Two elastic oscillations at frequencies f_1 and f_2 in an elastically nonlinear material can produce combination tones at the sum (f_1+f_2) and the difference (f_1-f_2) frequencies. If the nonlinearity in the earthu27s interior is sufficiently large, such phenomena can be observed in the earthu27s oscillations. Analyzing long-period seismograms recorded by a superconducting gravimeter installed at Syowa Station in Antarctica, we try to detect the combination tones between the earthu27s tides and the spheroidal free oscillations. The method employed here is bispectral analysis. Since the bispectrum represents the coherency among three components with frequencies of f_1, f_2 and f_1+f_2, it is a powerful tool to analyze the combination tones. The results suggest that the spheroidal oscillations _0S_0 and _0S_5 modes probably interact with the ter-diurnal and diurnal tides, respectively.
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