云南数字地震台站下方的S波速度结构研究

THE S WAVE VELOCITY STRUCTURE BENEATH DIGITAL SEISMIC STATIONS OF YUNNAN PROVINCE INFERRED FROM TELESEISMIC RECEIVER FUNCTION MODELLING

通过对云南数字地震台站的宽频带远震接收函数反演 ,获得了云南地区数字地震台站下方 0— 1 0 0km深度范围的S波速度结构 .结果表明 ,云南地区地壳厚度变化剧烈 ,中甸、丽江等西北部地区 ,地壳厚度达 62km左右 ,景洪、思茅和沧源等南部地区 ,地壳厚度仅为32— 34km .厚地壳从西北部向东南方向伸展 ,厚度和范围逐渐减小 ,至通海一带地壳厚度减为 42km ,其形态和范围与小江断裂和元江断裂围成的川滇菱形块体相一致 .地壳厚度较小的东、南部地区Moho面速度界面明显 ;在地壳厚度较大或变化剧烈的地区 ,Moho面大多表现为S波速度的高梯度带 .云南地区S波速度结构具有很强的横向不均匀性 .1 0km深度以上 ,北部地区S波速度明显低于南部地区 ,在 1 0— 2 0km深度范围内 ,北部地区的S波速度比南部地区高 .地壳内部S波速度界面的连续性较差 ,低速层的深度和范围不一 ,近一半的台站下方不存在明显的低速层 .受南部地区上地幔的影响 ,40— 50km深度范围内 ,S波速度南部高、北部低 ,高速区随深度增加逐渐向北推移 ,低速异常区形态与川滇菱形块体的形态趋向一致 .70— 80km深度的上地幔速度分布与云南地区大震分布具有一定的相关性 .

The S wave velocity structure within the depth of 0—100km beneath digital seismic stations of Yunnan Province was obtained from teleseismic receiver function modelling. The results show that the crustal thickness changes greatly in Yunnan,reaching about 62km in Zhongdian and Lijiang,and decreasing to 32—34km in Jinghong,Simao and Chang yuan in south. The thick crust extends from northwest to southeast,with decreasing of thickness and range,about 42km thick around Tong Hai. The shape and range of the thick crust consist with Chuan Dian diamond block bounded by Xiao Jiang and Yuan Jiang faults. In the eastern and southern regions,the crust is relatively thin, and the velocity contrast across Moho is obvious. In those areas with great variation of crustal thickness or thick crust area,the Moho is characterized by transition zone with high velocity gradient. In Yunnan region,S wave velocity structures show strong horizontal heterogeneity. Above the depth of 10km the S wave velocity in north area is obviously lower than that in south area,but within the depth of 10—20km,the S wave velocity in north is higher than that in south. The velocity interface within the crust is discontinuous,and the depth and range of lower velocity zones change with different seismic stations. There is no obvious crustal lower velocity zone beneath nearly half of the stations. Influenced by the upper mantle in south area S wave velocity within the depth of 40—50km in south area is higher than that in north,high velocity area extents to north, and the shape of lower velocity zone tends to consist with Chuan Dian diamond block. The upper mantle velocity distribution within depth of 70—80km correlate to the distribution of strong earthquakes to certain extend.