粘弹性数值模拟龙门山断裂带应力积累及大震复发周期
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摘要
2008年5月12日在低地形变速率的龙门山断裂带上突发汶川强震,引发人们对该地震孕震机制的思考。本文根据GPS观测资料确定边界条件,通过三维粘弹性数值模拟探讨了汶川地震的孕震机理,计算了该区域岩石圈的应力增加速率和积累过程,以及汶川地震同震应力变化与震后应力松弛,在此基础上估算了汶川8.0级大地震的复发周期。数值模拟结果表明:印度板块对欧亚板块的推挤造成青藏高原的物质东流,高原中、下地壳物质在龙门山断裂带处遭到相对坚硬的四川盆地的阻挡之后,部分中、下地壳物质在龙门山断裂带下堆积产生应力集中。两个重要因素为应力集中提供了重要控制作用:其一是青藏高原中、下地壳较低的粘滞系数与四川盆地中、下地壳较高的粘滞系数的差异,其二是从青藏高原到四川盆地的Moho面深度在龙门山断裂带的突变。低应变速率的龙门断裂带岩石圈在数千年时间尺度的应力积累过程中,脆性上地壳的应力随时间近乎线性增长,并且上地壳深部的应力增长率超过浅部,6000年内应力积累最大量达到-21.6MPa,应力增长速率为-0.0036MPa/a;而柔性的中、下地壳以及岩石圈上地幔的应力在增长一段时间之后趋于稳定。在空间上,龙门山断裂带受到的压应力从断层西南向北东方向逐渐减小,而剪应力从西南到北东方向逐渐增大,应力状态有利于地震发生时断层的破裂方式从西南的逆冲运动向北东的逆冲兼走滑运动的方式发展。通过应力积累与地震应力降的计算得到汶川8.0级大地震的复发周期约为5400年。
Wenchuan earthquake(Ms 8.0) occurred in the section of the Longmenshan fault where slow strain had accumulated.The stress change during this accumulation period is computed through 3-D finite-element modeling assuming visco-elasticity rheology and using the GPS data to quantify the boundary conditions.We calculate the stress accumulation in the lithosphere of the Longmenshan fault area,the co-seismic stress change and the post-seismic stress relaxation to estimate the Wenchuan earthquake recurrence interval.It is shown that the eastern movement of Tibet Plateau resulting from the India-Asia collision is obstructed in the Longmenshan fault area by the strong Sichuan Basin.The soft materials of the middle and lower crust of Tibet Plateau accumulate at the contact of the basin resulting in stress concentration.Two factors are controlling this stress concentration.The first is the sharp reduction of the Moho thickness at the Longmenshan fault as one moves from the Tibetan Plateau to the Sichuan Basin.The second factor is the large difference in the viscosity of the middle and the lower crust between the Tibetan Plateau and the Sichuan Basin.During the inter-seismic period,stress increases almost linearly in time in the brittle upper crust of the Longmenshan fault,and the stress rate is larger at greater depths in the upper crust.The maximum stress accumulation of-21.6 MPa,with a steady rate of-0.0036MPa/y at the bottom of the upper crust,is reached after 6000 years.However,stress increases exponentially in the ductile middle and lower crust and in the upper lithospheric mantle.The spatial distribution of the stress after these 6000 years is as follows.The normal stress decreases,but the shear stress increases from Southwest to Northeast in the upper crust on the Longmenshan fault.This stress distribution explains the motion of the fault in thrust in the SW and in strike-slip motion in the NE.The recurrence interval of Wenchuan earthquake is estimated to be 5400 years corresponding to the time needed to accumulate the magnitude of the stress drop of the earthquake based on visco-elasticity.
