2008年汶川8．0级地震发震断裂的滑动速率、复发周期和构造成因
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摘要
2008年5.12汶川大地震发生在中国大陆南北地震带中段.由于龙门山断裂带历史上只发生过3次6～6(?)级强震.而且其晚第四纪构造活动速率很低,以至于对其潜在地震危险性认识不足.为什么在龙门山地区突发大地震,该地震具有哪些特征?其成因机制是什么?本文在地震地质科学考察的基础上,利用震前的GPS观测结果,试图对上述问题进行一些初步的思考和探讨.结果表明,5.12汶川大地震是龙门山断裂带的映秀—北川断裂突发错动的结果,地表上形成200多公里长的地表破裂带;灌县—江油断裂在地震中也发生了破裂,形成的地表破裂带长达60多公里.震前的GPS观测表明,横跨整个龙门山断裂带的滑动速率不超过～2 mm/yr,单条断裂的活动速率不超过～1 mm/yr,与地震地质研究结果和历史地震记录相一致.利用地震地质考察和地震波反演得到的最大同震位移可以获得相当于5.12汶川大地震的强震复发周期为2000～6000年.龙门山断裂带发育在破裂强度很大的变质杂岩体中,断裂带本身在剖面上呈"犁形"或"铲形"结构,有利于能量积累,形成破坏性巨大的地震.所以,5.12汶川大地震是一次低滑动速率、长复发周期和高破坏强度的巨大地震,是一种值得高度重视和深入研究的新的地震类型.
The great Wenchuan earthquake of May 12,2008 occurs on the Longmen Shan fault zone which forms a prominent section of the seismically very active seismic belt called the North- South Trending Seismic Belt by Chinese seismologists.There have been 3 earthquakes with magmtudes from 6 to 61/2 along the Longmen Shan fault zone during more then 2000 year documented history of the Sichuan province.Previous active faulting studies indicate slow(less than 3 mm/yr)slip rate across the Longmen Shan fault zone.Why such a strong earthquake occurred in the Longmen Shah fault zone?What are the characteristics of the May 12 earthquake? What is the tectonic mechanism of the earthquake?On the basis of geological studies of the earthquake surface rupture zone and pre-earthquake GPS measurements in the region,we try to understand the questions mentioned above.The May 12 earthquake,2008 is caused by displacement along the Yingxiu-Beichuan fault along which a more than 200 km long surface ruptures was formed.Another strand of the Longmen Shan fault zone,Guanxian-Jiangyou fault also ruptured as indicated by more then 60 km long surface ruptures.GPS measurements before the earthquake suggest that slip rate across the entire Longmen Shan fault zone does not excess about 2 mm/yr,and does not excess 1 mm/yr across individual fault strand.These data agree with seismogeological studies and historical seismicity of the Sichuan province.Using maximum coseismic displacement obtained from seismogeological studies and inversions from seismic waves, the recurrence intervals of great earthquakes,such as the 2008 Wenchuan earthquake,can be estimated to be 2000～6000 years.The Longmen Shan fault zone has a high dipping angle(more than 50°～60°)near surface and low-angle at depth(15～20 kin).This kind of listric shape favors significant strain or energy accumulation to form great earthquake.The May 12 Wenchuan earthquake,2008 is characterized by slow strain accumulation,long recurrence interval,and significant damage power.It is a new type of earthquake event that deserves further studies.
