龙门山构造带及汶川震源区的S波速度结构
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摘要
利用四川地震台网的观测资料和体波地震层析成像方法反演了龙门山地区的S波速度结构,据此分析了龙门山断裂带的地壳结构和汶川震源区的深部构造特征.反演结果表明,地震破裂与龙门山断裂及其两侧的地壳结构差异存在明显的对应关系,汶川以北的龙门山上地壳具备较高的强度且明显抬升,灌县至江油是龙门山西侧应力积累的主要地区,汶川8.0级地震位于其南部边缘;四川盆地的刚性地壳向西俯冲于龙门山之下,其凸出部与造山带古老基底在坟川附近发生碰撞是汶川成为8.0级地震破裂起始点的主要原因.汶川以南的龙门山地区地壳上层具有较大的韧性,岩石强度相对减弱,与龙门山北部相比不易于应力积累和产生破裂,因而汶川以南的龙门山断裂缺少余震活动.龙门山地区地壳厚度明显增加,其原因与中下地壳具备较大的柔韧性有关.由于青藏东部向东挤出时受到四川盆地刚性岩石层的阻挡,龙门山中下地壳的塑性变形和垂向物质的增加导致地壳厚度加大和莫霍面下沉,以此方式吸收了龙门山地区的大部分地壳缩短量,地表则强烈褶皱抬升形成数千米的龙门山脉.
Using earthquake data from the Sichuan seismic network, the S-wave velocity structure of the Longmen Shan area was reconstructed by a body wave seismic tomography. It is followed by an analysis of the crustal structure along the Longmen Shan fault zone and in the Wenchuan earthquake area. Seismic rupture is well correlated with the Longmen Shan fault zone and the variation of crustal structure on the east and west of the fault zone. On the north of Wenchuan, the upper crust of the Longmen Shan has relatively high strength and is uplifted apparently. The Pengguan massif between Guanxian and Jiangyou is the main area for stress accumulation on the west of the Longmen Shan. The Wenchuan earthquake is located on the southern edge of this area. The rigid basement of the crust of the Sichuan basin underthrusts beneath the Longmen Shan, and the Wenchuan earthquake can be attributed to the collision of its western bulge with the base of the Pengguan massif along the Longmen Shan fault zone near Wenchuan. On the contrary, the Longmen Shan south of Wenchuan has a weak upper crust with lower mechanical strength, where stress is not easy to accumulate, compared with the northern Longmen Shan. It partly explains a lack of aftershocks along the Longmen Shan fault zone south of Wenchuan. Crustal thickening is observed beneath the Longmen Shan and this is because the strong lithosphere of the Sichuan basin obstructs the eastward flow of the weak crust of the eastern Tibetan. A large part of shortening is accommodated in a manner of ductile deformation, which causes the thickening of the deep crust and the downward flex of the Moho, while the upper crust is uplifted and folded, forming high mountains in the Longmen Shan.
Using earthquake data from the Sichuan seismic network, the S-wave velocity structure of the Longmen Shan area was reconstructed by a body wave seismic tomography. It is followed by an analysis of the crustal structure along the Longmen Shan fault zone and in the Wenchuan earthquake area. Seismic rupture is well correlated with the Longmen Shan fault zone and the variation of crustal structure on the east and west of the fault zone. On the north of Wenchuan, the upper crust of the Longmen Shan has relatively high strength and is uplifted apparently. The Pengguan massif between Guanxian and Jiangyou is the main area for stress accumulation on the west of the Longmen Shan. The Wenchuan earthquake is located on the southern edge of this area. The rigid basement of the crust of the Sichuan basin underthrusts beneath the Longmen Shan, and the Wenchuan earthquake can be attributed to the collision of its western bulge with the base of the Pengguan massif along the Longmen Shan fault zone near Wenchuan. On the contrary, the Longmen Shan south of Wenchuan has a weak upper crust with lower mechanical strength, where stress is not easy to accumulate, compared with the northern Longmen Shan. It partly explains a lack of aftershocks along the Longmen Shan fault zone south of Wenchuan. Crustal thickening is observed beneath the Longmen Shan and this is because the strong lithosphere of the Sichuan basin obstructs the eastward flow of the weak crust of the eastern Tibetan. A large part of shortening is accommodated in a manner of ductile deformation, which causes the thickening of the deep crust and the downward flex of the Moho, while the upper crust is uplifted and folded, forming high mountains in the Longmen Shan.
引文
[1] 陈运泰.汶川特大地震的震级和断层长度.科技导报,2008,26(10) :26-27 Chen Y T. On the magnitude and the fault length of the great Wenchuan earthquake. Science & Technology Review ( in Chinese), 2008,26(10) :26-27
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[6] Burchfiel B C, Royden L H, 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) :4-11 doi: 10． 1130/GSATG18A. 1
[7] 卢华复,贾 东,王良书等.关于汶川地震发震机制.高校地质学报,2008,14(2) :133-138 Lu H F, Jia D, Wang L S, et al. On the triggering mechanics of Wenchuan earthquake. Geological Journal of China Universities (in Chinese), 2008,14(2) :133-138
[8] 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
[9] Chen Z, Burchfiel B C, Liu Y, et al. Global Positioning System measurements from eastern Tibet and their implications for India/Eurasia intercontinental deformation. J. Geophys. Res. , 2000, 105(B7) :16215-16227
[10] Shen Z K, Zhao C, Yin A, et al. Contemporary crustal deformation in East Asia constrained by Global Positioning System measurements. J. Geophys. Res. , 2000,105:5721-5734
[11] Wang Q, Zhang P Z, Freymueller J T, et al. Present-day crustal deformation in China constrained by Global Positioning System measurements. Science, 2001, 294: 574-577
[12] 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
[13] 周仕勇.川西及邻近地区地震活动性模拟和断层间相互作用研究.地球物理学报,2008,51(1) :660-669 Zhou S Y. Seismicity simulation in Western Sichuan of China based on the fault interactions and its implication on the estimation of the regional earthquake risk. Chinese J.Geophys. (in Chinese), 2008, 51(1) :660-669
[14] 朱介寿.汶川地震的岩石圈深部结构与动力学背景.成都理工大学学报(自然科学版),2008,35(4) :348-356 Zhu J S. The Wenchuan earthquake occurrence background in deep structure and dynamics of lithosphere. Journal of Chengdu University of Technology (Science & Technology Edition) (in Chinese) , 2008,35(4) :348-356
[15] 蔡学林,曹家敏,朱介寿等.龙门山岩石圈地壳三维结构及汶川大地震成因浅析.成都理工大学学报(自然科学版),2008,35(4) :357-365 Cai X L, Cao J M, Zhu J S, et al. A preliminary study on the 3-D crust structure or the Longmen lithosphere and the genesis of the huge Wenchuan earthquake, Sichuan, China. Journal of Chengdu University of Technology (Science & Technology Edition) (in Chinese), 2008,35(4) :357-365
[16] 滕吉文,白登海,杨辉等.汶川M_s8． 0地震发生的深层过程和动力学响应.地球物理学报,2008,51(5) :1385-1402 Teng J W , Bai D H , Yang H , et al. Deep processes and dynamic responses associated with the Wenchuan M_s 8． 0 earthquake of 2008． Chinese J. Geophys. (in Chinese),2008,51(5) :1385-1402
[17] 中石化股份有限公司南方勘探开发分公司.松潘-阿坝地区地震层析成像与深部构造研究报告.2004 Exploration Southern Company of China Petroleum & Chemical Corporation.Research report on seismic tomography and deep tectonics of the Songpan-Aba region. 2004
[18] Jiang X, Yu J. Mapping the deep lithospheric structure beneath the eastern margin of the Tibetan Plateau from gravity anomalies. J. Geophys. Res., 2005,110, B07407,doi: 10． 1029/2004JB003394
[19] 江为为.调和级数法与重力资料反演地壳构造.中国科技大学研究生院学报,1989,6(1) :96-104 Jiang W W. The harmonious series and gravitational data used to inverse crustal structure. Journal of Graduate School, USTC (in Chinese), 1989 ,6(1) :96-104
[20] Wang C Y, Lou H, Lu Z Y, et al. S-wave crustal and upper mantle' s velocity structure in the eastern Tibetan Plateau-Deep environment of lower crustal flow. Science in China (Series D), 2008,51(2) :263-274
[21] Zhao G Z, Chen X B, Wang L F, et al. Evidence of crustal 'channel flow' in the eastern margin of Tibetan Plateau from MT measurements. Science in China (Series D) , 2008, 53(12) :1887-1893
[22] 王绪本,朱迎堂,赵锡奎等.青藏高原东缘龙门山逆冲构造深部电性结构特征.地球物理学报,2009,52(2) :564-571 Wang X B, Zhu Y T, Zhao X K, et al. Deep conductivity characteristics of the Longmen Shan, Eastern Qinghai-Tibet Plateau. Chinese J. Geophys. (in Chinese), 2009, 52(2) :564-571．
[23] Um J, Thurber C H. A fast algorithm for two-point seismic ray tracing. Bull. Seism. Soc. Am., 1987,77 :972-986
[24] Zhao D, Hasegawa A, Horiuchi S. Tomographic imaging of P and S wave velocity structure beneath northeastern Japan. J. Geophys. Res. , 1992,97(B13) :19909-19928
[25] Koketsu K, Sekine S. Pseudo-bending method for three-dimensional seismic ray tracing in a spherical earth with discontinuities. Geophys. J. Int. ,1998,132(2) :339-346
[26] Paige C C, Saunders M A. LSQR: An algorithm for sparse linear equations and least squares problems. ACM Trans. Math. Software, 1982, 8(1) :43-71
[27] Lees J M, Crosson R S. Tomographic inversion for three-dimensional velocity structure at Mount St. Helens using earthquake data. J. Geophys. Res. ,1989, 94:5716-5728
[28] Clark M K, Royden L H. Topographic ooze:Building the eastern margin of Tibet by lower crustal flow. Geology,2000,28(8) :703-706
[29] Royren L H, Burchfiel B C, Kin G R W, et al. Surface deformation and lowest crustal flow in eastern Tibet. Science, 1997, 276:788-790
[30] Clark M K, Bush J W M, Royden L H. Dynamic topography produced by lower crustal flow against rheological strength heterogeneities bordering the Tibetan Plateau. Geophy. J.Int. , 2005, 162: 575-590
[31] Schoenbohm L M, Burchfiel B C, Chen L. Propagation of surface uplift, lower crustal flow, and Cenozoic tectonics of the southeast margin of the Tibetan Plateau. Geology, 2006,34(10) :813-816
[2] 王卫民,赵连锋,李 娟等.四川汶川8． 0级地震震源过程.地球物理学报,2008,51(5) :1403-1410 Wang W M, Zhao L F, Li J, et al. Rupture process of the M_s 8． 0 Wenchuan earthquake of Sichuan, China. Chinese J. Geophys. (in Chinese), 2008,51(5) :1403-1410
[3] 胡幸平,俞春泉,陶开等.利用P波初动资料求解汶川地震及其强余震震源机制解.地球物理学报.2008,51(6) :1711-1718 Hu X P, Yu C Q, Tao K, et al. Focal mechanism solutions of Wenchuan earthquake and its strong aftershocks obtained from initial P wave polarity analysis. Chinese J . Geophys. (in Chinese), 2008,51(6) : 1711-1718
[4] Ji C. Preliminary result of the May 12, 2008 M_w 7． 9 eastern Sichuan, China earthquake, May 12, 2008: http://www. geol. ucsb. edu/faculty/ji/ big_earthquakes/2008/05/12/SiChuan. html (last accessed 8 June 2008)
[5] 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 Longmenshan 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) :66-1073
[6] Burchfiel B C, Royden L H, 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) :4-11 doi: 10． 1130/GSATG18A. 1
[7] 卢华复,贾 东,王良书等.关于汶川地震发震机制.高校地质学报,2008,14(2) :133-138 Lu H F, Jia D, Wang L S, et al. On the triggering mechanics of Wenchuan earthquake. Geological Journal of China Universities (in Chinese), 2008,14(2) :133-138
[8] 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
[9] Chen Z, Burchfiel B C, Liu Y, et al. Global Positioning System measurements from eastern Tibet and their implications for India/Eurasia intercontinental deformation. J. Geophys. Res. , 2000, 105(B7) :16215-16227
[10] Shen Z K, Zhao C, Yin A, et al. Contemporary crustal deformation in East Asia constrained by Global Positioning System measurements. J. Geophys. Res. , 2000,105:5721-5734
[11] Wang Q, Zhang P Z, Freymueller J T, et al. Present-day crustal deformation in China constrained by Global Positioning System measurements. Science, 2001, 294: 574-577
[12] 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
[13] 周仕勇.川西及邻近地区地震活动性模拟和断层间相互作用研究.地球物理学报,2008,51(1) :660-669 Zhou S Y. Seismicity simulation in Western Sichuan of China based on the fault interactions and its implication on the estimation of the regional earthquake risk. Chinese J.Geophys. (in Chinese), 2008, 51(1) :660-669
[14] 朱介寿.汶川地震的岩石圈深部结构与动力学背景.成都理工大学学报(自然科学版),2008,35(4) :348-356 Zhu J S. The Wenchuan earthquake occurrence background in deep structure and dynamics of lithosphere. Journal of Chengdu University of Technology (Science & Technology Edition) (in Chinese) , 2008,35(4) :348-356
[15] 蔡学林,曹家敏,朱介寿等.龙门山岩石圈地壳三维结构及汶川大地震成因浅析.成都理工大学学报(自然科学版),2008,35(4) :357-365 Cai X L, Cao J M, Zhu J S, et al. A preliminary study on the 3-D crust structure or the Longmen lithosphere and the genesis of the huge Wenchuan earthquake, Sichuan, China. Journal of Chengdu University of Technology (Science & Technology Edition) (in Chinese), 2008,35(4) :357-365
[16] 滕吉文,白登海,杨辉等.汶川M_s8． 0地震发生的深层过程和动力学响应.地球物理学报,2008,51(5) :1385-1402 Teng J W , Bai D H , Yang H , et al. Deep processes and dynamic responses associated with the Wenchuan M_s 8． 0 earthquake of 2008． Chinese J. Geophys. (in Chinese),2008,51(5) :1385-1402
[17] 中石化股份有限公司南方勘探开发分公司.松潘-阿坝地区地震层析成像与深部构造研究报告.2004 Exploration Southern Company of China Petroleum & Chemical Corporation.Research report on seismic tomography and deep tectonics of the Songpan-Aba region. 2004
[18] Jiang X, Yu J. Mapping the deep lithospheric structure beneath the eastern margin of the Tibetan Plateau from gravity anomalies. J. Geophys. Res., 2005,110, B07407,doi: 10． 1029/2004JB003394
[19] 江为为.调和级数法与重力资料反演地壳构造.中国科技大学研究生院学报,1989,6(1) :96-104 Jiang W W. The harmonious series and gravitational data used to inverse crustal structure. Journal of Graduate School, USTC (in Chinese), 1989 ,6(1) :96-104
[20] Wang C Y, Lou H, Lu Z Y, et al. S-wave crustal and upper mantle' s velocity structure in the eastern Tibetan Plateau-Deep environment of lower crustal flow. Science in China (Series D), 2008,51(2) :263-274
[21] Zhao G Z, Chen X B, Wang L F, et al. Evidence of crustal 'channel flow' in the eastern margin of Tibetan Plateau from MT measurements. Science in China (Series D) , 2008, 53(12) :1887-1893
[22] 王绪本,朱迎堂,赵锡奎等.青藏高原东缘龙门山逆冲构造深部电性结构特征.地球物理学报,2009,52(2) :564-571 Wang X B, Zhu Y T, Zhao X K, et al. Deep conductivity characteristics of the Longmen Shan, Eastern Qinghai-Tibet Plateau. Chinese J. Geophys. (in Chinese), 2009, 52(2) :564-571．
[23] Um J, Thurber C H. A fast algorithm for two-point seismic ray tracing. Bull. Seism. Soc. Am., 1987,77 :972-986
[24] Zhao D, Hasegawa A, Horiuchi S. Tomographic imaging of P and S wave velocity structure beneath northeastern Japan. J. Geophys. Res. , 1992,97(B13) :19909-19928
[25] Koketsu K, Sekine S. Pseudo-bending method for three-dimensional seismic ray tracing in a spherical earth with discontinuities. Geophys. J. Int. ,1998,132(2) :339-346
[26] Paige C C, Saunders M A. LSQR: An algorithm for sparse linear equations and least squares problems. ACM Trans. Math. Software, 1982, 8(1) :43-71
[27] Lees J M, Crosson R S. Tomographic inversion for three-dimensional velocity structure at Mount St. Helens using earthquake data. J. Geophys. Res. ,1989, 94:5716-5728
[28] Clark M K, Royden L H. Topographic ooze:Building the eastern margin of Tibet by lower crustal flow. Geology,2000,28(8) :703-706
[29] Royren L H, Burchfiel B C, Kin G R W, et al. Surface deformation and lowest crustal flow in eastern Tibet. Science, 1997, 276:788-790
[30] Clark M K, Bush J W M, Royden L H. Dynamic topography produced by lower crustal flow against rheological strength heterogeneities bordering the Tibetan Plateau. Geophy. J.Int. , 2005, 162: 575-590
[31] Schoenbohm L M, Burchfiel B C, Chen L. Propagation of surface uplift, lower crustal flow, and Cenozoic tectonics of the southeast margin of the Tibetan Plateau. Geology, 2006,34(10) :813-816
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