Seismic anisotropy estimated from long-period surface waves

地震层析成像技术的发展使对地球内部地震速度的三维结构成像成为可能。同时，三维结构的可视化有助于理解由于地幔对流和俯冲大洋板块的构造而引起的地球物质的运动。另一方面，地震波表现出各向异性的特征。地震各向异性被认为是地幔对流作用下地幔矿物择优取向的结果。通过体波到达时间、地表波和地球的自由振荡等地震资料估算各向异性横向结构和各向同性横向结构，可以了解地幔对流的流动模式。本文通过纵波走时分析、地表波形分析和地球自由振荡频谱反演，研究了地球各向同性和各向异性的横向结构。探讨了各向同性和各向异性横向结构对自由振荡和表面波形的影响。我们发现，非球面结构导致垂直分量的自由振荡谱和sedsmograms中出现准love波和准环面模态。此外，准love波在各向同性和各向异性边界处被Rayleigh-to-Love转换激发。准love波和准环面模态的激发对各向异性横向结构比各向同性横向结构更敏感。这意味着准love模式和准环面模式对各向异性结构以及各向同性地震速度的横向非均质结构提供了约束。因此，我们试图通过结合环面和球面模式混合耦合效应的频谱拟合技术来反演地球各向同性和各向异性的横向结构。通过数值实验，证实了光谱拟合技术可以正确地恢复各向同性和各向异性结构。此外，估计了菲律宾海板块俯冲带纵波的各向同性和各向异性横向结构。研究结果有助于认识板块构造和地幔对流的地幔过程。少

Development of the seismic tomography technique makes it possible to image three-dimensional structure of seismic velocities of the earth's interior. At the same time, visualizing the three-dimensional structure helps to understand the movement of the earth's material due to mantle convection and configuration of subducting oceanic slabs. On the other hand, seismic waves show characteristics of anisotropy. The seismic anisotropy is considered to arise from preferred orientation of mantle minerals due to the mantle convection. We can know the flow pattern of the mantle convection if anisotropic lateral structure, as wen as isotropic lateral structures, is estimated from seismic data such as body wave arrival times, surface waves, and free oscillations of the earth. In this study, we investigate the isotropic and anisotropic lateral structure of the earth by travel time analysis of the P-wave, analysis of surface waveforms, and spectral inversion of the earth's free oscillations. The eff … More ect of isotropic and anisotropic lateral structure on the free oscillations and surface waveforms are examined. We found that the aspherical structure causes quasi-Love wave and quasi-toroidal modes to appear in the free oscillation spectrum and sedsmograms of vertical component. In addition, the quasi-Love wave proved to be excited by Rayleigh-to-Love conversion at a boundary between isotropic and anisotropic regions. Excitation of the quasi-Love wave and quasi-toroidal modes is more sensitive to the anisotropic lateral structure than to isotropic lateral structure. This means that the quasi-Love and quasi-toroidal modes provides constraint on the anisotropic structure as well as laterally heterogeneous structure of isotropic seismic velocities. Thus we try to retrieve the isotropic and anisotropic lateral structure of the earth by a spectral fitting technique incorporating the effect of mixed-coupling between toroidal and spheroidal modes. By the numerical experiment, it is confirmed that the global isotropic and anisotropic structure can be correctly recovered by the spectral fitting technique. Moreover, isotropic and anisotropic lateral structure of P-wave is estimated in the subduction zone of the Philippine Sea plate. Hie results obtained in this study are helpful for understanding the mantle process of the plate tectonics and the mantle convection. Less