Skip to main content

Incessant excitation of the Earth’s free oscillations


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.


  1. Agnew, D. C., J. Berger, W. E. Farrell, J. F. Gilbert, G. Masters, and D. Miller, Project IDA: A decade in review, EOS, 67(16), 203–212, 1986.

    Article  Google Scholar 

  2. Benioff, H., J. C. Harrison, L. LaCoste, W. H. Munk, and L. B. Slichter, Searching for the Earth’s free oscillations, J. Geophys. Res., 64, 1334–1337, 1959.

    Article  Google Scholar 

  3. Benioff, H., F. Press, and S. Smith, Excitation of the free oscillations of the earth by earthquakes, J. Geophys. Res., 66, 605–619, 1961.

    Article  Google Scholar 

  4. Beroza, G. and T. Jordan, Searching for slow and silent earthquakes using free oscillations, J. Geophys. Res., 95, 2485–2510, 1990.

    Article  Google Scholar 

  5. Buland, R., J. Berger, and F. Gilbert, Observations from the IDA network of attenuation and splitting during a recent earthquake, Nature, 277, 358–362, 1979.

    Article  Google Scholar 

  6. Dziewonski, A. M. and D. L. Anderson, Preliminary reference Earth model (PREM), Phys. Earth Planet. Inter., 25, 297–356, 1981.

    Article  Google Scholar 

  7. Dziewonski, A. M., G. Ekström, and M. P. Salganik, Centroid-moment tensor solutions for October–December 1994, Phys. Earth Planet. Inter., 91, 187–201, 1995.

    Article  Google Scholar 

  8. Gilbert, F. and A. M. Dziewonski, An application of normal mode theory to the retrieval of structural parameters and source mechanisms from seismic spectra, Phil. Trans. R. Soc. London. A, 278, 187–269, 1975.

    Article  Google Scholar 

  9. Kanamori, H. and J. Mori, Harmonic excitation of mantle Rayleigh waves by the 1991 eruption of Mount Pinatubo, Philippines, Geophys. Res. Lett., 19, 721–724, 1992.

    Article  Google Scholar 

  10. Kanao, M. and T. Sato, Observation of tidal gravity and free oscillation of the earth with a LaCoste & Romberg gravity meter at Syowa Station, East Antarctica, Proc. 12th Int. Symp. Earth Tides, Beijing, edited by H. T. Hsu, pp. 571–580, 1995.

  11. Kobayashi, N., Oscillation of solid planets excited by atmospheric motions (abstract in Japanese), Fall meeting of the Jpn. Soc. Planet. Sci., 84, 1996.

  12. Park, J., C. R. Lindberg, and D. J. Thomson, Multiple-taper spectral analysis of terrestrial free oscillations: part I, Geophys. J. R. Astron. Soc., 91, 755–794, 1987.

    Article  Google Scholar 

  13. Sato, T., K. Shibuya, Y. Tamura, M. Kanao, M. Ooe, K. Okano, Y. Fukuda, N. Seama, K. Nawa, K. Kaminuma, Y. Ida, M. Kumazawa, and T. Yuku-take, One year observation with a superconducting gravimeter at Syowa Station, Antarctica, J. Geod. Soc. Jpn., 41, 75–89, 1995.

    Google Scholar 

  14. Sato, T., K. Shibuya, K. Nawa, K. Matsumoto, and Y. Tamura, On the diurnal and semidiurnal tidal factors at Syowa Station, Antarctica, J. Geod. Soc. Jpn., 42, 145–153, 1996.

    Google Scholar 

  15. Tamura, Y., A harmonic development of the tide-generating potential, Marées Terrestres Bulletin d’Informations, 99, 6813–6855, 1987.

    Google Scholar 

  16. Tamura, Y., Y. Aoyama, and K. Nawa, Gravimetric tidal factors at Syowa Station obtained from three-year observations with a superconducting gravimeter, Proc. NIPR Symp. Antarct. Geosci., 10, 1–10, 1997.

    Google Scholar 

  17. Watada, S., Near-source acoustic coupling between the atmosphere and the solid earth during volcanic eruptions, Ph.D. dissertation, Caltech, 1995.

  18. Widmer, R. and W. Zürn, Bichromatic excitation of long-period Rayleigh and air waves by the Mount Pinatubo and El Chichon volcanic eruptions, Geophys. Res. Lett., 19, 765–768, 1992.

    Article  Google Scholar 

Download references

Author information



Corresponding author

Correspondence to Kazunari Nawa.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Nawa, K., Suda, N., Fukao, Y. et al. Incessant excitation of the Earth’s free oscillations. Earth Planet Sp 50, 3–8 (1998).

Download citation


  • Free Oscillation
  • Synthetic Seismogram
  • nGal Level
  • Superconducting Gravimeter
  • Syowa Station