| Research Abstract |
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
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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
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