自由振動から見たコアの地震学的構造

Abstract

Analysis of the Earth's free oscillations using high-quality digital seismograms accumulated over the past 15 years has revealed the large-scale aspherical structure of the Earth. Recently modes of low harmonic degree have been analyzed using a technique of iterative inversion in which spectra of a narrow frequency band which contains a mode of interest are fitted. The modes analyzed fall into two categories: (1) normally split modes which are dominantly sensitive to the structure of the mantle and (2) anomalously split modes which are dominantly sensitive to the structure of the outer and inner cores. Relatively simple mantle models are capable of explaining the structure coefficients of the normally split modes, and these models are quite similar to the models derived from traveltime data. However, all the structure coefficients of the anomalously split modes have not yet been given good fits by a simple aspherical model. While a model including anisotropy of the inner core can give better fits to some of the most anomalous modes, what we can safely say at present is that there is degree-2, axisymmetric asphericity somewhere in the deep Earth. It should be also kept in mind that the core has not yet been given its correct spherically symmetric model.