汶川地震的发震构造与破裂机理
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摘要
通过对汶川地震同震地表破裂、余震分布、地壳形变等的考察以及震前构造变形的研究,提出了汶川地震孕育和发生的多单元组合模型,认为川西高原、龙门山和四川盆地岩石圈结构及性质的差异造成了它们在变形方式和应力积累上的差异,其共同作用导致了龙门山断裂带应力的高度积累和突发释放,形成了汶川特大地震。龙门山断裂带具有高摩擦强度和高角度铲型逆冲结构,使得其不仅不易发生变形(但能积累很高的应力)形成震前的"滑动亏损带",而且不易发生微破裂形成震前的"地震空区"。当应力积累超过龙门山断裂带的强度时,就突发破裂形成巨大地震,同震变形和能量释放主要发生在龙门山断裂带,补偿震前的"滑动亏损"、填满震前的"地震空区"。汶川地震发震断裂的高角度铲型逆冲结构对于汶川地震的孕育和发生起着重要的控制作用,断裂面上位移加速导致的正应力下降是高角度铲型逆冲断裂发生破裂的重要原因。
A composite multiple seismogenic source model is presented to interpret the preparation and generation of the Wenchuan earthquake,based on the results of investigations on the coseismic surface rupture,aftershock distribution and crustal deformation of the Wenchuan earthquake and the studies on the tectonic deformation conducted before the earthquake. It is assumed that the differences in the structure and property of the lithosphere beneath the west Sichuan plateau,Longmen Shan and Sichuan Basin led to the differences in deformation behavior and stress accumulation between them,and their joint actions resulted in the high accumulation and abrupt release of stress in the Longmen Shan Fault zone,witch eventually generated the Wenchuan mega-earthquake. As a high-friction high-angle listric thrust,the Longmen Shan Fault zone is not liable to deformation(but high stress accumulation)to form "slip deficit zones" before earthquake,or to micro-rupturing to form "seismic gaps" before earthquake. When stress accumulated exceeded the strength of the Longmen Shan Fault zone,burst occurred,creating the huge earthquake. Coseismic deformation and energy release were concentrated mainly in the Longmen Shan Fault zone to offset the pre-earthquake "slip deficit" and "seismic gap".The high-angle listric thrust structure of the seismogenic fault of the Wenchuan earthquake played an important role in controlling the preparation and generation of the Wenchuan earthquake. The decrease of normal stress resulting from increased displacement rate on the fault plane was the major contributor to the rupture of the high-angle listric thrust fault.
A composite multiple seismogenic source model is presented to interpret the preparation and generation of the Wenchuan earthquake,based on the results of investigations on the coseismic surface rupture,aftershock distribution and crustal deformation of the Wenchuan earthquake and the studies on the tectonic deformation conducted before the earthquake. It is assumed that the differences in the structure and property of the lithosphere beneath the west Sichuan plateau,Longmen Shan and Sichuan Basin led to the differences in deformation behavior and stress accumulation between them,and their joint actions resulted in the high accumulation and abrupt release of stress in the Longmen Shan Fault zone,witch eventually generated the Wenchuan mega-earthquake. As a high-friction high-angle listric thrust,the Longmen Shan Fault zone is not liable to deformation(but high stress accumulation)to form "slip deficit zones" before earthquake,or to micro-rupturing to form "seismic gaps" before earthquake. When stress accumulated exceeded the strength of the Longmen Shan Fault zone,burst occurred,creating the huge earthquake. Coseismic deformation and energy release were concentrated mainly in the Longmen Shan Fault zone to offset the pre-earthquake "slip deficit" and "seismic gap".The high-angle listric thrust structure of the seismogenic fault of the Wenchuan earthquake played an important role in controlling the preparation and generation of the Wenchuan earthquake. The decrease of normal stress resulting from increased displacement rate on the fault plane was the major contributor to the rupture of the high-angle listric thrust fault.
引文
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Zhang P,Shen Z,Wang M,et al.2004.Continuous deformation of the Tibetan Plateau from global positioning system da-ta[J].Geology,32:809—812.
Zhang P Z,Wen X Z,Shen Z K,et al.2010.Oblique high-angle listric-reverse faulting and associated straining proces-ses:The Wenchuan earthquake of12May2008,Sichuan,China[J].Annu Rev Earth Planet Sci,38:353—382.
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Zhao Z,Wang B,Long S,et al.2001.The tectonic active characteristic of the earthquake sequence with magnitude5.0in Mianzhu Qingping,Sichuan in1999[J].Seism and Geomagn Observ and Res,22:30—37.
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Zhu S B,Zhang P.2012.FEM simulation of interseismic and coseismic deformation associated with the2008WenchuanEarthquake[J].Tectonophysics.doi:10.1016/j.tecto.2012.06.024.
