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Summary.

The effect of modal coupling is examined for torsional eigenvibrations of an anelastic earth with oceanic and continental structure. While a coupling effect is hard to observe in the eigenspectrum, some eigenfunctions show significant coupled energy into a secondary neighbouring multiplet. The deeper the heterogeneity, the greater the coupling. Also, stronger coupling is observed in an elastic earth model than in an anelastic model because the poles of the two modes in the complex s-plane are closer in the elastic case than in the anelastic case. The scatter of Q values in a multiplet is less than 5 per cent in our examples but, in extreme cases, strong coupling produces Q values that range between the two Q values for the average earth models. The result indicates that caution is needed in determining Q values for modes that interfere.

The results strongly suggest that necessity of incorporating coupling effects in the formulation of inversion aimed at resolving heterogeneity in the Earth.