青藏高原东北缘地区Pn波速度的横向变化

搜集整理了中国国家地震台网和区域地震台网记录的Pn波到时资料，利用层析成像方法分析了青藏高原东北地区上地幔顶部P波速度及其各向异性的横向变化．结果表明，该地区Pn波的平均速度偏高，为8.09 km/s；构造上稳定的柴达木盆地下面，上地幔顶部Ｐ波速度较高，而构造上比较活跃的山西地堑地区波速则较低．1920年发生海原大地震的地区，上地幔顶部是Ｐ波速度偏低的地区．一个值得注意的结果是，与天山挤压带上地幔顶部Ｐ波速度偏低的情况不同，在青藏高原东北缘的祁连山现代挤压变形带地区，上地幔顶部是Ｐ波速度偏高的地区．鄂尔多斯地块下面，波速度不均一，存在高低速的变化．这与该地块四周皆有强地震活动、地块本身可能有整体的构造运动有关．

LATERAL VARIATION OF Pn VELOCITY BENEATH NORTHEASTERN MARGINAL REGION OF QINGZANG PLATEAU

Pn arrival time data are collected from the bulletins of both national and regional seismological network in China. These data are tomographically inverted to map the lateral variation and anisotropy of Pn velocity in the northeastern marginal region of Qingzang plateau. The average Pn velocity in this region is 8.09 km/s, being a little higher than the average for whole China. Higher velocity is found in tectonically stable Qaidam basin,while lower velocity is seen in and around tectonically active Shanxi graben. The region where the 1920 Haiyuan great earthquake occurred show a slightly low Pn velocity. A noticeable result is that, differing from the tectonically compressive Tianshan region, where Pn velocity is low (Pei et al., 2002), the Qilian Mountain region,where the Neo-tectonic deformation is also primarily compressive, shows high Pn velocity. In the uppermost mantle beneath the Ordos plateau Pn velocity is inhomogeneous, varying from higher velocity in southwestern part to lower one in northeastern part. This may be attributed to possible tectonic movement of the Ordos block, as there are strong earthquakes all around the block.