利用地核绕射波PcSdiff和地核反射波PcS研究地幔底部P波各向异性

Core-mantle boundary, separating the solid mantle from the liquid core, plays a leading role in the process of the exchange of heat and mass between the mantle and core. It is probably the most enigmatic and dynamic regime closely linked to the evolution of the Earth's interior. Primary features in the lowermost mantle, or the D" layer, include two large low velocity provinces (LLVPs) under Africa and Pacific and ultra-low velocity zones (ULVZs) near the edge of LLVPs. It has been recognized that, in the D" layer, the elastic property deviates from the rest of the lower mantle and strong radial and lateral heterogeneities prevail. The dynamics and the pattern of mantle flow cannot be resolved without observations of seismic anisotropy and a robust understanding of deformation mechanism or rheology. However, due to limitation of available data, the discussion on lowermost mantle anisotropy has been restricted to S-wave anisotropy based on shear wave splitting. Few study was done to determine P-wave anisotropy near the core-mantle boundary, which is crucial in distinguishing slip systems and deformation mechanisms. We propose a simple, effective and high-resolution method to investigate the D" layer using novel observations of core-diffracted PcSdiff waves and core-reflected PcS waves in the context of teleseismic receiver function methods. This approach will allow us to determine the P-wave azimuthal and radial anisotropy within the D" layer. The result of this proposed work will not only complement/validate existing S wave anisotropy models, but also serve as the basis of distinguishing different mechanisms, mantle flow patterns, as well as the nature of LLVPs and ULVZs.

作为固态地幔与液态地球外核之间的分界面，核幔边界在地幔与地核之间的热量和物质交换过程中扮演着重要角色，是与地球内部演化紧密联系的神秘地带。地幔底部的D"层，主要结构特征包括太平洋下方和非洲下方的大尺度低速异常体和其边界区域的超低速带。在D"层，物质的弹性性质与下地幔其余部分极为不同，且存在着很强的各向异性。若缺乏对该区域地震各向异性和形变机制的研究，则很难厘清其动力学特征和地幔流模式。而目前为止，由于受到可用数据的制约，对地幔底部的各向异性的讨论局限在利用横波分裂技术研究S波各向异性，而P波各向异性的研究为数不多。本项目拟用接收函数方法提取地核绕射波PcSdiff与地核反射波PcS来约束地幔底部特别是大尺度低速异常区内的P波方位各向异性和径向各向异性。本项目的研究结果将成为了解地幔底部的形变机制、地幔流模式以及LLVPs和ULVZs本质的重要途径，并对已有的S波各向异性模型提供补充和验证。

本项目使用接收函数方法提取地核绕射波PcSdiff与地核反射波PcS来约束地幔底部P波结构特征，进行了大量的地震数据分析处理和数值模拟工作，得到以下结果： . （1）收集了全球范围内大中型地震台网2000年以来所记录的6.0级以上地震的三分量波形数据，并提取了高质量地接收函数，叠加得到了清晰的PcS/PcSdiff震相；. （2）设计了不同的地幔底部P波速度结构模型，计算了各模型中的理论接收函数，系统讨论了地幔底部P波速度对PcS/PcSdiff的影响，发现只有当地幔底部存在P波低速异常时，才能显示出与观测数据类似的特征；. （3）测量了全球以及区域台网数据的PcS/PcSdiff走时，发现其走时特征与地幔底部存在负P波速度梯度的理论模型较为接近，表明可能在全球范围内地面底部均存在P波低速异常结构；. （4）计算了PcS/PcSdiff的有限频敏感核，发现因波形转换的存在，PcSdiff绕射波在其转换点附近敏感核十分靠近核幔边界，这与Pdiff、Sdiff绕射波仅对核幔边界上方速度结构敏感不同，表明PcSdiff可能反映地幔最底部的速度结构特征；. （5）对不同方位角的PcSdiff走时进行了分析，暂未发现显著的方位各向异性特征。. 本研究建立了丰富的数据，为进一步利用其研究地幔底部结构奠定了数据基础；并对PcSdiff的基本特征进行了数值模拟研究，对数据解释具有一定的理论指导意义。.