Spatial variations of excitation amplitudes of Earth's background free oscillations

The phenomenon of Earth's background free oscillations has now been confirmed firmly [Kobayashi and Nishida, 1998; Nawa et al., 1998; Suda et al., 1998]. Their excitation amplitudes and statistical features indicate that the excitation source is persistent disturbance at or just above the whole Earth's surface. The observed amplitudes also show clear annual variations and acoustic resonance between the solid Earth and the atmosphere, suggesting that the most likely excitation source is atmospheric disturbance although other possibilities, such as oceanic disturbance, cannot be ruled out. Fukao et al. [2002] calculated the synthetic spectra of Earth's background free oscillations, assuming that the atmospheric turbulence generates random pressure forces acting uniformly on the surface of the solid Earth. The calculated spectra were consistent with the observed ones. In order to constrain more narrowly the excitation mechanism of Earth's background free oscillations, we investigated the spatial distribution of the excited amplitudes more closely. We first calculated the cross-spectrum of background free oscillations for every pair of 55 stations from 3 to 6 mHz. We then stacked these cross-spectra for each of the surface points with intervals of 10 degrees in latitude and longitude after a correction for relative phase shift of Rayleigh wave propagation from every surface point to the two paired stations. The cross-spectral amplitudes show a clear spatial pattern dominant in harmonic degree 2. The amplitude maxima are on the Pacific ocean and its antipode, whereas the minima are on North America and its antipode. This result suggests that neither purely uniform atmospheric disturbance nor purely oceanic disturbance explains the observed amplitude pattern of Earth's background free oscillations.