2008年汶川M_s8.0地震发生过程的动力学机制研究
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摘要
2008年5月12日汶川地震突发在现今并不活动的龙门山断裂带上,该地震发生的动力学机制问题引起广泛关注.文中利用黏弹性接触问题的有限元方法,考虑重力作用,对青藏高原东缘的应力场空间分布及其随时间的演化进行了数值模拟,结果显示应力在空间由分散分布逐渐向龙门山及周边地区转移集中.基于前人的研究成果及计算分析,初步认为汶川地震孕育发生的动力学过程如下:青藏高原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(50°-70°)、犁状的龙门山断层面上的正应力随着川西高原向东运动而不断增大,导致该断层的闭锁性逐步加强,并且分布在断层附近的变质杂岩为存贮高密度弹性应变能提供物质保障.但另一方面随着青藏高原较柔软的下地壳物质的不断向东运动,囤积的东流物质对龙门山断裂带上盘的推挤作用会不断加强,从而导致断裂带上剪应力越来越大;当剪应力超过摩擦强度时,断层解锁产生滑动,发生地震.模拟结果还表明龙门山断层面上的摩擦系数较高,断裂带上地震的平均复发周期约为3163年,这与其他资料结果有一致性.
The unexpected Wenchuan earthquake occurred on the Longmen Shan Fault, which is not active recently. It will exert an important influence on the society and scientific community. Especially, the tectonic mechanisms of the Wenchuan earthquake will be paid more and more attention. In the paper, the distribution of the stress field and its evolution with time in the eastern Qinghai-Tibetan plateau is simulated by means of viscoelastic finite element method, with gravity being considered in the model. The result shows that the stresses are transferred to and concentrated in the Longmen Shan area from a state of decentralization in space. 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 more tightly. At the same time, the accumulated material will exert stronger and stronger forces to the hanging wall of the Longmen Shan fault, therefore the shear stresses on the fault are becoming greater and greater with the movement of eastern mass flow of the Tibet. Once the shear stresses exceed the friction strength, the fault will be unlocked, and it will slip suddenly. Therefore an earthquake occurs. Also, the simulation result suggests that the average recurrence time of earthquakes in the Longmen Shan area is about 3163 years, which is in agreement with the result by other methods.
The unexpected Wenchuan earthquake occurred on the Longmen Shan Fault, which is not active recently. It will exert an important influence on the society and scientific community. Especially, the tectonic mechanisms of the Wenchuan earthquake will be paid more and more attention. In the paper, the distribution of the stress field and its evolution with time in the eastern Qinghai-Tibetan plateau is simulated by means of viscoelastic finite element method, with gravity being considered in the model. The result shows that the stresses are transferred to and concentrated in the Longmen Shan area from a state of decentralization in space. 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 more tightly. At the same time, the accumulated material will exert stronger and stronger forces to the hanging wall of the Longmen Shan fault, therefore the shear stresses on the fault are becoming greater and greater with the movement of eastern mass flow of the Tibet. Once the shear stresses exceed the friction strength, the fault will be unlocked, and it will slip suddenly. Therefore an earthquake occurs. Also, the simulation result suggests that the average recurrence time of earthquakes in the Longmen Shan area is about 3163 years, which is in agreement with the result by other methods.
引文
[1] 张培震,徐锡伟,闻学泽等.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,2008,51(4) :1066-1073 Zhang P Z, Xu X W, Wen X Z, et al. Slip rates and recurrence intervals of the Longmen Shan active fault zone, and tectonic implications for the mechanism of the May 12 Wenchuan earthquake, 2008, Sichuan, China. Chinese J. Geophys. (in Chinese), 2008, 51(4) :1066-1073
[2] 刘启元,陈九辉,李顺成等.汶川M_s8． 0地震:川西流动地震台阵观测数据的初步分析.地震地质,2008,30(3) :584-595 Liu Q Y, Chen J H, Li S C, et al. The M_s8． 0 Wenchuan earthquake: preliminary results from the western Sichuan mobile seimic array observations. Seismology and Geology (in Chinese), 2008,30(3) :584-595
