青藏高原东构造结及周边地区上地幔顶部Pn速度结构和各向异性研究

东构造结是印度板块和欧亚板块碰撞的奇点，历来是地学研究的热点.本文在中国及区域地震台网资料所得结果的基础上，增加了1964～2006年的ISC数据，总共349475个Pn波到时资料，着重反演了东构造结及周边地区Pn波速度结构和各向异性.结果表明该区Pn速度结构与现代构造活动存在较明显的关联：四川盆地，印度板块是构造稳定的地块而具有高速异常；藏东南和南北地震带构造活动强烈而显示低速异常，云南南部、中南半岛北部因处于缅甸弧弧后扩张区其低速异常尤为明显. Pn波各向异性表明快波方向在东构造结有一顺时针旋转的趋势：在藏东南是北东方向，然后在南北地震带先转向东南，再转向南，最后在中南半岛北部突变为近东西向. 这是因为青藏高原物质在东流的过程中，相对于东构造结和四川盆地，在藏东南和南北地震带存在强烈的简单剪切变形，而在缅甸弧后扩张区为纯剪切变形所致. SKS快波方向也有类似的旋转趋势，但在中南半岛北部附近，具有近东西向Pn快波方向的区域，比云南南部东西向SKS区域偏南3°左右，这可能是由于该地区岩石圈上部运动比下部更快的结果.

Study on Pn velocity and anisotropy in the uppermost mantle of the Eastern Himalayan Syntaxis and surrounding regions

The Eastern Himalayan Syntaxis (EHS) is a singular point of continental collision between the Indian and Eurasian plates, and has been focused by many geoscientists. We added the ISC data of 1964~2006 to previous data set, Chinese national and provincial events data used by Pei et al, and 349475 Pn arrivals were selected in all. We have obtained seismic velocities and anisotropy in the uppermost mantle around EHS and surrounding regions by performing tomographic inversion of Pn travel times. Comparing Pn velocity variation with geological results, we find they have distinct correlation: particularly high velocities always exist in old stable regions such as Sichuan Basin and Indian Plate. Low Pn velocities lie in active tectonic regions, volcanic areas and magmatic rock regions, for example, eastern Tibetan Plateau and North-South Seismic Zone, southern Yunnan province and northern Indochina. Pn anisotropy results show that the fast directions of Pn are clockwise around the EHS from NE in southeastern Tibetan Plateau, then SE to NS in North-South Seismic Zone. But the direction sharply changes to EW in the northern Indochina. The clockwise Pn fast direction maybe results from movement of mass extruded from Tibet under simple shear, which is caused by mass flow in southeastern Tibetan Plateau and North-South Seismic Zone relative to stable EHS and Sichuan Basin. The EW Pn direction probably results from pure shear, which is caused by compression in NS and extension in EW in Burma back-arc extensional area. The polarization directions of SKS have similar rotation to Pn, but in northern Indochina the oriented EW fast direction area of Pn has a southward offset of about 3° than SKS, which maybe results from the faster movement of upper lithosphere in this area.