Detection of background acoustic free oscillations and development of a theory of background free oscillations in the system of the solid Earth and the atmosphere

点击翻译按钮获取中文摘要

In an eventual aim to detect background long period acoustic waves in the atmosphere that have to be excited incessantly by the same sources as for background free oscillations of the solid Earth, we installed a cross array of barometers in a 10 km--wide university forest in central Honshu. The array has 28 micro--barometers employing quartz crystal resonator technology with a station spacing of about 500 m. We analyzed 1-second continuous sampling records in a time period from March 2002 to November 2003 to obtain one--dimensional frequency--wavenumber spectra and two-dimensional frequency--slowness spectra to detect three kinds of waves in the seismic band from 1 mHz to 0.5 Hz. We measured the dispersion curves of these waves with an assumption of stochastic stationary plane waves. The waves we detected are (1)background internal gravity waves traveling from northwest at frequencies from 1 to 5 mHz with a phase velocity of about 50 m/s, (2)background acoustic waves traveling from nor … More thwest at frequencies from 0.01 to 0.1 Hz with a frequency-dependent phase velocity of about 400 m/s at 0.1 Hz and about 1000 m/s at 0.01 Hz, and (3)microbaroms traveling from southeast at frequencies from 0.1 to 0.5 Hz with a phase velocity of about 350 m/s. Internal gravity waves (1) dominate over the random component of atmospheric turbulence in the relevant frequency range. At higher frequencies the random component with coherent length shorter than 500m is dominant. Background acoustic waves (2) show clear annual variation with maxima in winter. The annual variation and their incident azimuths suggest their possible origins in mountain regions. Microbaroms (3) are most likely to be excited by standing ocean waves in nearby coastal regions. The power spectra of these acoustic waves were compared to the seismic spectra obtained in the same university forest. Their spectral shapes are mutually similar and change commonly with variable weather conditions, indicating that the microbaroms and microseisms have their common origins. Less