[3] 徐锡伟,闻学泽,叶建青等.汶川M_s8． 0地震地表破裂带及其发展构造.地震地质,2008,30(3) :597-629 Xu X W, Wen X Z, Ye J Q, et al. The M_s8． 0 Wenchuan earthquake surface ruptures and its seismogenic structure. Seismology and Geology (in Chinese), 2008,30(3) :597-629
[4] 冉勇康,陈立春,陈桂华等.汶川M_s8． 0地震发震断裂大地震原地重复现象初析.地震地质,2008,30(3) :630-643 Rang Y K, Chen L C, Chen G H, et al. Primary analyses of in-situ recurrence of large earthquake along seismogenic fault of the M_s8． 0 Wenchuan earthquake. Seismology and Geology (in Chinese), 2008,30(3) :630-643．
[5] 杨晓平,冯希杰,戈天勇等.龙门山断裂带北段第四纪活动的地质地貌证据.地震地质,2008,30(3) :644-657 Yang X P, Feng X J, Ge T Y, et al. Geological and Geomorphic evidence for the quaternary activity on the northeast segment of Longmenshan fault zone. Seismology and Geology (in Chinese), 2008 ,30(3) :644-657
[6] 何宏林,孙昭民,魏占玉等.汶川M_s8． 0地震地表破裂带白沙河段破裂及其位移特征.地震地质,2008,30(3) :658-673 He H L, Sun Z M, Wei Z Y, el al. Ruplure of the M_s8． 0 Wenchuan earthquake along Baishahe revier. Seismology and Geology (in Chinese), 2008,30(3) :658-673
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[33] 滕吉文,白登海,杨辉等.2008汶川M_s8． 0地震发生的深层过程和动力响应.地球物理学报,2008,51(5) :1385-1402 Teng J W, Bai D H, Yang H, el al. Deep processes and dynamic responses associated with the Wenchuan M_s8． 0 earthquake of 2008． Chinese J. Geophys. (in Chinese), 2008, 51(5) :1385-1402
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[35] 陈火红.Marc有限元实例分析教程.北京:机械工业出版社,2002 Chen H H. Case analysis of MARC Finite Element Method (in Chinese). Beijing: Mechanical Industry Publishing House, 2002
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[2] 刘启元,陈九辉,李顺成等.汶川M_s8． 0地震:川西流动地震台阵观测数据的初步分析.地震地质,2008,30(3) :584-595 Liu Q Y, Chen J H, Li S C, et al. The M_s8． 0 Wenchuan earthquake: preliminary results from the western Sichuan mobile seimic array observations. Seismology and Geology (in Chinese), 2008,30(3) :584-595
[3] 徐锡伟,闻学泽,叶建青等.汶川M_s8． 0地震地表破裂带及其发展构造.地震地质,2008,30(3) :597-629 Xu X W, Wen X Z, Ye J Q, et al. The M_s8． 0 Wenchuan earthquake surface ruptures and its seismogenic structure. Seismology and Geology (in Chinese), 2008,30(3) :597-629
[4] 冉勇康,陈立春,陈桂华等.汶川M_s8． 0地震发震断裂大地震原地重复现象初析.地震地质,2008,30(3) :630-643 Rang Y K, Chen L C, Chen G H, et al. Primary analyses of in-situ recurrence of large earthquake along seismogenic fault of the M_s8． 0 Wenchuan earthquake. Seismology and Geology (in Chinese), 2008,30(3) :630-643．
[5] 杨晓平,冯希杰,戈天勇等.龙门山断裂带北段第四纪活动的地质地貌证据.地震地质,2008,30(3) :644-657 Yang X P, Feng X J, Ge T Y, et al. Geological and Geomorphic evidence for the quaternary activity on the northeast segment of Longmenshan fault zone. Seismology and Geology (in Chinese), 2008 ,30(3) :644-657
[6] 何宏林,孙昭民,魏占玉等.汶川M_s8． 0地震地表破裂带白沙河段破裂及其位移特征.地震地质,2008,30(3) :658-673 He H L, Sun Z M, Wei Z Y, el al. Ruplure of the M_s8． 0 Wenchuan earthquake along Baishahe revier. Seismology and Geology (in Chinese), 2008,30(3) :658-673
[7] 朱艾斓,徐锡伟,刁桂苓等.汶川M_s8． 0地震部分余震重新定位及地震构造初步分析.地震地质,2008,30(3) :759-767 Zhu A L, Xu X W, Diao G L, et al. Relocation of the M_s8． 0 Wenchuan earthquake sequence in part: preliminary seismotectonic analysis. Seismology and Geology (in Chinese), 2008,30(3) :759-767
[8] England P C, Houseman GA. Finite Strain Calculations of Continental Deformation , 2, Comparison with the India-Asia Collision. J. Geophys. Res. ,1986,91:3664-3676
[9] Molnar P, Tapponnier. Cenozoic tenozoic tectonics of Asia; effects of continental collision. Science, 1975,(189) : 419-426
[10] England P C , Houseman G A. The Mechanics of the Tibetan Plateau. Phil Trans R Soc Lond, 1988, A326:3012320
[11] Parrison T M, Copeland P , Kidd W S F , et al. Raising Tibet. Science, 1992, 255:166-321,670
[12] Peter Bird. Later extrusion of lower crust from under high topography, in the isostatic limit. J. Geophys. Res. ,1991, 96:10275-10286
[13] 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.地震学报,1992,14(增刊):523-533 Zeng R S, Sun W G. Seismicity and focal mechanism in Tibetan Plateau and its implications to lithospheric flow. Acta Seismologica Sinica (in Chinese) , 1992, 14(Suppl):523-533
[14] Royden L H, Burchfiel B C, King R W, et al. Surface deformation and lower crustal flow in eastern Tibet. Science, 1997,276(2) :788-790
[15] 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.地壳形变与地震,2001,21(2) :1-6 Xiong X, Xu H Z, Teng J W. Investigation on lithospheric mechanical background of eastward massflow of Tibetan Plateau. Crustal Deformation and Earthquake (in Chinese), 2001, 21(2) :1-6
[16] Royden L H, Burchfiel B C, Robert D van der Hilst. The geological evolution of the tibetan plateau. Science, 2008,(321) :1054-1058
[17] 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.北京大学学报(自然科学版),2005,41(2) :225-234 Zhu S B, Shi Y L. Genetic algorithm-Finite element inversion of topographic spreading forces and drag forces of lower crust to upper crust in Tibetan Plateau. Acta Scientiarurn Naturalium, Universitatis Pekinensis (in Chinese), 2005,41(2) :225-234
[18] Clark M, L H Royden. Topographic ooze; building the eastern margin of Tibet by lower crustal flow. Geology, 2000． 28(8) :703-706
[19] 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. GSA Today, 2008, 18 (7) , doi:10． 1130/GSATG18A. 1
[20] Enkelmann E, Ratschbacher L, Jonckheere R, 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
[21] Zhu S B, Shi Y I,. Origin of tectonic stresses in the Chinese continent and adjacent areas. Science in China C Ser D),2007, 50(1) :67-74
[22] Gan W, Zhang P, Shen Z, et al. Present-day crustal motion within the Tibetan Plateau inferred from GPS measurements. J. Geophys. Res., 2007, 112, B08416, doi:10． 1029/2005JB004120
[23] 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.中国科学(D辑),2008,38(9) :1041-1056 Zhang P Z. The tectonic deformation, strain distribution and deep dynamic processes in the eastern margin of the Qinghai-Tibetan plateau. Science in China (Ser D), 2008, 38(9) :1041-1056
[24] 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. Uplift of the Longmen-Jinping orogenic belt along the eastern margin of the Qinghai-Tibet Plateau: Large-scale detachment faulting and extrusion mechanism. Geological Bulletin of China ( in Chinese), 2007,26(10) :1262-1276
[25] 尹祥础.固体力学.北京:地震出版社,1985 Yin X C. Solid Mechanics (in Chinese). Beijing: Seismological Press, 1985
[26] Belytschko T, Liu W K, Moran B. Nonlinear Finite Elements for Continua and Structures. London: John Wiley &. Sons Ltd. . 2000
[27] 周光泉.刘孝敏.粘弹性理论.合肥:中国科学技术大学出版社,1996 Zhou G Q, Liu X M. Theory of Viscoelasticity (in Chinese).Hefei:University of Science and Technology of China Press, 1996
[28] 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,地球物理学报, 2008, 51(2) :460-468 Zhu S B, Xing H L, Xie F R, et al. Simulation of earthquake processes by finite element method: The case of megathrust earthquakes on the Sumatra subduction zone. Chinese J.Geophys. (in Chinese), 2008, 51(2) :460-468
[29] MSC.Software. MSC.Software (Volume A), Theory and user information, 2005
[30] Wang C Y, Han W B, Wu J P, et al. Crustal structure beneath the eastern margin of the Tibetan plateau and its tectonic implications. J. Geophys. Res. , 2007, 112,B07307, doi:10． 1029/2005JB003873
[31] Li C, Wan der Hilst R D, Toksoz M N. Constraining P-wave velocity variations in upper mantle beneath southeast Asia. Phys Earth Planet Int, 2006,154:180-195
[32] 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. Geodynamical Processes of the Eastern Plateau and Its Geological Responses (in Chinese). Beijing:Geological Publishing House, 2006
[33] 滕吉文,白登海,杨辉等.2008汶川M_s8． 0地震发生的深层过程和动力响应.地球物理学报,2008,51(5) :1385-1402 Teng J W, Bai D H, Yang H, el al. Deep processes and dynamic responses associated with the Wenchuan M_s8． 0 earthquake of 2008． Chinese J. Geophys. (in Chinese), 2008, 51(5) :1385-1402
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