The great Wenchuan earthquake of May 12,2008 occurs on the Longmen Shan fault zone which forms a prominent section of the seismically very active seismic belt called the North- South Trending Seismic Belt by Chinese seismologists.There have been 3 earthquakes with magmtudes from 6 to 61/2 along the Longmen Shan fault zone during more then 2000 year documented history of the Sichuan province.Previous active faulting studies indicate slow(less than 3 mm/yr)slip rate across the Longmen Shan fault zone.Why such a strong earthquake occurred in the Longmen Shah fault zone?What are the characteristics of the May 12 earthquake? What is the tectonic mechanism of the earthquake?On the basis of geological studies of the earthquake surface rupture zone and pre-earthquake GPS measurements in the region,we try to understand the questions mentioned above.The May 12 earthquake,2008 is caused by displacement along the Yingxiu-Beichuan fault along which a more than 200 km long surface ruptures was formed.Another strand of the Longmen Shan fault zone,Guanxian-Jiangyou fault also ruptured as indicated by more then 60 km long surface ruptures.GPS measurements before the earthquake suggest that slip rate across the entire Longmen Shan fault zone does not excess about 2 mm/yr,and does not excess 1 mm/yr across individual fault strand.These data agree with seismogeological studies and historical seismicity of the Sichuan province.Using maximum coseismic displacement obtained from seismogeological studies and inversions from seismic waves, the recurrence intervals of great earthquakes,such as the 2008 Wenchuan earthquake,can be estimated to be 2000～6000 years.The Longmen Shan fault zone has a high dipping angle(more than 50°～60°)near surface and low-angle at depth(15～20 kin).This kind of listric shape favors significant strain or energy accumulation to form great earthquake.The May 12 Wenchuan earthquake,2008 is characterized by slow strain accumulation,long recurrence interval,and significant damage power.It is a new type of earthquake event that deserves further studies.
引文
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[3]徐锡伟．张培震，闻学泽等．川西及其邻近地区活动构造基本特征与强震复发模型．地震地质，2005，27(3)：446～461 Xu X W,Zhang P Z,Wen X Z,et al.Characteristics of active tectonics and models of earthquake recurrence in western Sichuan.Seismology and Geology(in Chinese), 2005,27(3):446～461
[4]徐锡伟．闻学泽，陈桂华等．巴颤喀拉地块东部龙日坝断裂带的发现及其大地构造意义．中国科学(D辑)，2008．38(5)：529～542 Xu X W,Wen X Z,Chen G H,et al.Discovery of the Longriba faults,eastern part of the Bayankela tectonic block and its geodynamic implications.Sciences in China(Series D) (in Chinese),2008,38(5):529～542
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[6]李勇．周荣军．Densemore A．Ellis M．青藏高原东缘大陆动力学过程与地质响应．北京：地质出版社，2006 Li Y,Zhou R J,Densemore A,Ellis M.Geodynamic Processes of Eastern Margin of the Tibetan Plateau and Its Geological Responses.Beijing:Geological Publishing House, 2006
[7]Zhou R,Li Y,Densmore A L,et al.Active tectonics of the Longmen Shah region of the eastern margin of the Tibetan plateau.Acta Geologica Sinica,2007,81:593～604
[8]许志琴，侯立玮，王宗秀等．中国松潘—甘孜造山带的造山过程．北京：地质出版杜．1992．1～60 Xu Z Q,Hou L W,Wang Z X,et al.Mountain building processes of the Songpan-Ganzi Orogeny,China,1992. Beijing(in Chinese):Geological Publishing House,1992.1～60
