汶川地震的岩石圈深部结构与动力学背景
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摘要
中国西部地区由于受到印度板块向北推移挤压,青藏高原强烈变形,高原内部及其边缘的活断层上经常发生强烈地震,是大陆内部最活跃的地震区。汶川8级地震就发生在青藏高原东缘的松潘-甘孜地块与扬子地块交界的龙门山主中央断裂带上。作者利用面波层析成像、跨龙门山的被动源地震观测、爆破地震剖面的结果对震源附近的岩石圈结构和动力学特征进行研究,发现松潘-甘孜地块与扬子地块的岩石圈结构与性质有重大差异。扬子地块岩石圈显示为高速、坚固和稳定特性,而松潘-甘孜地块为低速、软弱及易于破碎。在松潘-甘孜地块中,中地壳内普遍存在一个低速层,它是引起中上地壳推覆运动的滑脱层,龙门山的推覆构造就是上部地壳仰冲的结果。汶川地震震源深度为14 km,正好位于龙门山推覆体的映秀-北川主中央断裂带上。
Because the western region of China is pushed and pressed to the north by the Indian plate,the Qinghai-Tibet Plateau is extensively deformed,and strong earthquakes occur frequently on the active faults in the plateau interior and on its margin.This is a most active earthquake area in the intracontinent.The M=8.0 Wenchuan earthquake occurs in the main central fault zone of the Longmen Mountains belt tending towards NE.The main central fault zone separates the Songpan-Ganzi block and the Yangtze block on the east margin of the Qinghai-Tibet Plateau.This paper uses the results of the surface wave tomographic image,the passive seismological experiment profile and the explosive seismic profile across the Longmen Mountains to research the lithosphere structure and dynamic characteristics of the Songpan-Ganzi block and the Yangtze block.It is discovered that there is a significant difference in the lithosphere structure and the nature between the two blocks.The lithosphere of the Yangtze block has characteristics of high speed,firmness and stable state,but that of the Songpan-Ganzi block has characteristics of low speed,brittleness and easy break.Generally,a low velocity layer exists in the middle crust of the Songpan-Ganzi block.It is the slipping layer to push the upper and middle crust upthrust movement to form the Longmen Mountains belt.The depth of the main shock is 14 km,and the hypocenter is just located in the Yingxiu-Beichuan central fault zone.
Because the western region of China is pushed and pressed to the north by the Indian plate,the Qinghai-Tibet Plateau is extensively deformed,and strong earthquakes occur frequently on the active faults in the plateau interior and on its margin.This is a most active earthquake area in the intracontinent.The M=8.0 Wenchuan earthquake occurs in the main central fault zone of the Longmen Mountains belt tending towards NE.The main central fault zone separates the Songpan-Ganzi block and the Yangtze block on the east margin of the Qinghai-Tibet Plateau.This paper uses the results of the surface wave tomographic image,the passive seismological experiment profile and the explosive seismic profile across the Longmen Mountains to research the lithosphere structure and dynamic characteristics of the Songpan-Ganzi block and the Yangtze block.It is discovered that there is a significant difference in the lithosphere structure and the nature between the two blocks.The lithosphere of the Yangtze block has characteristics of high speed,firmness and stable state,but that of the Songpan-Ganzi block has characteristics of low speed,brittleness and easy break.Generally,a low velocity layer exists in the middle crust of the Songpan-Ganzi block.It is the slipping layer to push the upper and middle crust upthrust movement to form the Longmen Mountains belt.The depth of the main shock is 14 km,and the hypocenter is just located in the Yingxiu-Beichuan central fault zone.
引文
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[19]BURCHFIELD B C,CHEN Z,LIU Y,et al.Tec-tonics of the Longmen Shan and adjacent regions,Central China[J].International Geology Reviews,1995,37:661-735.
[20]CLARK M K,ROYDEN L H.Topographic ooze:building the eastern margin of Tibet by lower crustalflow[J].Geology,2000,28:703-706.
[21]朱介寿,曹家敏,蔡学林,等.东亚及西太平洋边缘海高分辨率面波层析成像[J].地球物理学报,2002,45(5):646-664.
[22]朱介寿.欧亚大陆及边缘海岩石圈的结构特性[J].地学前缘,2007,14(3):1-20.
[23]朱介寿,曹家敏,严忠琼.中国及邻区瑞利面波高分辨率层析成像及其地球动力学意义[J].中国地质,2007,34(5):759-767.
