汶川地震孕震机理的研究及其对地震预报的启示
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摘要
2008年5月12日汶川8.0级大地震突发在现今并不活动的龙门山断裂带上,引起社会及学术界的巨大反响,该地震的孕震机理及如何预报问题更引起广泛关注。本文试图从纵横两个侧面来研究汶川地震发生的机理,即:先利用GPS资料研究地表的变形场,然后运用数值方法对横跨龙门山断裂带的一个剖面进行模拟。文中的计算结果显示:①汶川地震前龙门山断裂带及附近地区地表的主应变率的方位与该区域的地应力方位基本一致,但形变量值很小,表明断层处于闭锁之中;②对地震发生过程进行有限元模拟的结果,显示等效应力及变形能量密度在空间分布由分散状态逐步向龙门山地区集聚,龙门山断裂带上地震复发周期约为3 163年。因此,根据前人的研究结果及本文的研究认为汶川大地震的动力成因可能为:随着印度洋板块对欧亚大陆的不断挤压碰撞,青藏高原隆升后形成的物质东流在向东运动过程中由于受到稳定的四川盆地的阻挡,一部分东流物质在川西地区囤积,造成龙门山隆升;高角度、铲状的龙门山断层面上的正应力随着川西高原的向东运动而不断增大,再加上断层附近介质刚性大,这就导致该断层的闭锁程度增高;但另一方面随着高原东流物质的不断向东运动,龙门山下部地壳囤积的东流物质不断地向斜上方推挤龙门山断层上盘,这样会导致龙门山断裂带上的剪应力越来越大;当剪应力超过摩擦强度时,断层解锁发生地震。最后,针对这种发震机理的新型地震,提出了今后关于地震预报的几点思考。
The sudden and unexpected Wenchuan earthquake occurred on the Longmenshan Fault,which is not active recently.It exerts an important influence on the society and scientific community.The author tries to study the preparation mechanisms of the Wenchuan earthquake from two different aspect: one is studying the strain rates on the earth surface,the other is modeling the earthquake process by finite element method.Firstly,Using GPS vectors,the strain rate field is obtained by means of two interpolation method proposed by Zhu.The results suggest that the orientations of the principal strain rates agree with those of the tectonic stresses in Longmenshan Fault and adjacent areas.However,the deformation in and near the Longmenshan Fault is small,suggesting that the Longmenshan Fault is locked.Then,the simulation result shows that the stresses and strain energy densities are transferred to and concentrated in the Longmenshan area from a state of decentralization in space.And the average recurrence time of earthquakes in the Longmenshan area is about 3 163 years,which is in agreement with the result by other methods.Based upon the results from previous studies and the present paper,the dynamical process of the Wenchuan earthquake is assumed as: with the movement of eastward mass flow of the Qinghai-Tibet Plateau,some part of material would accumulate on the west brim of the Sichuan Basin since the stable and strong Sichuan Basin blocks the eastward flow,resulting in the rise of the Longmenshan Mountains.The value of the normal stresses on the fault plane which is steep(the dip angle of 50°~70°) and of listric shape,increases with the eastward motion of the plateau of the western Sichuan,making the fault plane lock tighter.At the same time,the accumulated material will exert stronger and stronger forces to the hanging wall of the Longmenshan Fault;therefore the shear stresses on the fault are becoming greater and greater with the movement of eastern mass flow of the pleatau.Once the shear stresses exceed the friction strength,the fault will be unlocked,and it will slip suddenly.Hence an earthquake occurs.Lastly,some of the questions on earthquake prediction are put forward according to the new type of strong events such as the Wenchuan earthquake.
The sudden and unexpected Wenchuan earthquake occurred on the Longmenshan Fault,which is not active recently.It exerts an important influence on the society and scientific community.The author tries to study the preparation mechanisms of the Wenchuan earthquake from two different aspect: one is studying the strain rates on the earth surface,the other is modeling the earthquake process by finite element method.Firstly,Using GPS vectors,the strain rate field is obtained by means of two interpolation method proposed by Zhu.The results suggest that the orientations of the principal strain rates agree with those of the tectonic stresses in Longmenshan Fault and adjacent areas.However,the deformation in and near the Longmenshan Fault is small,suggesting that the Longmenshan Fault is locked.Then,the simulation result shows that the stresses and strain energy densities are transferred to and concentrated in the Longmenshan area from a state of decentralization in space.And the average recurrence time of earthquakes in the Longmenshan area is about 3 163 years,which is in agreement with the result by other methods.Based upon the results from previous studies and the present paper,the dynamical process of the Wenchuan earthquake is assumed as: with the movement of eastward mass flow of the Qinghai-Tibet Plateau,some part of material would accumulate on the west brim of the Sichuan Basin since the stable and strong Sichuan Basin blocks the eastward flow,resulting in the rise of the Longmenshan Mountains.The value of the normal stresses on the fault plane which is steep(the dip angle of 50°~70°) and of listric shape,increases with the eastward motion of the plateau of the western Sichuan,making the fault plane lock tighter.At the same time,the accumulated material will exert stronger and stronger forces to the hanging wall of the Longmenshan Fault;therefore the shear stresses on the fault are becoming greater and greater with the movement of eastern mass flow of the pleatau.Once the shear stresses exceed the friction strength,the fault will be unlocked,and it will slip suddenly.Hence an earthquake occurs.Lastly,some of the questions on earthquake prediction are put forward according to the new type of strong events such as the Wenchuan earthquake.
