川滇地块的震源机制解特征及其地球动力学解释

美国哈佛大学1977——2004年的矩心矩张量结果显示， 我国川西北次级地块、 滇中次级地块的西部及滇中次级地块的东部的应力场特征有明显的差别. 应用滑动矢量拟合法， 反演了这三个区域的应力场特征: 川西北次级地块以近南北向的水平主张应力轴和西倾的主压应力轴为特征； 滇中次级地块的西部以倾向北东东的主张应力轴以及近南北的水平中等主应力轴为特征； 滇中次级地块的东部以南西西——北东东向的水平主张应力轴以及北北西——南南东向的水平主压应力轴为特征. 有限元模拟结果清楚地显示出， 川滇地块在阿萨姆楔附近受到来自印度板块的强烈挤压， 随着远离阿萨姆楔， 这种挤压应力逐渐衰减； 同时， 该地区的主张应力方向明显地形成了围绕阿萨姆楔的环线. 其中， 内部物质性质均匀、 地表和底部边界自由、 侧部边界采用GPS观测约束的弹性有限元模拟显示， 在川西北次级地块， 模拟结果与震源机制解结果相一致； 在滇中次级地块， 模拟结果所显示的图象与震源机制解观测结果有差别， 不仅没有显示出与大面积的东部地区的震源机制解相一致的特征， 反而显示出与该地区西部震源机制解相一致的特征. 通过调节地块内部物质的弹性常数， 可以实现在滇中次级地块东部部分地区出现与震源机制 

FAULT PLANE SOLUTIONS IN SICHUAN YUNNAN RHOMBIC BLOCK AND THEIR DYNAMIC IMPLICATIONS

Harvard Centroid Moment Tensor(CMT)solutions for earthquakes from 1977 to 2004 showed that the stress fields are obviously different in northwestern Sichuan sub-block(NWSSB),western part of Central Yunnan Sub-Block(CYSB)and eastern part of CYSB.The characteristics of the mean stress fields in these three regions are obtained by fitting to CMT solutions.The stress state in NWSSB is characterized by its sub-horizontal tensile principal axis of stress(T axis)in roughly N-S direction and west dipping compressive principal axis of stress(P axis);the one in western part of CYSB is characterized by its ENE dipping T axis and sub-horizontal medium prin-cipal axis of stress(B axis)in roughly N-S direction;the one in eastern part of CYSB is characterized by its sub-horizontal P axis in roughly NNW-SSE direction and sub-horizontal T axis in roughly WSW-ENE direction.Finite element method simulation clearly shows that the Indian Plate imposes great extrusion on Sichuan-Yunnan rhombic block(SYRB)near Assam massif.The value of the simulated compressive principal stress decreases with the distance from Assam massif.The simulated directions of the T axes in SYRB form annular distribution encircling Assam.For a homogeneous elastic medium with free boundary conditions on the top and bottom surfaces as well as the displacement boundary conditions derived from the GPS observations on the lateral boundaries,the computation results are consistent with the Harvard CMT solutions in NWSSB and western part of CYSB,while inconsistent with the Harvard CMT solutions in eastern part of CYSB.The inconsistency in eastern part of CYSB can be reduced when it includes inhomogeneous elastic media.The stress states in NWSSB and western part of CYSB revealed by the Harvard CMT solutions are not local,which are mainly controlled by the boundary force on the whole region.On the other hand,the stress state in eastern part of CYSB given by the Harvard CMT solutions is local,which may be affected by local topography,material inhomogeneity,and the drag force underneath.