-tao,WU Zhen-han,et al.2005b.Active faults and their kinematics features at the Amdo-Tsona graben,central Xizang[J].Quaternary Sciences,25(4):490—502(in Chinese).
徐锡伟,陈文彬,于贵华,等.2002.2001年11月14日昆仑山库赛湖地震(MS8.1)地表破裂带的基本特征[J].地震地质,24(1):1—13.XU Xi-wei,CHEN Wen-bin,YU Gui-hua,et al.2002.Characteristic features of the surface ruptures of the Hou SaiHu(Kunlunshan)earthquake(MS8.1),northern Tibetan Plateau,China[J].Seismology and Geology,24(1):1—13(in Chinese).
Armijo R,Tapponnier P,Mercier L,et al.1986.Quaternary extension in southern Tibet:Field observation and tec-tonic implication[J].Journal of Geophysical Research,91(B14):13803—13872.
Blisniuk P M,Sharp W D.2003.Rates of late Quaternary normal faulting in central Tibet from U-series dating of ped-ogenic carbonate in displaced fluvial gravel deposits[J].Earth and Planetary Science Letters,215:169—186.
Molnar P,Lyou-caen H.1989.Fault plane solutions of earthquakes and active tectonics of the northern and easternparts of the Tibetan plateau[J].Geophysical Journal of International,99:123—153.
Molnar P,Chen Wang-ping.1983.Focal depths and fault plane solutions of earthquakes under the Tibetan Plateau[J].Journal of Geophysical Research,88(B2):1180—1196.
Molnar P,Tapponnier P.1978.Active tectonics of Tibet[J].Journal of Geophysical Research,83:5361—5375.
Rothery D A,Drury S A.1984.The neotectonics of the Tibetan Plateau[J].Tectonics,3(1):19—26.
Wu Zhenhan,Barosh P J,Hu Daogong,et al.2004.Hazards posed by active major faults along the Golmud-Lhasarailway route,Tibetan Plateau,China[J].Engineering Geology,74:163—182.
邓起东,于贵华,叶文华.1992b.地震地表破裂参数与震级关系的研究[A].见:国家地震局地质研究所编.活动断裂研究(2).北京:地震出版社.247—264.DENG Qi-dong,YU Gui-hua,YE Wen-hua.1992b.Relationship between earthquake magnitude and parameters ofsurface ruptures associated with historical earthquakes[A].In:Institute of Geology,SSB(ed).Research onActive Fault(2).Seismological Press,Beijing.247—264(in Chinese).
汪一鹏.1998.青藏高原活动构造基本特征[A].见:国家地震局地质研究所编.活动断裂研究(6).北京:地震出版社.135—144.WANG Yi-peng.1998.Principal features of the active tectonics in Qinghai-Xizang plateau[A].In:Institute of Geol-ogy,CSB(ed).Research on Active Fault(6).Seismological Press,Beijing.135—144(in Chinese).
吴章明,曹忠权,申屠炳明,等.1992.西藏中部活动断层[M].北京:地震出版社.183—192.WUZhang-ming,CAO Zhong-quan,SHENTUBing-ming,et al.1992.Active fault in the central Tibet[M].Seismo-logical Press,Beijing.183—192(in Chinese).
吴章明,邓起东.1989.西藏崩错8级地震地表破裂的变形特征及其破裂机制[J].地震地质,11(1):15—25.WU Zhang-ming,DENG Qi-dong.1989.Deformation features and fracture mechanism of surface rupture of1951BengCo,TibetMS=8earthquake[J].Seismology and Geology,11(1):15—25(in Chinese).
吴中海,胡道功,吴珍汉.2004a.青藏铁路邻侧昆仑山2001年MS8.1级地震地表破裂特征分析[J].地球学报,25(4):411—414.WUZhong-hai,HUDao-gong,WUZhen-han.2004a.Characteristics of co-seismic surface rupture of the2001MS8.1central Kunlun earthquake in the area adjacent to the Qingzang railway[J].Acta Geoscientica Sinica,25(4):411—414(in Chinese).
吴中海,叶培盛,刘琦胜,等.2004b.青藏高原中部温泉盆地西缘的晚新生代正断层作用[J].地震地质,26(4):658—675.WUZhong-hai,YE Pei-sheng,LIU Qi-sheng,et al.2004b.The normal faulting at the west side of Wenquan grabensince late Cenozoic,in central Qinghai-Tibet Plateau[J].Seismology and Geology,26(4):658—675(in Chi-nese).
吴中海,吴珍汉,胡道功,等.2005a.青藏高原中部温泉盆地西侧晚第四纪正断层作用的地貌标志及断裂活动速率[J].地质通报,24(1):48—57.WU Zhong-hai,WU Zhen-han,HU Dao-gong.2005a.Geomorphologic indicators and rates of late Quaternary normalfaulting at the west side of the Wenquan graben,central Qinghai-Tibet Plateau[J].Geological Bulletin of China,24(1):48—57(in Chinese).
吴中海,赵希涛,吴珍汉,等.2005b.西藏安多-错那湖地堑的第四纪地质、断裂活动及其运动学特征分析[J].第四纪研究,25(4):490—502.WU Zhong-hai,ZHAO Xi-tao,WU Zhen-han,et al.2005b.Active faults and their kinematics features at the Amdo-Tsona graben,central Xizang[J].Quaternary Sciences,25(4):490—502(in Chinese).
徐锡伟,陈文彬,于贵华,等.2002.2001年11月14日昆仑山库赛湖地震(MS8.1)地表破裂带的基本特征[J].地震地质,24(1):1—13.XU Xi-wei,CHEN Wen-bin,YU Gui-hua,et al.2002.Characteristic features of the surface ruptures of the Hou SaiHu(Kunlunshan)earthquake(MS8.1),northern Tibetan Plateau,China[J].Seismology and Geology,24(1):1—13(in Chinese).
Armijo R,Tapponnier P,Mercier L,et al.1986.Quaternary extension in southern Tibet:Field observation and tec-tonic implication[J].Journal of Geophysical Research,91(B14):13803—13872.
Blisniuk P M,Sharp W D.2003.Rates of late Quaternary normal faulting in central Tibet from U-series dating of ped-ogenic carbonate in displaced fluvial gravel deposits[J].Earth and Planetary Science Letters,215:169—186.
Molnar P,Lyou-caen H.1989.Fault plane solutions of earthquakes and active tectonics of the northern and easternparts of the Tibetan plateau[J].Geophysical Journal of International,99:123—153.
Molnar P,Chen Wang-ping.1983.Focal depths and fault plane solutions of earthquakes under the Tibetan Plateau[J].Journal of Geophysical Research,88(B2):1180—1196.
Molnar P,Tapponnier P.1978.Active tectonics of Tibet[J].Journal of Geophysical Research,83:5361—5375.
Rothery D A,Drury S A.1984.The neotectonics of the Tibetan Plateau[J].Tectonics,3(1):19—26.
Wu Zhenhan,Barosh P J,Hu Daogong,et al.2004.Hazards posed by active major faults along the Golmud-Lhasarailway route,Tibetan Plateau,China[J].Engineering Geology,74:163—182.
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