ON THE STABILITY OF THE EARTH'S FLUCTUATIONS SPECTRAL PEAKS AT THEIR LOWEST AMPLITUDE LEVEL

Abstract

We studied Earth's fluctuations at its lowest level of excitation, in an attempt to gain insight into the underlying features of such activity. We used a set of 42 worldwide broadband seismic stations that were individually analyzed for at least one year in successive 30-minute windows. The power spectrum of each window was computed and, for each station, the series of spectra were sequentially compared and the frequency bin of lowest amplitude was selected. This resulting "pseudo" spectrum contains the minimum contribution of signal that is present in all 30-minute windows (minimum amplitude for each frequency component). We refer to this "pseudo" spectrum as the Base Level Noise Seismic Spectrum. By comparing the minimum spectra at different stations, we find that for continental stations all spectra coalesce to a common shape and amplitude. This feature is notable for the northern hemisphere stations. Continental stations in the southern hemisphere share the same shape and predominant peaks as those of the northern hemisphere, but with higher amplitude. Stations at oceanic or continental margin sites differ slightly from continental stations, presenting strong fluctuations but sharing the same spectral peaks. These results confirms our previous findings that microseism activity at its lowest level can be interpreted as the resonant response of the Earth.