Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.s12a1185r&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #S12A-1185
Other
7255 Surface Waves And Free Oscillations
Scientific paper
Fundamental spheroidal free oscillations of the Earth appear, even on seismically quiet days, as continuous vertical straight lines on the frequency-time diagrams. The oscillations are persistent through the 10 years of data presented from the WUS (GEOSCOPE) station. Analysis of four years of TAM (GEOSCOPE) station data with simultaneous seismic-pressure recordings allows us to improve the resolution of these background oscillations. To reduce the atmospheric effect, we compute the pressure-acceleration transfer function and subtract the pressure effects from the acceleration signal. The 'cleaned' acceleration signal better resolves the free oscillation signal and allows us to isolate some very low frequen cy spheroidal fundamental modes (in the angular order range l = 2-15) and some radial modes after large earthquakes. The existence of a permanent free oscillation signal is verified, but the source of this phenomenon is not clearly defined. Before discarding a seismic source, we address the definition of "seismically quiet days". We found that severe selection criteria leads to only a few of these days. The favourite candidate for the permanent excitation of the Earth's normal modes is the coupling between the Earth and the atmosphere. We detect a small annual variation of the phenomenon, with an increased amplitude of some peaks in June- July that Terra and Tanimoto (1999), Tanimoto and Um (1999), Nishida et al. (1999, 2000) relate to solar activity. This hypothesis requires further analysis. If atmospheric forcing drives the excitation, this may be one of the rare phenomena indicating a direct solid earth-atmosphere interaction. Understanding this problem will clearly add another dimension to our understanding of the earth. It may also improve our understanding of other planets, because an atmosphere-surface coupling should also occur in other terrestrial planets. The existence of such phenomenon may allow us to study the interior structure of even tectonically quiet planets.
Clévédé Eric
Crawford Wayne
Roult Geneviève
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