西太平洋下D"区的剪切波各向异性

利用来自汤加—斐济及周边地区发生的47次地震并由中国地震台网中心和IRIS台站记录到的数据，与来自东亚北部及北太平洋地区发生的26次地震并由IRIS台站记录到的数据， 使用ScS-S相对走时分析方法研究了西太平洋下D区的剪切波各向异性， 得到了核幔边界反射点上ScS波径向分量和横向分量的分裂时间， 并计算了D区的地震波各向异性强度. 发现ScS波通过西太平洋下D区时普遍呈现出径向分量走时大于横向分量走时的模式. ScS波的分裂时间值为-0.91~3.21s， 平均值为1.1 s； 各向异性强度值为-0.45%~1.56%， 平均值为0.52%. 观测和分析结果表明， 西太平洋下D区水平流应占支配地位，各向异性结构主要是具有垂直对称轴的横向各向同性介质. 形成这种结构的可能原因是地幔底部水平流导致的核幔边界反应产物或部分熔融物质的形状优选方向, 或下地幔物质的晶格优选方向. 

SHEAR WAVE ANISOTROPY IN D" REGION BENEATH THE WESTERN PACIFIC

Using seismic shear phases from 47 Tonga-Fiji and its adjacent region events recorded by the CENC and IRIS,and from 26 northeast Asia and north Pacific events recorded by IRIS,we studied the shear wave anisotropy in D" region beneath the western Pacific utilizing the ScS-S differential travel time method and obtained the splitting time values between the radial and transverse components of each ScS wave corresponding to each core-mantle boundary(CMB)reflection point.We found that most shear waves involved horizontally polarized shear wave components traveling faster than vertically polarized shear wave components through the D" region.The splitting time values of ScS wave range from-0.91 s to 3.21 s with an average value of 1.1 s.The strength of anisotropy varies from-0.45% to 1.56% with an average value of 0.52%.The observations and analyses show that in the D" region beneath the western Pacific the lateral flow is expected to be dominant and the vertical transverse isotropy may be the main anisotropic structure.This structure feature may be explained by the shape preferred orientation of the CMB chemical reaction products or partial melt and the lattice preferred orientation of the lower mantle materials caused by the lateral flow at lowermost mantle.