北京—萨哈林剖面的地幔纵向速度结构

本文根据北京台网、大连、长春、牡丹江、海参威等26个台站的记录,选用了从北京地区到阿留申群岛西端的300多个地震,研究了北京—萨哈林剖面的地幔纵向速度分布。 用Herglotz-Wiechert公式计算得出的V-h作为初始模型,计算理论走时曲线,找出了与实际走时曲线符合较好的速度结构。其特点为:上地幔顶部的速度为7.8公里/秒;高速盖层中有正速度梯度;在约60—120公里深度范围内为低速层;130公里以下速度缓慢增大;在370—440公里、600—740公里有两个速度梯度较大的过渡层,其中以370—440公里的正速度梯度最大,与之相应的震中距为19°。在1060公里以下有微弱的正梯度。 用Kaila方法计算了深度为400公里左右的地震震源深度处的速度值,与上述结果一致。

THE VERTICLE VELOCITY STRUCTURE IN THE MANTLE OF BEIJING-SAHALIN PROFILE

In this paper,the authors study the velocity distribution with depth in the mantle of Beijing-Sahalin Profile,which is based on the records of more than 300 earthquakes in the area from Beijing to Western Aleutian Is.,selected from 26 stations consisting of Beijing Network,Dalian,Changchun,Mudan Jiang,Vladivoctok,etc.According to Herglotz-Wiechert's formula,the authors calculate the V-h relation and use it as an initial model to compute the theoretical time-distance curves,and find out the velocity structure which agrees well with the results obtained from the practical time-distance curve. The velocity at the top of the mantle is about 7.8 km/sec. In the covering layer with high velocity,there is a positive velocity gradient,and the low velocity layer appears at the depth about 60-120 km. The velocity increases slowly below 130 Km. There are two transitional layers with greater velocity gradient at the depths of 370-440 km and 600-740 km,the former one having the greater velocity gradient,and its corresponding epicentral distance is 19°. Below 1000 km,the velocity increases a little more rapidly.By the method of Kaila,the results from the calculation of the velocity in the sources of deep earthquakes,whose sources are in the depth about 400 km,agree with the results above.