HR: 16:30h
AN: S34B-03 INVITED     [Abstracts]
TI: The Earth's Hum: A Review
AU: * Fukao, Y
EM: fukao@jamstec.go.jp
AF: IFREE/JAMSTEC, 2-15 Natsushima, Yokosuka, 237-0061 Japan
AU: Nishida, K
EM: knishida@eri.u-tokyo.ac.jp
AF: ERI/University of Tokyo, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kobayashi, N
EM: shibata@geo.titech.ac.jp
AF: Dept Earth Planetary Sciences, Tokyo Inst Technology, Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AB: The Earth's background free oscillations (Earth's hum) is a global phenomenon involving both the solid Earth and the surrounding atmosphere and ocean. The excited modes of the hum are almost exclusively fundamental spheroidal modes, and fluctuate persistently in little correlation with their neighboring modes, implying that the hum is excited incessantly by random disturbances globally distributed near the Earth' surface. The intensity of the hum varies annually with some semiannual component, suggesting excitation sources outside the solid Earth. The modes theoretically expected to be coupled with those of acoustic free oscillations of the atmosphere show in fact anomalously large amplitudes. Assuming atmospheric random turbulence as the excitation source and using a coherent length value extrapolated from barometric observations outside the seismic band, the calculated synthetic spectra of the hum show a remarkable agreement with the observed spectra at frequencies below 8 mHz. Dispersion analysis of the hum indicates that the lowest-level ground noise in the entire seismic pass-band of 2-20 mHz comprises of spheroidal oscillations or Rayleigh waves of exotic origin. The spatial pattern of the hum is dominated in the spherical harmonic degrees of 0 and 1, with a greater power of the degree-0. The degree-1 component has its peak in the northern Pacific in northern-hemisphere winter and in the southern Pacific in southern-hemisphere winter. The observed anomalous amplitudes of the spheroidal modes resonant with the atmospheric acoustic modes can be explained quantitatively by random pressure sources placed near the bottom of the atmospheric layer. High resolution observations of atmospheric and oceanic disturbances in the seismic band are necessary to constrain more narrowly the possible origin of the Earth's hum.
DE: 3384 Acoustic-gravity waves
DE: 7255 Surface waves and free oscillations
SC: Seismology [S]
MN: Fall Meeting 2005