地核和下地幔底部结构和动力学探测

The project focuses on structure and dynamics of the Earth’s core and lowermost mantle. This is an exciting time for deep earth research with the explosion of high-quality broadband digital data, emerging of powerful new imaging techniques, ever increasingly computational power, as well as surprising results. Specifically, we will focus on four related areas. (1) Mapping the 3D structure of inner core anisotropy. Previous studies have shown complex structure of the inner core. The detailed picture of 3D inner core anisotropy and heterogeneity is far from clear. We will greatly expand our PKP data set from data sources available to us. We will also explore a new method of using ambient noise correlation to extrace core phases. (2) Modeling regional and local anisotropic structure of inner core. We will use waveform modeling and inversion methods to to sharpen up possible sharp boundaries or transition zones inside the inner core. (3) Determining the mode of the inner core motion. A steady inner core rotation is not consistent with the observation of hemispherical variation of inner core strucutre. The project seeks to address the mode of inner core motion (e.g., variable rotation, oscillation, or deformation). (4) Joint inversion of the inner core structure and the lowermost mantle structure. Recent exciting results have shown strong evidence that the inner core, outer core, and the lower mantle are a coupled dynamic system. We propose to use joint inversion methods to improve resolution of both inner core anisotropy and lowermost mantle P-wave heterogeniety, which in turn is important in understanding the natue of the core-mantle boundary. The project has important implications for understanding the origin of the anisotropy, and the composition, dynamics, and evolution of the Earth’s core and inner core, and on the geodynamo, geomagnetism, Earth’s rotation, and core-mantle coupling.

本项目重点研究地核和下地幔底部结构和动力学。我们的目标主要放在以下四个相关领域：1）研究内核的三维各向异性结构。前人研究揭示了内核的复杂结构，但其三维各向异性结构还不清楚。我们将获取大量的PKP相对到时差的数据，并尝试使用环境噪声相关方法提取地核体波震相的新方法。2）拟合内核局部各向异性结构变化。主要通过波形拟合和反演方法提高内核里可能的间断面或过渡带的分辨率。3）确定内核运动模式。内核稳定旋转模式与内核结构半球变化观测并不一致，我们将探索内核运动模式(如变速旋转、摆动、变形)。４）内核和下核幔底部速度结构联合反演。近年研究结果表明内核、外核及下地幔是一个动力学耦合系统。我们计划通过联合反演以提高内核各向异性、下地幔底部结构的分辨率。本项目对我们理解内核各向异性的起源，地核的组成、演化和发展，以及研究地球磁流体发电机、地球磁场、地球转动和核幔耦合都有着非常重要的意义。

本研究项目目标已经基本完成，同时在项目执行过程中我们在某些方面取得的杰出的成果。利用相似地震，PKPdf与PKPcd残差时间变化尺度上变化，发现在中美洲和非洲下方内外核边界存在局部起伏。通过噪声互相关的方法首次提取到PKP三重震相（PKPab, PKPbc, PKPdf）(Xia et al., 2016)。 利用大地震尾波自相关的方法提出到了PKIKP2 和PKIIKP2震相，并利用其发现地球内内核近赤道面的各项异性快轴。与此同时发现太平洋北部核幔边界存在异常的高速分布,非洲西北部核幔边界存在异常低速区域。