汶川8.0级地震陡坎(北川以北段)探槽的记录特征
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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。
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.
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.
引文
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李传友,叶建青,谢富仁,等.汶川MS8·0地震地表破裂带北川以北段的基本特征[J].地震地质,30(3):683-696.LI Chuan-you,YE Jian-qing,XIE Fu-ren,et al.2008.Characteristics of the surface rupture zone of theMS8.0Wen-chuan earthquake,China along the segment of Beichuan[J].Seismology and Geology,30(3):683-696(in Chi-nese).
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徐锡伟,张培震,闻学泽,等.2005.川西及其邻近地区活动构造基本特征与强震复发模型[J].地震地质,27(3):446-461.XU Xi-wei,ZHANG Pei-zhen,WEN Xue-ze,et al.2005.Features of active tectonics and recurrence behaviors of strongearthquakes in the western Sichuan Province and its adjacent regions[J].Seismology and Geology,27(3):446-461(in Chinese).
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徐锡伟,闻学泽,陈桂华,等.2008b.巴颜喀拉地块东部龙日坝断裂带的发现及其大地构造意义[J].中国科学(D辑):38(5):529-542.XU Xi-wei,WEN Xue-ze,CHENGui-hua,et al.2008b.Discovery of the Longriba faults in the eastern part of the Bay-ankela tectonic block and its geodynamic implications[J].Sciences of China(ser D),38(5):529-542(in Chi-nese).
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许志琴,李化启,侯立玮,等.2007.青藏高原东缘龙门山-锦屏造山带的崛起:大型拆离断层和挤出机制[J].地质通报,26(10):1262-1276.XU Zhi-qin,LI Hua-qi,HOU Li-wei,et al.2007.Uplift of the Longmen-Jinping orogenic belt along the eastern marginof the Qinghai-Tibet Plateau:Large-scale detachment faulting and extrusion mechanism[J].Geolgoical Bulletinof China,26(10):1262-1276(in Chinese).
张培震,徐锡伟,闻学泽,等.2008.5·12汶川8·0级大地震的特点与成因:低速率、长周期、高强度的新地震类型[J].地球物理学报,51(4):1066-1073.ZHANG Pei-zhen,XU Xi-wei,WEN Xue-ze,et al.2008.Slip rate and recurrence intervals of the Longmenshan activefault zone,and tectonic implications for the mechanism of the May122008,Wenchuan earthquake,Sichuan,China[J].Chinese Journal of Geophysics,51(4):1066-1073(in Chinese).
Burchfiel B C,Chen Z,Liu Y,et al.1995.Tectonics of the Longmen Shan and adjacent regions,central China[J].In-ternational Geology Review,37:661-738.
Burchfiel B C,Royden L H,Van der Hilst R D,et al.2008.A geological and geophysical context for the Wenchuanearthquake of12May2008,Sichuan,People s Republic of China[J].GSA Today,18(7):4-11.
Densmore A L,Ellis M A,Li Y,et al.2007Active tectonics of the Beichuan and Pengguan faults at the eastern marof the Tibetan Plateau[J].Tectonics,26:1-17.
Wang Q,Zhang P,Freymueller J T,et al.2001.Present-day crustal deformation in China constrained by Global Positio-ning System(GPS)measurements[J].Science,294:574-577.
Zhang P Z,Deng QD,Zhang G M,et al.2003.Active tectonic blocks and strong earthquakes in the continent of China[J].Science in China(ser D),46(suppl):13-24.
Zhang P,Shen Z,Wang M,et al.2004.Continuous deformation of the Tibetan Plateau from Global Positioning Systemdata[J].Geology,32:809-812.
高玄余,李勇.2008.岷江中游河流下蚀速率的研究[J].水土保持通报,28(1):39-45.GAO Xuan-yu,LI Yong.2008.Study of incision rate in the middle reaches of Minjiang River[J].Bulletin of Soil andWater Conservation,28(1):39-45(in Chinese).
李传友,叶建青,谢富仁,等.汶川MS8·0地震地表破裂带北川以北段的基本特征[J].地震地质,30(3):683-696.LI Chuan-you,YE Jian-qing,XIE Fu-ren,et al.2008.Characteristics of the surface rupture zone of theMS8.0Wen-chuan earthquake,China along the segment of Beichuan[J].Seismology and Geology,30(3):683-696(in Chi-nese).
