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Comment on “Incessant excitation of the Earth’s free oscillations” by Nawa et al.


The report of Nawa et al. (1998) that incessant excitation of the Earth’s free oscillations was discovered in the low frequency spectra of the superconducting gravimeter at the Syowa station in Antarctica is reexamined. The mean spectra for Syowa are compared with those for Matsushiro, another superconducting gravimeter station in an extremely calm circumstance, hence showing a much lower noise level than Syowa. Although the spectra for Syowa have many peaks below 3 mHz corresponding to those present in the time-frequency diagram of Nawa et al., no similar peaks are found in the spectra for Matsushiro. This result suggests that the peaks below 3 mHz in the Syowa spectra are not real signals of the Earth’s free oscillations.


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Correspondence to Yuichi Imanishi.

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Imanishi, Y. Comment on “Incessant excitation of the Earth’s free oscillations” by Nawa et al.. Earth Planet Sp 50, 883–885 (1998).

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  • Gravity Data
  • Free Oscillation
  • Lower Noise Level
  • Superconducting Gravimeter
  • Syowa Station