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Free oscillations of a fluid sphere in an infinite elastic medium and long-period volcanic earthquakes

Earth, Planets and Space volume 54, pages 91–106 (2002)Cite this article

Abstract

A source model of long-period volcanic earthquakes is presented. We consider that a fluid-filled spherical cavity surrounded by an infinite elastic medium is excited into resonance like the Earth’s free oscillations. The eigenequation of this system is derived in a general manner, making use of the spherical harmonic and spherical Bessel expansions. The solution is given as a complex number; its real part is the eigenfrequency and the imaginary part represents the attenuation coefficient of the oscillation. The eigenmodes are classified into five groups: (1) the compressional modes in a fluid sphere, (2) the compressional modes in a solid medium, (3) the shear modes in a solid medium, (4) the Stoneley modes, and (5) the torsional modes. We apply them to the long-period volcanic earthquake observed at Asama volcano, Japan. Estimating the characteristic frequencies and attenuation coefficients of the observed vibrations and assuming that the primary component (f = 1.73 Hz) corresponds to the fundamental translation mode of a fluid sphere as one of the compressional modes in fluid, we conclude that the resonator which is a spherical cavity of diameter 220 m filled with steam of temperature 500°C and pressure 170 atm is favorable.

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Authors and Affiliations

  1. Department of Earth and Planetary Physics, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, 113-0033, Japan

    Ataru Sakuraba

  2. Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032, Japan

    Jun Oikawa

  3. Ocean Research Institute, University of Tokyo, Minamidai 1-15-1, Nakano-ku, Tokyo, 164-8639, Japan

    Yuichi Imanishi

Authors
  1. Ataru Sakuraba
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  2. Jun Oikawa
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  3. Yuichi Imanishi
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Correspondence to Ataru Sakuraba.

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Sakuraba, A., Oikawa, J. & Imanishi, Y. Free oscillations of a fluid sphere in an infinite elastic medium and long-period volcanic earthquakes. Earth Planet Sp 54, 91–106 (2002). https://doi.org/10.1186/BF03351710

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  • DOI: https://doi.org/10.1186/BF03351710

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