HR: 0800h
AN: S11A-1009    [Abstracts]
TI: Atmospheric Pressure Oscillations Forced by Surface Waves From the 2003 Tokachi-Oki Earthquake
AU: * Watada, S
EM: watada@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Nishida, K
EM: knishida@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Sekiguchi, S
EM: seki@bosai.go.jp
AF: NIED, 3-1 Tennodai, Tsukuba, Iba 305-0006 Japan
AB: Clear atmospheric pressure changes associated with the 2003 Tokachi-Oki Earthquake with M 8.3 were recorded by 8 microbarographs along Japan. The maximum oscillatory pressure change is about 2 Pascal with dominant period is about 15-20 second, and lasted for more than 30 minutes. Comparing the pressure change with broadband seismic records observed near or at the microbarogram, the pressure change starts at the arrival of seismic waves and reaches its maximum amplitude at the arrival of Rayleigh waves. Four microbarographs, co-located with STS-1 broadband seismographs and suffering less atmospheric wind noise, show that peaks in vertical ground velocity records correspond to the peaks of atmospheric pressure records. Similar pressure changes were observed during the largest aftershock (M 7.4). All ground motion analyzed in this paper were recorded by STS-1 broadband sensors. Spectrum analysis in the frequency domain supports that the vertical ground velocity and the pressure change has the same phase and the amplitude ratio is constant up to a period of about 50 second. The constant amplitude ratio is about (atmospheric density) times (sound velocity in the atmosphere), indicating that the surface ground in vertical ground motion compresses or inflates the air above the ground locally and low-frequency sound waves are generated. Pressure change recorded after the passage of Rayleigh waves does not well correlate with the ground velocity. Through the precise atmospheric pressure and ground motion measurement at the same sites, we witnessed the process of low-frequency sound generation by the vertical ground surface motion acted as a vibrating plate of a speaker. The radiated low-frequency sound waves propagates upward and reaches to the ionosphere with large amplitude because of the energy conservation. The ionospheric turbulence reported in the past researches were originated from this low-frequency sound at the ground surface.
DE: 7255 Surface waves and free oscillations
DE: 7299 General or miscellaneous
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting