2008年汶川8.0级地震前横跨龙门山断裂带的震间形变
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摘要
利用区域GPS和水准测量资料,结合地震构造背景的分析,本文研究2008年汶川8.0级地震前横跨龙门山断裂带地区的震间地壳形变,探讨引起这种形变的活动构造与动力学模式,并由此认识汶川地震的孕育与成因机制.主要结果表明:1997～2007年期间,自龙门山断裂带中段朝北西约230 km的地带内存在垂直于断裂的水平缩短变形、以及平行于断裂的水平右旋剪切变形,缩短率为1.3×10~(-8)/a(即:0.013 mm/km/a),角变形速率为2.6×10~(-8)/a;同一地带在1975～1997年期间还表现出垂直上隆变形,上隆速率在龙门山前山断裂与中央断裂之间仅0.6mm/a,而至龙门山后山断裂及其以西达2～3 mm/a.这些反映了在汶川地震之前至少10～30余年,龙门山断裂带中段的前山与中央断裂业已闭锁、并伴有应变积累.造成这种形变的主要原因是:以壳内的低速层为"解耦"带,巴颜喀拉地块上地壳朝南东的水平运动在四川盆地西缘受到华南地块的阻挡、转换成龙门山断裂带中段的逆冲运动;由于该断裂段的震间闭锁,致使西侧的巴颜喀拉地块的上地壳发生横向缩短以及平行断裂的右旋剪切变形.然而,龙门山断裂带北段在1997～2007年期间除了有大约0.9 mm/a的右旋剪切变形外,横向的缩短变形极微弱,这可能与该断裂段西侧的岷江、虎牙、龙日坝等断裂带吸收了巴颜喀拉地块朝东水平运动的大部分有关.另外,汶川地震前,横跨龙门山断裂带中段与北段的地壳形变特征的差异,与汶川地震时能量释放的空间分布吻合.
By using regional GPS and leveling data and through an analysis considering seismotectonic background, this paper studies interseismic crustal deformation in the region across the Longmenshan fault zone before the 2008 M8. 0 Wenchuan earthquake, discusses the active tectonic and geodynamic model that caused the interseismic deformation, and, from these, analyzes the mechanism of brewing and producing the Wenchuan earthquake. The result mainly shows that, in the period from 1997 to 2007 horizontal shortening in the direction perpendicular to the fault zone and horizontal right-lateral shearing parallel to the fault zone occurred in the area from the middle segment of the fault zone to about 230 km away northwest. The shortening rate is estimated to be 1. 3×10~(-8)/a (that is 0. 013 mm/km/a) and the distortion rate to be 2. 6×10-8/a. Vertical uplifting also happened in the same area during the period from 1975 to 1997, with uplifting rates changing from 0. 6 mm/a in between the frontal-range fault and the central fault of the fault zone to 2 to 3 mm/a near and northwest of the back-range fault of the fault zone. All these reflect that at least in the last 10 to over 30 years before the Wenchuan earthquake the frontal-range and central faults of the middle segment of the Longmenshan fault zone had been locked with strain building up. The main reason to cause such deformation is believed to be that, taking a low-velocity zone in the crust as an "uncoupling layer", the horizontal southeast-ward movement of the upper crust of the Bayan Har block was counterworked from the South China block at the western edge of the Sichuan basin and transformed into the thrust movement along the middle segment of the fault zone. The interseismic locking of this fault segment made both the horizontal shortening perpendicular to the fault and the right-lateral shearing parallel to the fault occur in the upper crust of the Bayan Har block. However, during the period from 1997 to 2007, horizontal shortening across the northern segment of the Longmenshan fault zone was very weak, but a right-lateral distortion deformation of 0.9 mm/a happened there. Such weak activity of the northern Longmenshan fault segment should apparently be attributed to that those fault zones of Minjiang, Huya and Longriba on the west of this fault segment have absorbed most of the horizontal east-ward movement of the Bayan Har block. In addition, the difference of the crustal deformation features between the middle and northern segments of the Longmenshan fault zone before the Wenchuan earthquake is consistent with the difference of along-fault distribution of the coseismic energy release during the earthquake.
By using regional GPS and leveling data and through an analysis considering seismotectonic background, this paper studies interseismic crustal deformation in the region across the Longmenshan fault zone before the 2008 M8. 0 Wenchuan earthquake, discusses the active tectonic and geodynamic model that caused the interseismic deformation, and, from these, analyzes the mechanism of brewing and producing the Wenchuan earthquake. The result mainly shows that, in the period from 1997 to 2007 horizontal shortening in the direction perpendicular to the fault zone and horizontal right-lateral shearing parallel to the fault zone occurred in the area from the middle segment of the fault zone to about 230 km away northwest. The shortening rate is estimated to be 1. 3×10~(-8)/a (that is 0. 013 mm/km/a) and the distortion rate to be 2. 6×10-8/a. Vertical uplifting also happened in the same area during the period from 1975 to 1997, with uplifting rates changing from 0. 6 mm/a in between the frontal-range fault and the central fault of the fault zone to 2 to 3 mm/a near and northwest of the back-range fault of the fault zone. All these reflect that at least in the last 10 to over 30 years before the Wenchuan earthquake the frontal-range and central faults of the middle segment of the Longmenshan fault zone had been locked with strain building up. The main reason to cause such deformation is believed to be that, taking a low-velocity zone in the crust as an "uncoupling layer", the horizontal southeast-ward movement of the upper crust of the Bayan Har block was counterworked from the South China block at the western edge of the Sichuan basin and transformed into the thrust movement along the middle segment of the fault zone. The interseismic locking of this fault segment made both the horizontal shortening perpendicular to the fault and the right-lateral shearing parallel to the fault occur in the upper crust of the Bayan Har block. However, during the period from 1997 to 2007, horizontal shortening across the northern segment of the Longmenshan fault zone was very weak, but a right-lateral distortion deformation of 0.9 mm/a happened there. Such weak activity of the northern Longmenshan fault segment should apparently be attributed to that those fault zones of Minjiang, Huya and Longriba on the west of this fault segment have absorbed most of the horizontal east-ward movement of the Bayan Har block. In addition, the difference of the crustal deformation features between the middle and northern segments of the Longmenshan fault zone before the Wenchuan earthquake is consistent with the difference of along-fault distribution of the coseismic energy release during the earthquake.
引文
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[3] 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_S8． 0 Wenchuan earthquake of Sichuan, China. Chinese J. Geoph ys. (in Chinese), 2008, 51(5) :1403-1410
[4] 陈运泰,许力生,张勇等.2008年5月12日汶川特大地震震源特性分析报告.http://www.cea-igp.ac.cn/汶川地震专题/地震情况/初步研究及考察结果(一).pdf,2008 Chen Y T,Xu L S,Zhang Y,et al. Analysis report on focal characteristics of the Wenchuan strong earthquake on May 12,2008． http://www.cea-igp.ac.cn/Wenchuan earthquake special subject/Earthquake situation/Preliminary study and investigation results (Ⅰ). pdf (in Chinese) ,2008
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[6] Thatcher W.Horizontal crustal deformation from historic geodetic measurements in southern California. J. Geophys. Res. ,1979, 84:2351-2370
[7] Prescott WH,Savage JC, Kinoshita W T. Strain accumulation rates in the western United States between 1970 and 1978． J. Geophys. Res. , 1979, 84:5423-5436
[8] Savage J C, Lisowski M. Interseismic deformation along the San Andreas fault in southern California. J. Geophys. Res. , 1995,100(12) :703-717
[9] Savage J C. Interseismic uplift at the Nankai subduction zone, southwest Japan, 1951-1990． J. Geophys. Res. , 1995,100:6339-6350
[10] Bilham R, Larson K, Freymueller J, et al. GPS measurements of present-day convergence across the Nepal Himalaya. Nature, 1997, 386． 61-64
[11] Burgmann R, Larson K, Bilham R. Model inversion of GPS and leveling measurements across the Himalaya; Implications for earthquakehazardsandfuturegeodeticnetworks. Himalayan Geology , 1999, 20． 59-72
[12] Jouanne F, Mugnier J L, Gamond J F, et al.Current shortening across the Himalayas of Nepal.Geophysical Journal International, 2004, 157:1-14
[13] Prawirodirdjo L, Bock Y, McCaffrey R, et al. Geodetic observations of interseismic strain segmentation atthe Sumatra subduction zone.Geophysical Research Letters,1997,24(21) :2601-2604
[14] Hsu Y J, Simons M, Yu S B, et al. A two-dimensional dislocation model for interseismic deformation of the Taiwan mountain belt. Earth and Planetary Science Letters , 2003,211:287-294
[15] 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 Longmen shan active fault zone and tectonic implications for mechanism of the May 12 Wenchuan earthquake, 2008, Sichuan, China. Chinese J. Geophys (in Chinese), 2008, 51(4) :1066-1073
[16] Tapponnier P, Xu Z Q, Francoise R, et al. Oblique stepwise rise and growth of the Tibet Plateau. Science, 2001, 294:1671-1677
[17] Royden L H, Burchifiel B C, King R W, et al. Surface deformation and lower crustal flow in eastern Tibet. Science, 1997, 276:788-790
[18] Royden LH, Burchfiel B C, van der Hilst R D.The geological evolution of the Tibetan Plateau. Science, 2008, 321:1054-1958
[19] Burchfiel B C, Royden L H , van der Hilst R D, et al. A geological andgeophysicalcontext forthe Wenchuan earthquake of 12 May 2008, Sichuan, People's Republic of China. GSA Today, 2008, 18:4-11
[20] Hubbard J, Shaw J H. Uplift of the Longmen Shan and
[21] 刘启元,李昱,陈九辉等.汶川M_S8． 0地震:地壳上地幔S波速度结构的初步研究.地球物理学报,2009,52(2) :309-319 Liu Q Y, Li Y, Chen J H, et al. Wenchuan M_S8． 0 earthquake: Preliminary study of the S wave velocity structure of the crust and upper mantle.Chinese J. Geophys (in Chinese), 2009, 52(2) :309-319
[22] 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
[23] 王敏,沈正康,牛之俊等.现今中国大陆地壳运动与活动块体模型.中国科学(D辑)-地球科学,2003,33(增刊):21-32 Wang M, Shen Z K, Niu Z J, et al. Contemporary crustal deformation of the Chinese continent and tectonic block model. Science in China (Series D) (in Chinese) , 2003,33 (Suppl. ):25-40
[24] 徐锡伟,闻学泽,陈桂华等.巴颜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)-地球科学,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. Science in China (Series D), 2008, 51(9) :1209-1223
[25] Shen Z K, Lu J N, Wang M, et al. Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau. J. Geophy. Res., 2005, 110(B11409) , doi:10． 1029/2004JB003421
[26] 张培震,邓起东,张国民等.中国大陆的强震活动与活动地块.中国科学(D辑),2003,33(增刊):12-20 Zhang P Z, Deng Q D, Zhang G M, et al. Active tectonic blocks and strong earthquake activities in the continent of China. Science in China (Series D) , 2003, 33(Suppl.) :13-24
[27] 王椿镛,韩渭宾,吴建平等.松潘-甘孜造山带地壳速度结构.地震学报,2003,25(3) :229-242 Wang C Y, Han W B, Wu J P, et al. Crustal strcture beneath the Songpan-Garze orogenic belt. Acta Seismologica Sinica, 2003, 16(3) :237-250
[28] 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
[29] 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_S8． 0 earthquake of 2008． Chinese J. Geophys(in Chinese), 2008, 51(5) :1385-1402
[30] Tapponnier P, Peltzer G, Le Dain A Y, et al. Propagating extrusion tectonics in Asia: New insights from simple experiments with plasticine. Geology, 1982,10(12) :611-616
[31] Zhang P Z, Shen Z K, Wang M, et al. Continuous deformation of the Tibetan Plateau from Global Positioning System data. Geology, 2004, 32 (9) : 809-812, doi: 10． 1130/G20554． 1
[32] 黄媛,吴建平,张天中等.汶川8． 0级大地震及其余震序列重定位研究.中国科学(D辑),2008,38(10) :1242-1249 Huang Y, Wu J P, Zhang T Z, et al. Relocation of the M8． 0 Wenchuan earthquake and its aftershock sequence. Science in China(Series D), 2008, 51(12) :1703-1711
[33] 李勇,周荣军,Densmore A L等.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． 1-148 Li Y, Zhou R J, Densmore A L, et al. Geodynamic Processes of Eastern Margion of the Tibetan Plateau and Its Geological Responses (in Chinese). Beijing: Geological Publishing House, 2006． 1-148
[34] Maddy D, Bridgland D R, Green C P. Crustal uplift in southern England: Evidence from the river terrace records. Geomorphology, 2000, 33:167-181
[35] 张勇,冯万鹏,许力生等.2008年汶川大地震的时空破裂过程.中国科学(D辑),2008,38(10) :1186-1194 Zhang Y, Feng W P, Xu L S, et al. Rupture process of space-time of the Wenchuan strong earthquake in 2008． Sciencein China (Series D) (in Chinese) , 2008, 38(10) : 1186-1194
[2] Ji C, Hayes G. Preliminary result of the May 12, 2008 Mw7． 9 eastern Sichuan, China earthquake. Page Last Modified: May 12, 2008 11:07:35 UTC,http://earthquake,usgs. gov/eqcenter/eqinthenews/2008/us 2008ryan/finite _fault. php
[3] 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_S8． 0 Wenchuan earthquake of Sichuan, China. Chinese J. Geoph ys. (in Chinese), 2008, 51(5) :1403-1410
[4] 陈运泰,许力生,张勇等.2008年5月12日汶川特大地震震源特性分析报告.http://www.cea-igp.ac.cn/汶川地震专题/地震情况/初步研究及考察结果(一).pdf,2008 Chen Y T,Xu L S,Zhang Y,et al. Analysis report on focal characteristics of the Wenchuan strong earthquake on May 12,2008． http://www.cea-igp.ac.cn/Wenchuan earthquake special subject/Earthquake situation/Preliminary study and investigation results (Ⅰ). pdf (in Chinese) ,2008
[5] ]Thatcher W.Strain accumulation on the northern San Andreas fault zone since 1906． J. Geophys. Res. , 1975, 80: 4873-4880
[6] Thatcher W.Horizontal crustal deformation from historic geodetic measurements in southern California. J. Geophys. Res. ,1979, 84:2351-2370
[7] Prescott WH,Savage JC, Kinoshita W T. Strain accumulation rates in the western United States between 1970 and 1978． J. Geophys. Res. , 1979, 84:5423-5436
[8] Savage J C, Lisowski M. Interseismic deformation along the San Andreas fault in southern California. J. Geophys. Res. , 1995,100(12) :703-717
[9] Savage J C. Interseismic uplift at the Nankai subduction zone, southwest Japan, 1951-1990． J. Geophys. Res. , 1995,100:6339-6350
[10] Bilham R, Larson K, Freymueller J, et al. GPS measurements of present-day convergence across the Nepal Himalaya. Nature, 1997, 386． 61-64
[11] Burgmann R, Larson K, Bilham R. Model inversion of GPS and leveling measurements across the Himalaya; Implications for earthquakehazardsandfuturegeodeticnetworks. Himalayan Geology , 1999, 20． 59-72
[12] Jouanne F, Mugnier J L, Gamond J F, et al.Current shortening across the Himalayas of Nepal.Geophysical Journal International, 2004, 157:1-14
[13] Prawirodirdjo L, Bock Y, McCaffrey R, et al. Geodetic observations of interseismic strain segmentation atthe Sumatra subduction zone.Geophysical Research Letters,1997,24(21) :2601-2604
[14] Hsu Y J, Simons M, Yu S B, et al. A two-dimensional dislocation model for interseismic deformation of the Taiwan mountain belt. Earth and Planetary Science Letters , 2003,211:287-294
[15] 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 Longmen shan active fault zone and tectonic implications for mechanism of the May 12 Wenchuan earthquake, 2008, Sichuan, China. Chinese J. Geophys (in Chinese), 2008, 51(4) :1066-1073
[16] Tapponnier P, Xu Z Q, Francoise R, et al. Oblique stepwise rise and growth of the Tibet Plateau. Science, 2001, 294:1671-1677
[17] Royden L H, Burchifiel B C, King R W, et al. Surface deformation and lower crustal flow in eastern Tibet. Science, 1997, 276:788-790
[18] Royden LH, Burchfiel B C, van der Hilst R D.The geological evolution of the Tibetan Plateau. Science, 2008, 321:1054-1958
[19] Burchfiel B C, Royden L H , van der Hilst R D, et al. A geological andgeophysicalcontext forthe Wenchuan earthquake of 12 May 2008, Sichuan, People's Republic of China. GSA Today, 2008, 18:4-11
[20] Hubbard J, Shaw J H. Uplift of the Longmen Shan and
[21] 刘启元,李昱,陈九辉等.汶川M_S8． 0地震:地壳上地幔S波速度结构的初步研究.地球物理学报,2009,52(2) :309-319 Liu Q Y, Li Y, Chen J H, et al. Wenchuan M_S8． 0 earthquake: Preliminary study of the S wave velocity structure of the crust and upper mantle.Chinese J. Geophys (in Chinese), 2009, 52(2) :309-319
[22] 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
[23] 王敏,沈正康,牛之俊等.现今中国大陆地壳运动与活动块体模型.中国科学(D辑)-地球科学,2003,33(增刊):21-32 Wang M, Shen Z K, Niu Z J, et al. Contemporary crustal deformation of the Chinese continent and tectonic block model. Science in China (Series D) (in Chinese) , 2003,33 (Suppl. ):25-40
[24] 徐锡伟,闻学泽,陈桂华等.巴颜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)-地球科学,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. Science in China (Series D), 2008, 51(9) :1209-1223
[25] Shen Z K, Lu J N, Wang M, et al. Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau. J. Geophy. Res., 2005, 110(B11409) , doi:10． 1029/2004JB003421
[26] 张培震,邓起东,张国民等.中国大陆的强震活动与活动地块.中国科学(D辑),2003,33(增刊):12-20 Zhang P Z, Deng Q D, Zhang G M, et al. Active tectonic blocks and strong earthquake activities in the continent of China. Science in China (Series D) , 2003, 33(Suppl.) :13-24
[27] 王椿镛,韩渭宾,吴建平等.松潘-甘孜造山带地壳速度结构.地震学报,2003,25(3) :229-242 Wang C Y, Han W B, Wu J P, et al. Crustal strcture beneath the Songpan-Garze orogenic belt. Acta Seismologica Sinica, 2003, 16(3) :237-250
[28] 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
[29] 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_S8． 0 earthquake of 2008． Chinese J. Geophys(in Chinese), 2008, 51(5) :1385-1402
[30] Tapponnier P, Peltzer G, Le Dain A Y, et al. Propagating extrusion tectonics in Asia: New insights from simple experiments with plasticine. Geology, 1982,10(12) :611-616
[31] Zhang P Z, Shen Z K, Wang M, et al. Continuous deformation of the Tibetan Plateau from Global Positioning System data. Geology, 2004, 32 (9) : 809-812, doi: 10． 1130/G20554． 1
[32] 黄媛,吴建平,张天中等.汶川8． 0级大地震及其余震序列重定位研究.中国科学(D辑),2008,38(10) :1242-1249 Huang Y, Wu J P, Zhang T Z, et al. Relocation of the M8． 0 Wenchuan earthquake and its aftershock sequence. Science in China(Series D), 2008, 51(12) :1703-1711
[33] 李勇,周荣军,Densmore A L等.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． 1-148 Li Y, Zhou R J, Densmore A L, et al. Geodynamic Processes of Eastern Margion of the Tibetan Plateau and Its Geological Responses (in Chinese). Beijing: Geological Publishing House, 2006． 1-148
[34] Maddy D, Bridgland D R, Green C P. Crustal uplift in southern England: Evidence from the river terrace records. Geomorphology, 2000, 33:167-181
[35] 张勇,冯万鹏,许力生等.2008年汶川大地震的时空破裂过程.中国科学(D辑),2008,38(10) :1186-1194 Zhang Y, Feng W P, Xu L S, et al. Rupture process of space-time of the Wenchuan strong earthquake in 2008． Sciencein China (Series D) (in Chinese) , 2008, 38(10) : 1186-1194
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