引文
[1]张培震,徐锡伟,闻学泽.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[J].地球物理学报,2008,51(4):1066-1073.
[2]朱守彪,蔡永恩,石耀霖.青藏高原现今构造应变率场的计算及其结果的地球动力学意义[J],地球物理学报,2005,22(3):292-302.
[3]Zhu Shoubiao,Yongen Cai,Yaolin Shi.The contemporary tectonic strain rate field of continental China predicted from GPS measurements and its geodynamics implications[J].Pure Appl.Geophys.2006,163:1477-1493.
[4]程万正,杨永林.川滇地块边界构造带形变速率变化与成组强震[J].大地测量与地球动力学,2002,2(4):21-24.
[5]England P C,Houseman GA.Finite Strain Calculations of Continental Deformation,2,Comparison with the India-Asia Collision[J].J.Geophys.Res.,1986,91:3664-3676.
[6]Molnar P,Tapponnier.Cenozoic tenozoic tectonics of Asia:effects of continental collision[J].Science,1975,(189):419-426.
[7]England P C,Houseman GA.The Mechanics of the Tibetan Plateau[J].Phil Trans R Soc Lond,1988,A326:3012320.
[8]Parrison TM,Copeland P,Kidd W S F,et al.Raising Tibet[J].Science,1992,255:166-321670.
[9]Peter Bird.Later extrusion of lower crust from under high topography,in the isostatic limit[J].J.Geophys.Res.1991,96:10275-10286.
[10]曾融生,孙为国.青藏高原岩石圈及其东部邻区的地震活动性和震源机制以及高原物质东流的讨论[J].地震学报,1992,14(增刊):523-533.
[11]Royden L H,Burchfiel B C,King R W,et al.Surface deformation and lower crustal flow in eastern Tibet[J].Science,1997,276,2:788-790.
[12]熊熊,许厚泽,滕吉文.青藏高原物质东流的岩石层力学背景探讨[J].地壳形变与地震,2001,21(2):1-6.
[13]Royden L H,Burchfiel B C,Robert D van der Hilst.The Geological Evolution of the Tibetan Plateau[J].Science,2008,(321):1054-1058.
[14]朱守彪,石耀霖.青藏高原地形扩展力及下地壳对上地壳拖曳力的的遗传有限单元法反演[J].北京大学学报(自然科学版),2005,41(2):225-234.
[15]Clark M,L H Royden.Topographic ooze:building the eastern margin of Tibet by lower crustal[J].flow.Geology,2000,28(8):703-706.
[16]Burchfiel B C,Royden L H,vander Hilst R D,et al.A geological and geophysical context for the Wenchuan earthquake of 12 May 2008,Sichuan,People’s Republic of China[J].GSA Today,2008,18(7),doi:10.1130/GSATG18A.1.
[17]Enkelmann E,L Ratschbacher,R Jonckheere,et al.Cenozoic exhumation and deformation of northeastern Tibet and the Qinling:Is Tibetan lower crustal flow diverging around the Sichuan Basin?GSA Bulletin,2006,118(5/6):651–671,doi:10.1130/B25805.1.
[18]朱守彪,石耀霖.中国大陆及邻区构造应力场成因的研究[J].中国科学(D辑),2006,36(12):1077-1083.
[19]Gan W,Zhang P,Shen Z,et al.Present-day crustal motion within the Tibetan Plateau inferred from GPS measurements[J].J.Geophys.Res.,2007,112,B08416,doi:10.1029/2005JB004120.
[20]张培震.青藏高原东缘川西地区的现今构造变形、应变分配与深部动力过程[J].中国科学(D辑),2008,38(9):1041-1056.
