汶川地震的岩石圈深部结构与动力学背景

The Wenchuan earthquake occurrence background in deep structure and dynamics of lithosphere

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.