Spectral analysis of body waves from the earthquake of February 18, 1956

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Abstract
The earthquake which occurred south of Honshu, Japan, on February 18, 1956 is studied by means of Fourier analysis. The focal depth of the shock is about 450 km and the magnitude is 714 to 712. Three theoretical models of the source mechanism, that is, Type Ia, Type Ib, and Type II, are examined by the observed amplitude spectra of S and ScS waves. It is found that the observed amplitude ratios of the Fourier components between two horizontal components of the S wave and of the ScS wave, respectively, agree well with the theoretical ratios for a Type II source. Under the assumption that spectral structures should be the same at all observing points, the scattering from the mean amplitude is calculated. The result shows that the Type II model is preferable to either of the Type I models. Assuming Honda's volume model, whose radiation pattern corresponds to that of a Type II point source, the radius of the source region is estimated by making use of the amplitude ratio of the Fourier component of the S wave to that of the P wave. The radius of the source is found to be 11 km ± 2 km.

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