Study of the atmospheric pressure change as a n excitation source of ground motion by the co-located array of microbarographs and broadband seismographs

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Small but clear atmospheric pressure changes associated with the 2003 Tokachi-Oki earthquake with M 8.3 were recorded by the microbarographs installed along Japanese Island ; at Enoshima in Miyagi, Tsukuba in Ibaragki, Nokogiriyama in Chiba, Kirishima in Miyagaki, Muroto in Kochi, Sugeno in Yamashina, and Nakaizu in Shizuoka. The pressure change starts at the arrival of seismic waves and reaches its maximum amplitude at the arrival of Rayleigh waves. The ground motion is about 5 mm/sec and pressure change is about 2 Pa. The dominant period in the pressure record is about 10 to 15 seconds. The propagation speed of the waves in barograms and seismograms are both about 3.2 km/sec. The transfer function between the vertical ground motion and the pressure change for periods between 10 sec through 50 sec is constructed from the co-locating barograph and seismograph records. The transfer function reveals that the pressure change is propotinal to the vertical ground velocity. Comparison of the observed transfer function with the theoretical one including the finite frequency and wavelength effects for a gravitationally stratified isothermal atmosphere confirms that the observed amplitude and phase of the pressure change is explained by the acoustic coupling between the atmosphere and the ground just beneath the sensors.