Wenchuan earthquake(Ms 8.0) occurred in the section of the Longmenshan fault where slow strain had accumulated.The stress change during this accumulation period is computed through 3-D finite-element modeling assuming visco-elasticity rheology and using the GPS data to quantify the boundary conditions.We calculate the stress accumulation in the lithosphere of the Longmenshan fault area,the co-seismic stress change and the post-seismic stress relaxation to estimate the Wenchuan earthquake recurrence interval.It is shown that the eastern movement of Tibet Plateau resulting from the India-Asia collision is obstructed in the Longmenshan fault area by the strong Sichuan Basin.The soft materials of the middle and lower crust of Tibet Plateau accumulate at the contact of the basin resulting in stress concentration.Two factors are controlling this stress concentration.The first is the sharp reduction of the Moho thickness at the Longmenshan fault as one moves from the Tibetan Plateau to the Sichuan Basin.The second factor is the large difference in the viscosity of the middle and the lower crust between the Tibetan Plateau and the Sichuan Basin.During the inter-seismic period,stress increases almost linearly in time in the brittle upper crust of the Longmenshan fault,and the stress rate is larger at greater depths in the upper crust.The maximum stress accumulation of-21.6 MPa,with a steady rate of-0.0036MPa/y at the bottom of the upper crust,is reached after 6000 years.However,stress increases exponentially in the ductile middle and lower crust and in the upper lithospheric mantle.The spatial distribution of the stress after these 6000 years is as follows.The normal stress decreases,but the shear stress increases from Southwest to Northeast in the upper crust on the Longmenshan fault.This stress distribution explains the motion of the fault in thrust in the SW and in strike-slip motion in the NE.The recurrence interval of Wenchuan earthquake is estimated to be 5400 years corresponding to the time needed to accumulate the magnitude of the stress drop of the earthquake based on visco-elasticity.
引文
安其美,丁立丰,王海忠,赵仕广.2004.龙门山断裂带的性质与活动性研究.大地测量与地球动力学,24(2):115～119.
安美建,石耀霖.2007.中国大陆地壳和上地幔三维温度场.中国科学(D辑),37(6):736～745.
邓起东.2008.关于四川汶川8.0级地震的思考.地震地质,30(4):811～827.
李勇,周荣军,Densmore L A.2006.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.
李传友,叶建青,谢富仁.2008.汶川Ms8.0地震地表破裂带北以北段的基本特征.地震地质,30(3):683～696.
胡圣标,何丽娟,汪集旸.2001.中国大陆地区大地热流数据汇编(第三版).地球物理学报,44(5):611～626
石玉涛,高原,赵翠萍.2009.汶川地震余震序列的地震各向异性.地球物理学报,52(2):398～407.
石耀霖,曹建玲.2008.中国大陆岩石圈等效粘滞系数的计算和讨论.地学前缘,15(3):83～95.
滕吉文,白登海,辉杨,闫雅芬,张洪双,张永谦,阮小敏.2008.2008汶川MS8.0地震发生的深层过程和动力学响应.地球物理学报,51(5):1385～1402.
吴建平,明跃红,王椿镛.2004.云南地区中小地震震源机制及构造应力场研究.地震学报,26(5):457～465.
王俊,刘启元,陈九辉,李顺成,郭飚,李昱.2009.首都圈地区地壳厚度与泊松比.地球物理学报,52(1):57～66
王有学,Mooney W D,韩果花.2005.台湾—阿尔泰地学断面阿尔金—龙门山剖面的地壳纵波速度结构.地球物理学报48(1):98～105.
王连捷,李朋武,崔军文,丁原辰.2005.中国大陆科学钻探主孔声发射法现今地应力状态的确定.中国地质,32(2):259～264.
许志琴,李化启,侯立玮.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.地质通报,26(10):1262～1276.
张勇,冯万鹏,许力生,周成虎,陈运泰.2008.2008年汶川大地震的时空破裂过程.中国科学(D辑),38(10):1186～1194.
张晁军,曹建玲,石耀霖.2008.从震后形变探讨青藏高原下地壳黏滞系数.中国科学(D辑),38(10):1250～1257.
张培震.2008.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.中国科学(D辑),38(9):1041～1056.
张培震,徐锡伟,闻学泽,冉勇康.2008.2008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.地球物理学报,51(4):1066～1073.
张国民,汪素云,李丽.2002.中国大陆地震震源深度及其构造含义.科学通报,47(9):663～668.
朱介寿,李海丰,Gerad J.2007.龙门山及青藏高原被动源地震剖面观测.地球探测与信息技术教育部与四川省重点实验室2007年学术年会论文集.成都:成都理工大学.
朱守彪,张培震.2009.2008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,52(2):418～427.
朱艾斓,徐锡,刁桂苓.2008.汶川Ms8.0地震部分余震重新定位及地震构造初步分析.地震地质,30(3):759～767.
朱艾斓,徐锡伟,周永胜.2005.川西地区小震重新定位及其活动构造意义.地球物理学报,48(3):629～636.
Bendick R,Flesch L.2007.Reconciling lithospheric deformationand lower crustal flow beneath central Tibet.Geology,35(10):895～898.
Brudy M,Zoback M D.,Fuchs K,Rummel F,Baumgartner J.1997.Estimation of the complete stress tensor to8km depth inthe KTB scientific drill holes:Implications for crustal strength.J.Geophys.Res.,102(B8):18,453～18,475.
Burchfiel B C,Royden L H,Hilst R D V,Hagera B H,Chen Z,King RW,Li C,L J,Yao H,Kirby E.2008.A geological andgeophysical context for the Wenchuan earthquake of12May2008,Sichuan,People's Republic of China.GSA Today,18(7):4～11.
Byerlee J.1978.Friction of Rocks.Pageoph,161:615～626.
Chen K H,Nadeau R M,Rau Ruey-Juin.2007.Towards auniversal rule on the recurrence interval scaling of repeatingearthquakes?Geohysical Research Letters,34(L16308):1～5.
Clark M K,Royden L H.2000.Topographic ooze:Building theastern margin of Tibet by lower crustal flow.Geology28(8)703～706.
Densmore L A,Ellis A.M,Li Yong,Zhou Rongjun,Gregory S H,Richardson N.2007.Active tectonics of the Beichuan andPengguan faults at the eastern Margin of the Tibet Plateau.Tectonics,26(TC4005):1～17.
Huang Jinli,Zhao Dapeng,Zheng Sihua.2007.Lithosphericstructure and it's relationship to seismic and volcanic activity insouthwest China.J.Geophys.Res.,107(B10):1～14.
Nadeau R M,Foxall W,Mcevilly T V.1995.Clustering andPeriodic Recurrence of Microearthquakes on the San AndreasFault at Parkfield,California.Science,267(5197):503～507.
Royden L H,Burchfiel B C,Hilst R D V.2008.The geologicalevolution of the Tibetan Plateau.Science,321:1054～1058.
Royden L H,Burchfiel B C,King W R,Chen Z,Shen F.1997.Surface Deformation and Lower Crustal Flow in Eastern Tibet.Science,276(5313):788～790.
Shen Z K,L J N,Wang M,Burgmann R.2005.Contemporarycrustal deformation around the southeast borderland of theTibetan Plateau.J.Geophys.Res.,110(B11409):1～17.
Shao X Z,Zhang J R,Liu Y R,Zhang S Y.1986.The commonfeatures of deep structures in some large-earthquakes areas ofNorth China Plain and their implication for earthquake siteprediction.J.Phys.Earth,34(suppl.):S223～S239
Thatcher W,Rundle B.1979.A Model for the Earthquake Cycle inUnderthrust Zones.J.Geophys.Res.,84(B10):5540～5556.
Wang C Y,Flesch L M,Silver P G,Chang L J.2008.Evidence formechanically coupled lithosphere in central Asia and resultingimplications.Geology,36(5):363～366.
Wang R J,Lorenzo-Martin F,Roth F.2006.PSGRN/PSCMP-Anew code for calculating co-and post-seismic deformation,geoidand gravity changes based on the viscoelastic-gravitationaldislocation theory.Comput Geosci,32(4):527～541.
安美建,石耀霖.2007.中国大陆地壳和上地幔三维温度场.中国科学(D辑),37(6):736～745.
邓起东.2008.关于四川汶川8.0级地震的思考.地震地质,30(4):811～827.
李勇,周荣军,Densmore L A.2006.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.
李传友,叶建青,谢富仁.2008.汶川Ms8.0地震地表破裂带北以北段的基本特征.地震地质,30(3):683～696.
胡圣标,何丽娟,汪集旸.2001.中国大陆地区大地热流数据汇编(第三版).地球物理学报,44(5):611～626
石玉涛,高原,赵翠萍.2009.汶川地震余震序列的地震各向异性.地球物理学报,52(2):398～407.
石耀霖,曹建玲.2008.中国大陆岩石圈等效粘滞系数的计算和讨论.地学前缘,15(3):83～95.
滕吉文,白登海,辉杨,闫雅芬,张洪双,张永谦,阮小敏.2008.2008汶川MS8.0地震发生的深层过程和动力学响应.地球物理学报,51(5):1385～1402.
吴建平,明跃红,王椿镛.2004.云南地区中小地震震源机制及构造应力场研究.地震学报,26(5):457～465.
王俊,刘启元,陈九辉,李顺成,郭飚,李昱.2009.首都圈地区地壳厚度与泊松比.地球物理学报,52(1):57～66
王有学,Mooney W D,韩果花.2005.台湾—阿尔泰地学断面阿尔金—龙门山剖面的地壳纵波速度结构.地球物理学报48(1):98～105.
王连捷,李朋武,崔军文,丁原辰.2005.中国大陆科学钻探主孔声发射法现今地应力状态的确定.中国地质,32(2):259～264.
