Studies on the origin of Earth's background free oscillations

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We have analyzed records from superconducting gravimeters at four stations : Canberra (Australia), Esashi (Japan), Metsahovi (Finland) and Syowa Station (Antarctica). We were able to detect the Earth's background free oscillations from the records at Canberra and Metsahovi in addition to Syowa Station. The background free oscillations were detected most consistently and distinctly at Canberra since the noise level was the lowest among the SG stations used in this study. Although the noise level at Canberra was higher than that at one of the quietest GEOSCOPE stations CAN (Canberra), it was comparable to those at two of the quietest IDA stations ESK (Scotland) and SUR (South Africa). At Esashi it was too high to detect the oscillations. All of the anomalous oscillation features observed at Syowa Station were absent at the other SG stations, and we conclude that they arise from some local phenomena.We have developed a normal mode theory of atmospheric excitation of the Earth's spheroidal oscillations to derive the expression that can be directly compared to the observed spectrum of background free oscillations. The calculated spectrum exhibits distinct peaks of fundamental modes and complex troughs consisting of overtone modes, which well simulate the peaks and troughs of the observed spectrum. The observed and calculated spectra are in marked contrast to the synthetic free oscillation spectrum for seismically quiet days not only in terms of the amplitude level but in terms of the spectral behavior. These results clearly rule out the possibility of excitation source of earthquake origin and strongly support the idea of atmospheric excitation of background free oscillations.