地球内核顶部300km速度和衰减各向异性的区域变化

Regional variations of velocity and attenuation anisotropy at the top 300km of the inner core

衰减结构是地球内核的重要性质,它可以与地球内核的速度结构结合,对内核的形成和演化机制提供更全面的信息.本文系统收集了1991年到2014年全球、区域和临时地震台网的PKPDF和PKPBC数据,研究了澳大利亚、非洲和太平洋中部下方内核顶部300km的速度和衰减各向异性结构.速度结果表明,澳大利亚下方内核的速度没有明显的各向异性,但是非洲和太平洋中部下方的内核具有明显的各向异性,且非洲的速度各向异性强于太平洋中部.同时,相对于AK135模型,澳大利亚的平均速度快0.5%,而非洲和太平洋中部的平均速度与参考模型没有明显差异.对于内核的衰减结构,我们得到以下结果:1)在东西方向,内核顶部200km左右的区域,澳大利亚的衰减最强(Q值在400左右),非洲和太平洋中部的Q值分别在600和500左右.2)澳大利亚下方的内核衰减没有明显的各向异性,非洲和太平洋中部下方的内核衰减存在明显的各向异性.此外,内核在非洲地区的衰减各向异性强于太平洋中部的各向异性.3)最后,内核中三个区域的速度和衰减具有良好的相关性,即高/低速对应于高/低衰减.考虑到以上结果以及三个区域的位置,我们认为内核顶部的速度和衰减结构都存在区域变化,而不是简单的半球变化.这种区域变化很可能是由于核幔边界热结构的不均一性和内核耦合,使得内核顶部的不同区域在形成过程中受不同的变形影响,从而形成铁晶体不同的生长和排列,引发了不同的各向异性特征.

The attenuation structure is an important parameter of the Earth′s core,which can be combined with the velocity structure to provide more comprehensive information on the formation and evolution of the inner core.In this study,we systematically collect PKPDF and PKPBC data from global,regional and temporary seismic networks from year 1991 to 2014,and investigate both the velocity and attenuation anisotropy structures at the top 300 km of inner core.Based on data availability,we select three regions in the inner core,which are beneath Australia,Africa and Central Pacific regions.Our results show regional variations in both velocity and attenuation anisotropies in these regions.There is no obvious velocity anisotropy beneath Australian,but there exist strong velocity anisotropies beneath Africa and Central Pacific regions.Meanwhile,the average velocity is 0.5%faster underneath Australia than AK135 model,but it is similar to that of the reference model underneath Africa and Central Pacific.For the inner core attenuation structure,we have the following results:1)In equatorial direction,at the top 200 km of the inner core,Australia has the strongest attenuation,with Q value around 400.Africa and the Central Pacific have Qvalues at around 600 and 500,respectively.2)Attenuation shows no/weak anisotropy in Australia,but shows obvious anisotropy in Africa and Central Pacific regions.3)Finally,we find that the velocity and attenuation in all three areas have good correlation,with fast/slow velocity corresponds to high/low attenuation.Considering the locations of three regions,we conclude that inner core attenuation exhibit regional variations rather than simply hemispherical pattern.The core-mantle boundary thermal heterogeneities may couple with the inner core heat flow,generate different deformation and growth process at the inner core boundary,and create different iron crystal alignment in different regions,which causes different anisotropy in the inner core.