李勇,周荣军,Densmore A L,et al.2006.青藏高原东缘大陆动力学过程与地质响应[M].北京:地质出版社.LI Yong,ZHOU Rong-jun,Densmore A L,et al.2006.Continental Dynamics and Geological Response of the EasternMargin of Qinghai-Tibet Plateau[M].Geologic Publishing House,Beijing(in Chinese).
徐锡伟,张培震,闻学泽,等.2005.川西及其邻近地区活动构造基本特征与强震复发模型[J].地震地质,27(3):446-461.XU Xi-wei,ZHANG Pei-zhen,WEN Xue-ze,et al.2005.Features of active tectonics and recurrence behaviors of strongearthquakes in the western Sichuan Province and its adjacent regions[J].Seismology and Geology,27(3):446-461(in Chinese).
徐锡伟,闻学泽,叶建青,等.2008a.汶川MS8·0地震地表破裂带及其发震构造[J].地震地质,30(3):597-629.XU Xi-wei,WEN Xue-ze,YE Jian-qing,et al.2008a.TheMS8.0Wenchuan earthquake surface ruptures and its seis-mogenic structure[J].Seismology and Geology,30(3):597-629(in Chinese).
徐锡伟,闻学泽,陈桂华,等.2008b.巴颜喀拉地块东部龙日坝断裂带的发现及其大地构造意义[J].中国科学(D辑):38(5):529-542.XU Xi-wei,WEN Xue-ze,CHENGui-hua,et al.2008b.Discovery of the Longriba faults in the eastern part of the Bay-ankela tectonic block and its geodynamic implications[J].Sciences of China(ser D),38(5):529-542(in Chi-nese).
许志琴,侯立玮,王宗秀,等.1992.中国松潘-甘孜造山带的造山过程[M].北京:地质出版社.XU Zhi-qin,HOU Li-wei,WANG Zong-xiu,et al.1992.Mountain Building Processes of the Songpan-Ganzi Orogeny,China[M].Geologic Publishing House,Beijing(in Chinese).
许志琴,李化启,侯立玮,等.2007.青藏高原东缘龙门山-锦屏造山带的崛起:大型拆离断层和挤出机制[J].地质通报,26(10):1262-1276.XU Zhi-qin,LI Hua-qi,HOU Li-wei,et al.2007.Uplift of the Longmen-Jinping orogenic belt along the eastern marginof the Qinghai-Tibet Plateau:Large-scale detachment faulting and extrusion mechanism[J].Geolgoical Bulletinof China,26(10):1262-1276(in Chinese).
张培震,徐锡伟,闻学泽,等.2008.5·12汶川8·0级大地震的特点与成因:低速率、长周期、高强度的新地震类型[J].地球物理学报,51(4):1066-1073.ZHANG Pei-zhen,XU Xi-wei,WEN Xue-ze,et al.2008.Slip rate and recurrence intervals of the Longmenshan activefault zone,and tectonic implications for the mechanism of the May122008,Wenchuan earthquake,Sichuan,China[J].Chinese Journal of Geophysics,51(4):1066-1073(in Chinese).
Burchfiel B C,Chen Z,Liu Y,et al.1995.Tectonics of the Longmen Shan and adjacent regions,central China[J].In-ternational Geology Review,37:661-738.
Burchfiel B C,Royden L H,Van der Hilst R D,et al.2008.A geological and geophysical context for the Wenchuanearthquake of12May2008,Sichuan,People s Republic of China[J].GSA Today,18(7):4-11.
Densmore A L,Ellis M A,Li Y,et al.2007Active tectonics of the Beichuan and Pengguan faults at the eastern marof the Tibetan Plateau[J].Tectonics,26:1-17.
Wang Q,Zhang P,Freymueller J T,et al.2001.Present-day crustal deformation in China constrained by Global Positio-ning System(GPS)measurements[J].Science,294:574-577.
Zhang P Z,Deng QD,Zhang G M,et al.2003.Active tectonic blocks and strong earthquakes in the continent of China[J].Science in China(ser D),46(suppl):13-24.
Zhang P,Shen Z,Wang M,et al.2004.Continuous deformation of the Tibetan Plateau from Global Positioning Systemdata[J].Geology,32:809-812.
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