许志琴,李化启,侯立玮.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.地质通报,26(10):1262～1276.
张勇,冯万鹏,许力生,周成虎,陈运泰.2008.2008年汶川大地震的时空破裂过程.中国科学(D辑),38(10):1186～1194.
张晁军,曹建玲,石耀霖.2008.从震后形变探讨青藏高原下地壳黏滞系数.中国科学(D辑),38(10):1250～1257.
张培震.2008.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.中国科学(D辑),38(9):1041～1056.
张培震,徐锡伟,闻学泽,冉勇康.2008.2008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.地球物理学报,51(4):1066～1073.
张国民,汪素云,李丽.2002.中国大陆地震震源深度及其构造含义.科学通报,47(9):663～668.
朱介寿,李海丰,Gerad J.2007.龙门山及青藏高原被动源地震剖面观测.地球探测与信息技术教育部与四川省重点实验室2007年学术年会论文集.成都:成都理工大学.
朱守彪,张培震.2009.2008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,52(2):418～427.
朱艾斓,徐锡,刁桂苓.2008.汶川Ms8.0地震部分余震重新定位及地震构造初步分析.地震地质,30(3):759～767.
朱艾斓,徐锡伟,周永胜.2005.川西地区小震重新定位及其活动构造意义.地球物理学报,48(3):629～636.
Bendick R,Flesch L.2007.Reconciling lithospheric deformationand lower crustal flow beneath central Tibet.Geology,35(10):895～898.
Brudy M,Zoback M D.,Fuchs K,Rummel F,Baumgartner J.1997.Estimation of the complete stress tensor to8km depth inthe KTB scientific drill holes:Implications for crustal strength.J.Geophys.Res.,102(B8):18,453～18,475.
Burchfiel B C,Royden L H,Hilst R D V,Hagera B H,Chen Z,King RW,Li C,L J,Yao H,Kirby E.2008.A geological andgeophysical context for the Wenchuan earthquake of12May2008,Sichuan,People's Republic of China.GSA Today,18(7):4～11.
Byerlee J.1978.Friction of Rocks.Pageoph,161:615～626.
Chen K H,Nadeau R M,Rau Ruey-Juin.2007.Towards auniversal rule on the recurrence interval scaling of repeatingearthquakes?Geohysical Research Letters,34(L16308):1～5.
Clark M K,Royden L H.2000.Topographic ooze:Building theastern margin of Tibet by lower crustal flow.Geology28(8)703～706.
Densmore L A,Ellis A.M,Li Yong,Zhou Rongjun,Gregory S H,Richardson N.2007.Active tectonics of the Beichuan andPengguan faults at the eastern Margin of the Tibet Plateau.Tectonics,26(TC4005):1～17.
Huang Jinli,Zhao Dapeng,Zheng Sihua.2007.Lithosphericstructure and it's relationship to seismic and volcanic activity insouthwest China.J.Geophys.Res.,107(B10):1～14.
Nadeau R M,Foxall W,Mcevilly T V.1995.Clustering andPeriodic Recurrence of Microearthquakes on the San AndreasFault at Parkfield,California.Science,267(5197):503～507.
Royden L H,Burchfiel B C,Hilst R D V.2008.The geologicalevolution of the Tibetan Plateau.Science,321:1054～1058.
Royden L H,Burchfiel B C,King W R,Chen Z,Shen F.1997.Surface Deformation and Lower Crustal Flow in Eastern Tibet.Science,276(5313):788～790.
Shen Z K,L J N,Wang M,Burgmann R.2005.Contemporarycrustal deformation around the southeast borderland of theTibetan Plateau.J.Geophys.Res.,110(B11409):1～17.
Shao X Z,Zhang J R,Liu Y R,Zhang S Y.1986.The commonfeatures of deep structures in some large-earthquakes areas ofNorth China Plain and their implication for earthquake siteprediction.J.Phys.Earth,34(suppl.):S223～S239
Thatcher W,Rundle B.1979.A Model for the Earthquake Cycle inUnderthrust Zones.J.Geophys.Res.,84(B10):5540～5556.
Wang C Y,Flesch L M,Silver P G,Chang L J.2008.Evidence formechanically coupled lithosphere in central Asia and resultingimplications.Geology,36(5):363～366.
Wang R J,Lorenzo-Martin F,Roth F.2006.PSGRN/PSCMP-Anew code for calculating co-and post-seismic deformation,geoidand gravity changes based on the viscoelastic-gravitationaldislocation theory.Comput Geosci,32(4):527～541.
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