Mapping seismic anisotropy in the mantle transition zone

智力优势：本研究的目标是通过绘制布伦过渡带（深度400-1000公里）地震各向异性，促进对地球地幔对流过程的基本认识。本研究发展了有限频率理论来解释多模地震表面波的波衍射效应，并应用该理论获得了布伦过渡带的高分辨率、全球各向异性三维结构。本研究解决了全球尺度上地幔动力学的以下重要问题，包括：(1)过渡带是否存在明显的径向各向异性？各向异性结构如何与各向同性速度结构相关联？(2)在过渡带的上方、内部和下方，径向各向异性是否有显著变化？它是否支持过渡区上下的流的耦合或解耦？(3)与板块和羽流相关的布伦过渡带各向异性是否存在显著变化？(4)全地幔对流模型能否解释过渡带径向各向异性的存在和分布规律？布伦过渡带的地震各向异性如何适应整体（或分层）地幔对流？本研究包括以下主要项目：(1)建立多模面波的三维有限频率理论和地幔过渡带不连续深度扰动的二维边界灵敏度核，充分考虑地球的径向各向异性。年代地幔;（ii）建立全球多模面波测量初步数据集；（iii）利用全三维速度核和二维边界核进行衍射层析成像，获得地幔最上层~1000 km地震各向异性的高分辨率全球模型。更广泛的影响：本研究开发的多模面波灵敏度核可以应用于区域研究。层析成像模型将在网上提供。本文所获得的布伦过渡带各向异性模型将为地幔地球动力学建模提供重要参考，并为过渡带水过滤模型等地幔对流假说提供重要的地震学约束[Bercovici and Karato， 2003]。本项目经费将用于培养1名博士研究生。

Intellectual Merit: The goal of this research is to advance fundamental understanding of the Earth's convection processes in the mantle through mapping seismic anisotropy in the Bullen transition zone (depth 400-1000 km). This research develops finite-frequency theory to account for wave diffraction effects in multi-mode seismic surface waves, and applies the theory to obtain high-resolution, global anisotropic 3-D structure in the Bullen transition zone. This research addresses the following important questions on mantle dynamics at a global scale including: (1) Is there significant radial anisotropy in the transition zone? How does the anisotropic structure correlate with the isotropic velocity structure? (2) Is the there any significant change in radial anisotropy across (above, within, below) the transition zone? does it support coupling or decoupling of the flow above/below the transition zone? (3) Are there any significant variations in anisotropy in the Bullen transition zone correlated with slabs and plumes? (4) Can a whole-mantle convection model explain the presence/absence and the pattern of radial anisotropy in the transition zone? How does seismic anisotropy in the Bullen transition zone fit into whole (or layered) mantle convection? This research includes the following main projects: (i) develop 3-D finite-frequency theory for multi-mode surface waves and 2-D boundary sensitivity kernels for mantle transition zone discontinuity depth perturbations, fully account for radial anisotropy in the Earth?s mantle; (ii) build a preliminary global dataset of multi-mode surface-wave measurements; and (iii) diffractional tomography using full 3-D velocity kernels and 2-D boundary kernels to obtain high-resolution global models of seismic anisotropy in the uppermost ~1000 km of the mantle. Broader impacts: The multi-mode surface-wave sensitivity kernels developed in this research can be applied to regional studies. Tomographic models will be made available on-line. The anisotropic model in the Bullen transition zone obtained in this proposed work will provide important references for mantle geodynamical modelings, and provide important seismological constraints on mantle convection hypothesis such as the transition-zone water filter model [Bercovici and Karato, 2003]. The support requested in this project will be used to educate one Ph.D graduate student.