Open Access

Incessant excitation of the Earth’s free oscillations

  • Kazunari Nawa1Email author,
  • Naoki Suda1,
  • Yoshio Fukao2,
  • Tadahiro Sato3,
  • Yuichi Aoyama4 and
  • Kazuo Shibuya5
Earth, Planets and Space201450:BF03352080

DOI: 10.1186/BF03352080

Received: 5 November 1997

Accepted: 10 December 1997

Published: 6 June 2014


We, for the first time, report the evidence of incessant excitation of the Earth’s free oscillations, mainly the fundamental spheroidal modes in a frequency range from 0.3 to 5 mHz, based on the three year record of a superconducting gravimeter at Syowa Station, East Antarctica. The frequency-time spectrogram of this record is striped by more than 30 lines at nGal level parallel to the time axis, mostly corresponding to the fundamental spheroidal modes. This spectrogram is characterized by relatively efficient excitation of gravest fundamental modes, enhancement of signal intensities in the austral winter and amplification of signal in the frequency band from 3 to 4 mHz. Assuming that earthquakes are only the sources for the free oscillations, we calculate the synthetic spectrograms, which have not shown such a series of parallel lines as observed. The result of this synthetic test and characteristics of the observed spectrogram suggest that the mode signals we found are not of earthquake origin. We tentatively suggest atmospheric or oceanic origin for this newly discovered phenomenon of the solid Earth.