[9]许志琴．李化启，侯立玮等．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．地质通报，2007，26(10)：1262～1276 Xu Z Q,Li H Q,Hou L W,et al.Uplifting of Longmen Shan and Jinping Shan,eastern margin of the Tibetan Plateau:large scale detachment faults and their extrusion mechanism.GeologicalBall.of China(in Chinese),2007, 26(10):1262～1276
[10]Burchfiel B C,Chen Z,Liu Y,Royden L H.Tectonics of the Longmenshan and adjacent regions,central China.Int. Geol.Rev.1995,37(8):661～735
[11]Burchfiel B C,Royden L H,van der Hilst R D,et al.A geological and geophysical context for the Wenchuan earthquake of 12 May 2008,Sichuan,People's Republic of China.GSA Today,2008,15(7),doi:10.1130/ GSATG18A.1
[12]Shimazaki K,Nakata T.Time-predictable recurrence model for large earthquakes.Geophys.Res.Lett.,1980,7:278～282
[13]Schwartz D P,Coppersmith K J.Fault behavior and characteristic earthquakes:Examples from the Wasatch and Anderson Fault Zones.J.Geophys.Res.,1984,90:5681～5698
[14]Zhang P Z,Deng Q,Zhang G,et al.Active tectonic blocks and strong earthquakes in continental China.Science in China (Series D),2003,46(Suppl.):13～24
[15]钱辉．姜枚，Chen Wangping等．青藏高原吉隆-鲁谷(Hi- Climb)层析成像与印藏碰撞的消减作用．地球物理学报，2007，50(5)：1427～1436 Qian H,Jiang M,Chen Wangping,et al.Tomography of Gyirong-Lugu profile(Hi-Climb)and the subduction of Indian-Tibet collision.Chinese J.Geophys.(in Chinese), 2007,50(5):1427～1436
[16]张东宁，袁松涌，沈正康．青藏高原现代地壳运动与活动断裂带关系的模拟实验．地球物理学报，2001．50(1)：153～162 Zhang D N,Yuan S Y,Shen Z K.Numerical simulation of the recent crust movement and the fault activities in Tibetan Plateau.Chinese J.Geophys.(in Chinese),2007,50(1): 153～162
[17]贺日政，高锐，郑洪伟等．青藏高原中西部航磁异常的匹配虑波分析与构造意义．地球物理学报，2007，50(4)：1131～1140 He R Z,Gao R,Zheng H W,et al.Matched-filter analysis of aeromagnetic anomaly in mid-western Tibetan Plateau and its tectonic imlication.Chinese J.Geophys.(in Chinese), 2007,50(4):1131～1140
[18]孙洁，晋光文，白登海等．青藏高原东缘地壳、上地幔电性结构探测及其构造意义．中国科学(D辑)，2003，33(增刊)：173～180 Sun J,Jin G W,Bai D H,et al.Sounding of electrical structure of the crust and upper mantle along the eastern border of Qinghai-Tibet Plateau and its tectonic significance. Sciences in China(Series D):2003,46(Suppl.):243～253
[19]王椿镛，吴建平，楼海等．川西—藏东地区的地壳P波速度结构．中国科学(D辑)，2003，33(增刊)：181～189 Wang C Y,Wu J P,Lou H,et al.P-wave velocity structure in western Sichuan and eastern Tibetan region.Sciences in China(Series D),2003,46(Suppl.):254～265
[20]王椿镛．楼海，吕智勇等．青藏高原东部地壳上地幔S波速度结构—下地壳流的深部环境．中国科学(D辑)，2008，38 (1)：22～32 Wang C Y,Lou H,LüZ Y,et al.S-wave crustal and upper mantle's velocity structure in the eastern Tibetan Plateau—Deep environment of lower crustal flow.Sciences in China (Series D),2008,51(2):263～274
[21]Xu L,Rondenay S,Van der Hilst R D.Structure of the crust beneath the Southeastern Tibetan Plateau from teleseismic receiver functions.Phys.Earth Planet.Int., 2007,doi:10.1016/j.pepi.2007.09.002
[22]Yao H,Van der Hilst R D,de Hoop M V.Surface-wave array tomography in SE Tibet from ambient seismic noise and two-station analysis-Ⅰ.Phase velocity maps.Geophys.J. Int.,2006,166:732～744
[23]Yao H,Beghein C,van der Hilst R D.Surface wave array tomography in SE Tibet from ambient seismic noise and two- station analysis-Ⅱ.Crustal and upper-mantle structure. Geophys.J.Int.,2008,doi:10.1111/j.1365-246X.2007. 03696.x
[24]Molnar P,England P,Martinod J.Mantle dynamics,uplift of the Tibet plateau,and the Indian monsoon.Review of Geophys,1993,31:357～396
[25]Molnar P.Mio-Pliocene growth of the Tibetan Plateau and evolution of East Asian climate.Palaeontologia Electronics, 2005,8(1):1～23
