体波波形反演对青藏高原上地幔速度结构的研究

采用波形反演方法对青藏高原地区震中距8°-38°范围内的宽频带炸波波形进行拟合，研究该地区上地幔平均速度结构以及上地幔纵、横波速度的横向不均匀性结果表明青藏高原地区的平均地壳厚度约为68km，上地幔盖层平均厚度约为30－40km，速度约为8.10km／s雅鲁藏布江附近地壳厚度最大，约80km，相应的上地幔Pn速度为8．15km／s左右,青藏高原中部地区的地壳平均厚度约68-70km．位于拉萨地块北部的羌塘地块S波速度相对较低，其地壳和上地慢的平均S波速度分别比拉萨地块低1％和2％以上34°N以北，90°E附近的区域存在明显的上地幔P波低速异常区，P波的平均速度小于7．8km／s据此结果及前人工作，推断印度板块的俯冲可能以雅鲁藏布江缝合带附近为界，青藏高原巨大的地壳厚度是由于欧亚板块碰撞造成地壳缩短与增厚引起.

UPPER MANTLE VELOCITY STRUCTURE IN THE QINGHAI-XIZANG PLATEAU FROM INVERSION OF BODY WAVEFORMS

Based on the broadband they waveform data within the range of 8°-38° in the Qinghai-Xizang plateau, the average velocity and its lateral inhomogeneity of upper mantle are investigated by waveform inversion. The results show that in the Qinghai-Xizang plateau, the average crustal thickness is about 68km, and the average thickness of the lid is about 30-40kTn with P wave velocity of 8. 10km/s. The crust is thickest near Yarlung Zangha River, about 80km, the P wave velocity of uppermost mantle reachs about 8. 15 km/s, and S wave velocity is about 4. 7 km/s. In the central of the Lahsa block, the crust is about 68-70 km thick. In Qiangtang block, the S wave velocity of crustal and upper mantle is relatively low, the average velocities of crust and upper mantle are about 1% and 2% less than the in Lahsa block respectively. There is a low velocity zone in upper mantle with P velocity less than 7. 8km/s to the north of 34°N and near 90°E. Based on our and former results, we infer that the subduction of the indian plate is delimitted by the Yarlung Zangbe River, and the thick crust is caused by the crustal shortening and thickening from the continental collision between the indian and Eurasian plates.