常時地球自由振動

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タイトル別名
  • Earth’s Background Free Oscillations
  • ジョウジ チキュウ ジユウ シンドウ
  • Earth^|^rsquo;s Background Free Oscillations

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抄録

It has long been believed that only large earthquakes excite free oscillations of the solid Earth. In 1998 a few Japanese groups reported existence of Earth’s background free oscillations even on seismically quiet days. The excited modes are almost exclusively fundamental spheroidal modes with amplitudes of about 0.5 nGal (10-11 m/s2), and they fluctuate persistently in little correlation with their neighboring modes. Cumulative effects of many small earthquakes are too small to explain the amplitudes. These features suggest that the background free oscillations are excited persistently by random disturbances globally distributed near the Earth’ surface. The intensities of these modes clearly show annual and semiannual variations with the largest peak in July and a secondary peak in January. The observed amplitudes of some modes are anomalously large relative to the adjacent modes. These are the modes that are theoretically expected to be coupled with the acoustic modes of the atmospheric free oscillations. All of these features suggest that atmospheric disturbance is one of the most likely excitation sources of this phenomenon. Assuming that atmospheric pressure disturbance acting on the Earth’s surface is a primary excitation source, some quantitative comparison has been made between the atmospheric pressure disturbance and Earth’s background free oscillations. Shortly after the discovery, some groups proposed oceanic excitation mechanism. This mechanism is based on two observations; (1) The typical frequency of Earth's background free oscillations of about 0.01 Hz coincides with that of ocean bottom pressure sensors. (2) The excitation sources are dominated in the northern Pacific in winter of the northern hemisphere and in the circum-Antarctic in winter of the southern hemisphere. The source distribution is consistent with oceanic wave height data. Based on these observations oceanic excitation mechanism by ocean infragravity waves through their nonlinear interaction was proposed. However, the proposed mechanisms work efficiently only in shore regions, and hence may not be consistent with the observed spatial extent of the excitation sources, and they cannot explain observed acoustic resonance. At present there is little consensus about the excitation mechanism that can explain all of the observed features. These observations suggest that excitation sources are superposition of oceanic and atmospheric phenomena. The phenomena of background free oscillations should be understood as those in a single system of the atmosphere, ocean and solid Earth.

収録刊行物

  • 地震　第２輯

    地震　第２輯 61 (Supplement), 115-121, 2009

    公益社団法人 日本地震学会

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