[26]Zhang P Z,Shen Z K,Wang M,et al.Continuous deformation of the Tibetan Plateau from global positioning system data.Geology,2004,32:809～812
[27]Shen Z K,LüJ N,Wang M,Bürgmann R.Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau.J.Geophys.Res.,2005,110,B11409, doi:10.1029/2004.1B003421
[28]Gan W,Zhang P Z,Shen Z K,et al.Present-day crustal motion within the Tibetan Plateau inferred from GPS measurements.J.Geophys.Res.,2007,112,B08416,doi: 10.1029/2005JB004120
[29]Royden L H,Burchfiel B C,King R W,et al.Surface deformation and lower crustal flow in eastern Tibet.Science, 1997,276:788～790
[30]Clark M,Royden L H.Topographic ooze:building the eastern margin of Tibet by lower crustal flow.Geology, 2000,28(8):703～706
[31]Kirby E,Reiners P W,Kro M A,et al.Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from ~(40)Ar/~(39)Ar and(U-Th)/He thermo chronology.Tectonics,2002,21(1),10.1029/2000TC0 01246
[2]闻学泽．活动断裂地震潜势的定量评估．北京：地震出版社，1995 Wen X Z.Quantitative Assessments of Earthquake Potential on Active Faults(in Chinese).Beijing:Seismological Press, 1995
[3]徐锡伟．张培震，闻学泽等．川西及其邻近地区活动构造基本特征与强震复发模型．地震地质，2005，27(3)：446～461 Xu X W,Zhang P Z,Wen X Z,et al.Characteristics of active tectonics and models of earthquake recurrence in western Sichuan.Seismology and Geology(in Chinese), 2005,27(3):446～461
[4]徐锡伟．闻学泽，陈桂华等．巴颤喀拉地块东部龙日坝断裂带的发现及其大地构造意义．中国科学(D辑)，2008．38(5)：529～542 Xu X W,Wen X Z,Chen G H,et al.Discovery of the Longriba faults,eastern part of the Bayankela tectonic block and its geodynamic implications.Sciences in China(Series D) (in Chinese),2008,38(5):529～542
[5]Densemore A L,Ellis M,Li Y,et al,Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the Tibetan Plateau.Tectonics,2007,26:TC4005,doi:10. 1029/2006TC001987
[6]李勇．周荣军．Densemore A．Ellis M．青藏高原东缘大陆动力学过程与地质响应．北京：地质出版社，2006 Li Y,Zhou R J,Densemore A,Ellis M.Geodynamic Processes of Eastern Margin of the Tibetan Plateau and Its Geological Responses.Beijing:Geological Publishing House, 2006
[7]Zhou R,Li Y,Densmore A L,et al.Active tectonics of the Longmen Shah region of the eastern margin of the Tibetan plateau.Acta Geologica Sinica,2007,81:593～604
[8]许志琴，侯立玮，王宗秀等．中国松潘—甘孜造山带的造山过程．北京：地质出版杜．1992．1～60 Xu Z Q,Hou L W,Wang Z X,et al.Mountain building processes of the Songpan-Ganzi Orogeny,China,1992. Beijing(in Chinese):Geological Publishing House,1992.1～60
[9]许志琴．李化启，侯立玮等．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．地质通报，2007，26(10)：1262～1276 Xu Z Q,Li H Q,Hou L W,et al.Uplifting of Longmen Shan and Jinping Shan,eastern margin of the Tibetan Plateau:large scale detachment faults and their extrusion mechanism.GeologicalBall.of China(in Chinese),2007, 26(10):1262～1276
[10]Burchfiel B C,Chen Z,Liu Y,Royden L H.Tectonics of the Longmenshan and adjacent regions,central China.Int. Geol.Rev.1995,37(8):661～735
[11]Burchfiel B C,Royden L H,van der Hilst R D,et al.A geological and geophysical context for the Wenchuan earthquake of 12 May 2008,Sichuan,People's Republic of China.GSA Today,2008,15(7),doi:10.1130/ GSATG18A.1
[12]Shimazaki K,Nakata T.Time-predictable recurrence model for large earthquakes.Geophys.Res.Lett.,1980,7:278～282
[13]Schwartz D P,Coppersmith K J.Fault behavior and characteristic earthquakes:Examples from the Wasatch and Anderson Fault Zones.J.Geophys.Res.,1984,90:5681～5698
[14]Zhang P Z,Deng Q,Zhang G,et al.Active tectonic blocks and strong earthquakes in continental China.Science in China (Series D),2003,46(Suppl.):13～24
