【摘要】 汶川-映秀8.0级大地震发生在由3条NE向断裂带组成的龙门山逆冲走滑断裂系上,造成了以汶川、映秀为中心和其周边地域的严重破坏和人员的重大伤亡。然而强烈地震发生前却未见有可能的确切征兆或浅表层异常活动的迹象(相对于短期和临震预测),即浅层过程与地震发生的深层过程并不匹配。那么为什么在龙门山地区突然发生了这次大地震,它具有哪些特征?其成因机制又是什么?为此必须对这次强烈地震"孕育"、发生和发展的壳、幔速度结构以及其深层动力过程进行深入研究和探讨。通过对松藩-甘孜、龙门山造山带和四川盆地地域人工源地震深部探测以及天然地震层析成像、接收函数与面波频散反演研究分析,初步结果表明:1)在印度洋板块与欧亚板块陆-陆碰撞和挤压作用下,形成了喜马拉雅造山带和东构造结,该东构造结似一尖楔向NNE方向顶挤并插入青藏高原东北缘,故迫使高原深部物质向东流展,在受到以龙门山为西北边界的四川盆地高速"刚性"物质阻隔下,一部分物质则转而向东南侧向运移;2)龙门山造山带与其西北侧地带地形高差达3500±500m左右,地壳厚度在龙门山西北部为65±5km左右,四川盆地为40±2km左右,而龙门山地带与其东、西两侧相比则其地壳厚度变化幅度达15～20km,这里恰为应力作用的耦合与集中地带;3)下地壳和地幔盖层物质以上、中地壳(深20～25km)中的低速层为第一滑移面,以上地幔软流层顶面为第二滑移面,且在四川盆地深部"刚性"物质阻隔下,下地壳和上地幔盖层物质以高角度在龙门山断裂系与四川盆地的耦合地带向上运移(或称逆冲),且在龙门山地表3条断裂构成的断裂系向下收敛到15±5km左右深处汇聚,二者强烈碰撞、挤压和震源介质破裂;即在物质与能量的强烈交换下,高度集中的应力得到急速释放,故形成了这次8.0级大地震。为此通过该区的壳、幔速度结构变异,初步揭示了这次8.0级大地震"孕育"、发生和发展的深部介质和构造环境及其深层动力过程。更多还原

【Abstract】 The 2008 Wenchuan M_S8.0 earthquake occurred in the Longmenshan fault system had no noticeable precursors.It might imply that the shallow process was matched with the deep process in this gigantic shock.Then why such a strong earthquake occurred in the Longmenshan fault system? What are the characteristics of this earthquake? What is its seismogenic mechanism? This article attempts preliminarily to answer some of these problems through method such as deep seismic sounding,nature earthquake tomography,receiver function,and surface wave dispassion.The India-Eurasia plate collision is the most important dynamic setting of major earthquakes in South Western China.The eastern end of the Himalayan chain is rapidly moving in NNE direction into the Tibetan Plateau,forcing the deep material to flow eastward.Owing to the obstruction of the NE trending Longmenshan fault system on the northwestern margin of the Sichuan Basin,a part of the flow material turns to southeast.The Longmenshan orogenic belt is a conspicuous boundary of topography and crust structure,across which the elevation difference is as large as 3500±500m,with crustal thickness 60±5km in its west and 40±2km in its east.Thus it must experience intense deformation and stress accumulation.It can be inferred from available data that there is the lower crust and upper mantle materials to flow eastward along two deep slide surfaces: one is a low velocity and low-resistivity layer(20～25km deep);the other is the top of the asthenosphere.Hindered by the high velocity rigid material beneath the Sichuan Basin,the lower crust and upper mantle materials move up by high angle overthrusting in Longmenshan.On the other hand,three thrust faults visible west of Longmenshan extend downward to the depth of 15±5km,where they converge,into one fault.Therefore the Longmenshan tectonic zone is a place of the intense exchange of material and energy.It is the sudden large-scale rupture of source medium beneath Longmenshan that released a huge amount of elastic strain and then resulted in the Wenchuan M_S8.0 earthquake.更多还原