Array observation of background atmospheric waves in the seismic band from 1 mHz to 0.5 Hz

Abstract

SUMMARY 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 microbarometers employing quartz crystal resonator technology with a station spacing of about 500 m. We analysed 1‐s continuous sampling records in a time period from 2002 March to 2003 November to obtain 1‐D frequency–wavenumber spectra and 2‐D 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 travelling from northwest at frequencies from 1 to 5 mHz with a phase velocity of about 50 m s−1, (2) background acoustic waves traveling from northwest at frequencies from 0.01 to 0.1 Hz with a frequency‐dependent phase velocity of about 400 m s−1 at 0.1 Hz and about 800 m s−1 at 0.01 Hz and (3) microbaroms travelling from southeast at frequencies from 0.1 to 0.5 Hz with a phase velocity of about 350 m s−1. 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 500 m 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.

On the minimum of vertical seismic noise near 3 mHz

“…An important atmospheric phenomenon in the frequency band under discussion are acoustic‐gravity waves (AGW, e.g. Gossard & Hooke 1975; Nappo 2002; Nishida et al 2005). Neumann (1997), Neumann & Zürn (1999) and Zürn (2002) calculated the signals produced by a plane sinusoidal density wave where , and the resulting pressure distribution with travelling in x ‐direction along the surface of an elastic half‐space (λ and μ as above) with horizontal phase velocity c h and horizontal wave number k h =ω/ c h .…”

“…The examples above suffer from the fact that only local pressure is available to study them. A sensitive small microbarograph array with a typical dimension of only very few km s (Egger et al 1993; Nishida et al 2005) would be very helpful to improve the understanding of these noise sources in terms of propagation direction and horizontal speeds. Such an array is in the preparation stage at BFO.…”

On reduction of long-period horizontal seismic noise using local barometric pressure

“…It can be shown (e.g. Gossard & Hooke 1975; Neumann 1997; Nappo 2002; Nishida et al 2005) that the atmosphere supports AGW. If such a wave travels along the surface and its amplitude decays exponentially with height z it is called a Lamb wave and its density distribution can be described by where is the peak density at the bottom of this model atmosphere, k h the horizontal wavenumber, , and ω the angular frequency.…”

Stochastic excitation of seismic waves by a hurricane

“…Under the assumption that the correlation length in the surface pressure field is much smaller than the wavelength of seismic waves, we can simplify equation further. This condition is satisfied in our problem because the wavelengths of seismic waves are over 100 km for the frequency range 0.01–0.02 Hz, whereas the correlation lengths of pressure are of the order of 1 km or smaller for this frequency range [e.g., Herron et al ., ; McDonald et al ., , Nishida et al ., ]. Figure lends some support for this assumption.…”