Source distribution of Earth's background free oscillations

Recently a few groups reported existence of Earth's background free oscillations even on seismically quiet days. Observed features suggest that they are persistently excited on or just above the Earth's surface owing most likely to atmospheric and/or oceanic disturbances. To constrain their excitation mechanisms we developed a new method for estimation of spatial distribution of their excitation sources by modeling cross spectra between pairs of stations. The method is to calculate synthetic cross spectra for spatially homogeneous distribution of random sources and invert with them the observed cross spectra to the heterogeneous source distribution. We applied this method to the IRIS records at 54 stations during 1988–2000. The result showed clear temporal variations of spatial patterns. From November to April the spatial distribution shows a degree 1 pattern with the maximum in the North Pacific Ocean. From May to October strong excitation sources are located along the eastern and western Pacific rims through the Indian Ocean. In all the time periods, excitation sources on continents are weaker than in oceanic areas. This temporal variation of the spatial pattern is qualitatively consistent with that reported by Rhie and Romanowicz (2004). However, the excitation sources are not localized in shallow seas, as might be expected from the hypothesis of excitation by ocean infragravity waves, but must be distributed on the whole sea surface.