[15]钱辉．姜枚，Chen Wangping等．青藏高原吉隆-鲁谷(Hi- Climb)层析成像与印藏碰撞的消减作用．地球物理学报，2007，50(5)：1427～1436 Qian H,Jiang M,Chen Wangping,et al.Tomography of Gyirong-Lugu profile(Hi-Climb)and the subduction of Indian-Tibet collision.Chinese J.Geophys.(in Chinese), 2007,50(5):1427～1436
[16]张东宁，袁松涌，沈正康．青藏高原现代地壳运动与活动断裂带关系的模拟实验．地球物理学报，2001．50(1)：153～162 Zhang D N,Yuan S Y,Shen Z K.Numerical simulation of the recent crust movement and the fault activities in Tibetan Plateau.Chinese J.Geophys.(in Chinese),2007,50(1): 153～162
[17]贺日政，高锐，郑洪伟等．青藏高原中西部航磁异常的匹配虑波分析与构造意义．地球物理学报，2007，50(4)：1131～1140 He R Z,Gao R,Zheng H W,et al.Matched-filter analysis of aeromagnetic anomaly in mid-western Tibetan Plateau and its tectonic imlication.Chinese J.Geophys.(in Chinese), 2007,50(4):1131～1140
[18]孙洁，晋光文，白登海等．青藏高原东缘地壳、上地幔电性结构探测及其构造意义．中国科学(D辑)，2003，33(增刊)：173～180 Sun J,Jin G W,Bai D H,et al.Sounding of electrical structure of the crust and upper mantle along the eastern border of Qinghai-Tibet Plateau and its tectonic significance. Sciences in China(Series D):2003,46(Suppl.):243～253
[19]王椿镛，吴建平，楼海等．川西—藏东地区的地壳P波速度结构．中国科学(D辑)，2003，33(增刊)：181～189 Wang C Y,Wu J P,Lou H,et al.P-wave velocity structure in western Sichuan and eastern Tibetan region.Sciences in China(Series D),2003,46(Suppl.):254～265
[20]王椿镛．楼海，吕智勇等．青藏高原东部地壳上地幔S波速度结构—下地壳流的深部环境．中国科学(D辑)，2008，38 (1)：22～32 Wang C Y,Lou H,LüZ Y,et al.S-wave crustal and upper mantle's velocity structure in the eastern Tibetan Plateau—Deep environment of lower crustal flow.Sciences in China (Series D),2008,51(2):263～274
[21]Xu L,Rondenay S,Van der Hilst R D.Structure of the crust beneath the Southeastern Tibetan Plateau from teleseismic receiver functions.Phys.Earth Planet.Int., 2007,doi:10.1016/j.pepi.2007.09.002
[22]Yao H,Van der Hilst R D,de Hoop M V.Surface-wave array tomography in SE Tibet from ambient seismic noise and two-station analysis-Ⅰ.Phase velocity maps.Geophys.J. Int.,2006,166:732～744
[23]Yao H,Beghein C,van der Hilst R D.Surface wave array tomography in SE Tibet from ambient seismic noise and two- station analysis-Ⅱ.Crustal and upper-mantle structure. Geophys.J.Int.,2008,doi:10.1111/j.1365-246X.2007. 03696.x
[24]Molnar P,England P,Martinod J.Mantle dynamics,uplift of the Tibet plateau,and the Indian monsoon.Review of Geophys,1993,31:357～396
[25]Molnar P.Mio-Pliocene growth of the Tibetan Plateau and evolution of East Asian climate.Palaeontologia Electronics, 2005,8(1):1～23
[26]Zhang P Z,Shen Z K,Wang M,et al.Continuous deformation of the Tibetan Plateau from global positioning system data.Geology,2004,32:809～812
[27]Shen Z K,LüJ N,Wang M,Bürgmann R.Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau.J.Geophys.Res.,2005,110,B11409, doi:10.1029/2004.1B003421
[28]Gan W,Zhang P Z,Shen Z K,et al.Present-day crustal motion within the Tibetan Plateau inferred from GPS measurements.J.Geophys.Res.,2007,112,B08416,doi: 10.1029/2005JB004120
[29]Royden L H,Burchfiel B C,King R W,et al.Surface deformation and lower crustal flow in eastern Tibet.Science, 1997,276:788～790
[30]Clark M,Royden L H.Topographic ooze:building the eastern margin of Tibet by lower crustal flow.Geology, 2000,28(8):703～706
[31]Kirby E,Reiners P W,Kro M A,et al.Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from ~(40)Ar/~(39)Ar and(U-Th)/He thermo chronology.Tectonics,2002,21(1),10.1029/2000TC0 01246
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