[21]许志琴,李化启,侯立玮.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,2007,26(10):1262-1276.
[22]朱守彪,张培震.2008年汶川MS8.0地震发生过程的动力学机制研究.地球物理学报,2009,52(2):418-427.
[23]郭增建,秦保燕.震源物理[M].北京:地震出版社,1979.
[24]朱守彪,邢会林,谢富仁等.地震发生过程的有限单元法模拟-以苏门答腊俯冲带上的大地震为例[J].地球物理学报,2008,51(2):460-468.
[2]朱守彪,蔡永恩,石耀霖.青藏高原现今构造应变率场的计算及其结果的地球动力学意义[J],地球物理学报,2005,22(3):292-302.
[3]Zhu Shoubiao,Yongen Cai,Yaolin Shi.The contemporary tectonic strain rate field of continental China predicted from GPS measurements and its geodynamics implications[J].Pure Appl.Geophys.2006,163:1477-1493.
[4]程万正,杨永林.川滇地块边界构造带形变速率变化与成组强震[J].大地测量与地球动力学,2002,2(4):21-24.
[5]England P C,Houseman GA.Finite Strain Calculations of Continental Deformation,2,Comparison with the India-Asia Collision[J].J.Geophys.Res.,1986,91:3664-3676.
[6]Molnar P,Tapponnier.Cenozoic tenozoic tectonics of Asia:effects of continental collision[J].Science,1975,(189):419-426.
[7]England P C,Houseman GA.The Mechanics of the Tibetan Plateau[J].Phil Trans R Soc Lond,1988,A326:3012320.
[8]Parrison TM,Copeland P,Kidd W S F,et al.Raising Tibet[J].Science,1992,255:166-321670.
[9]Peter Bird.Later extrusion of lower crust from under high topography,in the isostatic limit[J].J.Geophys.Res.1991,96:10275-10286.
[10]曾融生,孙为国.青藏高原岩石圈及其东部邻区的地震活动性和震源机制以及高原物质东流的讨论[J].地震学报,1992,14(增刊):523-533.
[11]Royden L H,Burchfiel B C,King R W,et al.Surface deformation and lower crustal flow in eastern Tibet[J].Science,1997,276,2:788-790.
[12]熊熊,许厚泽,滕吉文.青藏高原物质东流的岩石层力学背景探讨[J].地壳形变与地震,2001,21(2):1-6.
[13]Royden L H,Burchfiel B C,Robert D van der Hilst.The Geological Evolution of the Tibetan Plateau[J].Science,2008,(321):1054-1058.
[14]朱守彪,石耀霖.青藏高原地形扩展力及下地壳对上地壳拖曳力的的遗传有限单元法反演[J].北京大学学报(自然科学版),2005,41(2):225-234.
[15]Clark M,L H Royden.Topographic ooze:building the eastern margin of Tibet by lower crustal[J].flow.Geology,2000,28(8):703-706.
[16]Burchfiel B C,Royden L H,vander Hilst R D,et al.A geological and geophysical context for the Wenchuan earthquake of 12 May 2008,Sichuan,People’s Republic of China[J].GSA Today,2008,18(7),doi:10.1130/GSATG18A.1.
[17]Enkelmann E,L Ratschbacher,R Jonckheere,et al.Cenozoic exhumation and deformation of northeastern Tibet and the Qinling:Is Tibetan lower crustal flow diverging around the Sichuan Basin?GSA Bulletin,2006,118(5/6):651–671,doi:10.1130/B25805.1.
[18]朱守彪,石耀霖.中国大陆及邻区构造应力场成因的研究[J].中国科学(D辑),2006,36(12):1077-1083.
[19]Gan W,Zhang P,Shen Z,et al.Present-day crustal motion within the Tibetan Plateau inferred from GPS measurements[J].J.Geophys.Res.,2007,112,B08416,doi:10.1029/2005JB004120.
[20]张培震.青藏高原东缘川西地区的现今构造变形、应变分配与深部动力过程[J].中国科学(D辑),2008,38(9):1041-1056.
[21]许志琴,李化启,侯立玮.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,2007,26(10):1262-1276.
[22]朱守彪,张培震.2008年汶川MS8.0地震发生过程的动力学机制研究.地球物理学报,2009,52(2):418-427.
[23]郭增建,秦保燕.震源物理[M].北京:地震出版社,1979.
[24]朱守彪,邢会林,谢富仁等.地震发生过程的有限单元法模拟-以苏门答腊俯冲带上的大地震为例[J].地球物理学报,2008,51(2):460-468.
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