[2]滕吉文,张中杰,王光杰,等.喜马拉雅碰撞造山带的深层动力过程与陆-陆碰撞新模型[J].地球物理学报,1999,42(4):481-493.
[3]赵文津,NELSON K D,车敬凯,等.深反射地震揭示喜马拉雅地区地壳上地幔的复杂结构[J].地球物理学报,1996,39(6):615-628.
[4]周兵,朱介寿,秦建业.青藏高原及邻近区域S波三维速度结构[J].地球物理学报,1991,34(4):426-441.
[5]ZHU L,HELMBERGER D V.Moho offset acrossthe northern margin of the Tibetan plateau[J],Sci-ence,1998,281:1170-1172.
[6]KOSAREV G,KIND R,SOBOLEV S V,et al.Seismic evidence for a detached Indian lithosphericmantle beneath Tibet[J].Science,1999,283:1306-1309.
[7]BRANDON C,ROMANOWICA B.A“no-lid”zone inthe central Chang-Thang Platform of Tibet:Evidencefrom pure path velocity measurements of long periodRayleigh waves[J].J Geophys Res,1986,91:6547-6564.
[8]MOLNAR P.A review of geophysical constraints onthe deep structure of the Tibetan Plateau,the Hima-laya,and the Karakoram,and their tectonic implica-tions[J].Phil Trans R Soc Lond,1988,A326:33-88.
[9]OWENS T J,RANDALL G E,WU F T,et al.PASSCAL instrument performance during the Tibet-an Plateau passive seismic experiment[J].Bull Seis-mol Soc Am,1993,83:1959-1970.
[10]NELSON D,et al.Partially molten middle crust be-neath southern Tibet:Synthesis of project INDE-PTH results[J].Science,1996,274:1684-1687.
[11]ZHU L,OWENS T J,RANDALL G E.Lateralvariation in crustal structure of the northern TibetanPlateau inferred from teleseismic receiver functions[J].Bull Seismol Soc Am,1995,85:1531-1540.
[12]McNAMARA D E.Lithospheric structure of theTibetan Plateau[D].Univ of S C,Columbia,1995.
[13]ZHU J S,CAO J M,YAN Z Q.High resolusionsurface wave tomography in East Asia and West Pa-cific marginal sea:structure of lithosphere and as-thenosphere and dynamics[J].EOS Trans AGU,2003,84(46):F1443.
[14]MONSALVE G A,SHEEHAN V,SCHULTE-PELKUM,et al.Seismicity and one dimensionalvelocity structure of the Himalayan collision zone:Earthquakes in the crust and upper mantle[J].JGeophys Res,2006,111,B10301,doi:10.1029/2005JB004062.
[15]BREWER I D,BURBANK D W.Thermal and kine-matic modeling of bedrock and detrital cooling agesin the central Himalaya[J].J Geophys Res,2006,111,B09409,doi:10.1029/2004JB003304.
[16]CHEN Z,BURCHFIEL B C,LIU Y,et al.Globalpositioning system measurements from eastern Tibetand their implications for India/Eurasia interconti-nental deformation[J].J Geophys Res,2000,105(B7):16215-16227.
[17]KIRBY E,WHIPPLE K X,TANG W,et al.Dis-tribution of active rock uplift along the eastern mar-gin of the Tibetan Plateau:Inferences from bedrockchannel longitudinal profiles[J].J Geophys Res,2003,108(B4),2217,doi:10.1029/2001JB000861,2003.
[18]ROYDEN L H,BURCHFIEL B C,KING R W,etal.Surface deformation and lowest crustal flow ineastern Tibet[J].Science,1997,276:788-790.
[19]BURCHFIELD B C,CHEN Z,LIU Y,et al.Tec-tonics of the Longmen Shan and adjacent regions,Central China[J].International Geology Reviews,1995,37:661-735.
[20]CLARK M K,ROYDEN L H.Topographic ooze:building the eastern margin of Tibet by lower crustalflow[J].Geology,2000,28:703-706.
[21]朱介寿,曹家敏,蔡学林,等.东亚及西太平洋边缘海高分辨率面波层析成像[J].地球物理学报,2002,45(5):646-664.
[22]朱介寿.欧亚大陆及边缘海岩石圈的结构特性[J].地学前缘,2007,14(3):1-20.
[23]朱介寿,曹家敏,严忠琼.中国及邻区瑞利面波高分辨率层析成像及其地球动力学意义[J].中国地质,2007,34(5):759-767.
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