HR: 0800h
AN: S31A-0272    [Abstracts]
TI: On excitation of the Earth's background free oscillations
AU: * Kobayashi, N
EM: shibata@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551 Japan
AB: Recently it has been established that the Earth's free oscillations are continuously excited by phenomena other than earthquakes. The excited modes are almost fundamental spheroidal modes with amplitudes slightly less than 1 ngal and their intensities clearly show annual variations (e.g. Nishida et al. 2000) with some biannual component. In particular the mode _0S₂₉ located at the branch crossing with the acoustic modes has excess amplitudes having an annual variation of 30 %. As the cause of such oscillations, randum disturbances in the atmosphere and/or the oceans has been considered. In the previous works, pressure disturbances are treated as random vertical traction forces on the bottom floors of the atmosphere or the oceans. Such disturbances has successfully explained the observed amplitudes of the free oscillations. However the treatments are insufficient for detailed features of the observed spectra. In this study, we treat the pressure disturbances extending in the bottom atmosphere or the oceans and use normal mode eigenfunctions of an earth model including the atmosphere in the calculation of the excitation problem. We also calculated ocean infragravity waves in a range from 7 mHz to 25 mHz excited by the same mechanism. The results show that only the atmospheric disturbances can explain the observed amplitudes of _0S₂₉ and the detailed features of the background spectra in the mHz band. This means that the volume effect of the atmospheric disturbances is necessary for the details. And they also show that the ocean infragravity waves excited by the same mechanism can be responsible for the background spectra in the calculation range. From those results, we conclude that atmospheric disturbances play an important role in the long period background noises.
DE: 3379 Turbulence (4490)
DE: 3384 Acoustic-gravity waves
DE: 4560 Surface waves and tides (1222)
DE: 4568 Turbulence, diffusion, and mixing processes (4490)
DE: 7255 Surface waves and free oscillations
SC: Seismology [S]
MN: Fall Meeting 2005