张培震,闻学泽,徐锡伟,等.2009.2008年汶川8.0级特大地震孕育和发生的多单元组合模式[J].科学通报,54(7):944—953.ZHANG Pei-zhen,WEN Xue-ze,XU Xi-wei,et al.2009.Tectonic model of the great Wenchuan earthquake of May12,2008,Sichuan,China[J].Chinese Sci Bull,54(7):944—953(in Chinese).
Avouac J P.2003.Mountain building,erosion,and the seismic cycle in the Nepal Himalaya[J].Adv Geophys,46:11—80.doi:10.1016/S0065-2687(03)46001—9.
Burchfiel B C,Royden L H,van der Hilst R D,et al.2008.A geological and geophysical context for the Wenchuanearthquake of12May2008,Sichuan,People's Republic of China[J].GSA Today,18(7):4—11.doi:10.1130/GSATG18A.1
Chen J,Liu Q,Li S,et al.2009.Seismotectonics study by relocation of the Wenchuan MS8.0earthquake sequence[J].Chinese J Geophys,52:390—397(in Chinese).
Densemore A L,Ellis M,Li Y,et al.2007.Active tectonics of the Beichuan and Pengguan Faults at the eastern marginof the Tibetan Plateau[J].Tectonics,26:TC4005.doi:10.1029/2006TC001987.
Feldl N,Bilham R.2006.Great Himalayan earthquakes and the Tibetan Plateau[J].Nature,444:165—170.doi:10.1038/nature05199.
Li S,Lai X,Yao Z,et al.2009.Study on fault zone structures of northern and southern portions of the main central faultgenerated by MS=8.0Wenchuan earthquake using fault zone trapped waves[J].Acta Seismologica Sinica,22:417—424.
Liu Zeng J,Zhang Z,Wen L,et al.2009.Co-seismic ruptures of the12May2008,MS8.0Wenchuan earthquake,Si-chuan:East-west crustal shortening on oblique,parallel thrusts along the eastern edge of Tibet[J].Earth PlanetSci Lett,286:355—370.
Scholtz C H.1990.The Mechanics of Earthquakes and Faulting[M].Cambridge Univ Press.
Scholz C H.1998.Earthquakes and friction laws[J].Nature,391(1):36—42.
Shen Z K,LüJ N,Wang M,et al.2005.Contemporary crustal deformation around the southeast borderland of the Tibet-an Plateau[J].J Geophys Res,110(B11409).doi:10.1029/2004JB003421.
Shen Z K,Sun J,Zhang P,et al.2009.Slip maxima at fault junctions and rupturing of barriers during the12May2008Wenchuan earthquake[J].Nature Geosciences,2:718—724.
Wang Q L,Cui D,Zhang X,et al.,2009,Coseismic vertical deformation of the MS8.0Wenchuan earthquake from re-peated levelings and its constraint on listric fault geometry[J].Earthquake Sciences,22:595—602.
Xu Z,Ji S,Li H,et al.2008.Uplift of the Longmen Shan range and the Wenchuan earthquake[J].Episodes,31:291—301.
Xu X,Wen X,Yu G,et al.,2009,Coseismic reverse-and oblique-slip surface faulting generated by the2008MW7.9Wenchuan earthquake,China[J].Geology,37:515—518.
Zhang P,Shen Z,Wang M,et al.2004.Continuous deformation of the Tibetan Plateau from global positioning system da-ta[J].Geology,32:809—812.
Zhang P Z,Wen X Z,Shen Z K,et al.2010.Oblique high-angle listric-reverse faulting and associated straining proces-ses:The Wenchuan earthquake of12May2008,Sichuan,China[J].Annu Rev Earth Planet Sci,38:353—382.
Zhang Y,Feng W P,Xu LS,et al.2009.Temporal-spatial rupture process of MS8.0Wenchuan earthquake of2008[J].Sci China(Ser D):Earth Sci,38.doi:10.1007/s11430-008-0148—7.
Zhao Z,Wang B,Long S,et al.2001.The tectonic active characteristic of the earthquake sequence with magnitude5.0in Mianzhu Qingping,Sichuan in1999[J].Seism and Geomagn Observ and Res,22:30—37.
Zhu S B,Zhang P Z.2010.Numeric modeling of the strain accumulation and release of the2008Wenchuan,Sichuan,China,earthquake[J].Bull Seism Soc Am,100:2825—2839.
Zhu S B,Zhang P.2012.FEM simulation of interseismic and coseismic deformation associated with the2008WenchuanEarthquake[J].Tectonophysics.doi:10.1016/j.tecto.2012.06.024.
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