HR: 09:30h
AN: S11D-07    [Abstracts]
TI: Forcing by Infragravity Waves Interacting Over the Deep Ocean Basins is Important to Maintaining the Earth's Seismic Normal Mode Background (the 'Earth's Hum')
AU: * Webb, S C
EM: scw@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, Columbia University, 61 Rt. 9W, Palisades, NY 10964, United States
AB: The recent discovery that the seismic normal modes of the Earth are excited to a nearly constant level during seismically quiet days (‘the hum of the Earth') has lead to much speculation as to what drives the observed background in the absence of large Earthquakes. Other authors have shown that the hum cannot be explained by the many small earthquakes occurring each day and have also precluded excitation by "slow" earthquakes as the source. The nonlinear interaction of low frequency ocean waves (infragravity waves) generates high phase velocity components that excite the seismic normal modes of the Earth. I show the expected forcing of Earth normal modes by infragravity waves interacting over the deep ocean basins is sufficient to explain the background seismic acceleration spectrum observed during seismically quiet days. Previously, I had shown that waves interacting over the continental shelves could also explain the background spectrum. It is likely that both pelagic and shelf ocean wave sources contribute significantly to the forcing of the Earth's hum and both coastal and pelagic source regions have been identified using seismic techniques. Infragravity wave spectral amplitudes in deep water over the Pacific basin are relatively constant, thus explaining the small observed variation in the hum. However, time varying source regions are also observed in the hum, and may be best explained by intense near shore sources. Observations suggesting enhanced spectral levels for two seismic modes that couple strongly with atmospheric modes (OS27 and OS39) have been cited as evidence that the source of the hum lies in the atmosphere. However, the wave interaction mechanism also couples energy into infrasound and thus may explain these observations. A calculation of the coupling between ocean waves and normal modes expected from interacting ocean waves reproduces the vertical acceleration spectrum observed at quiet seismic sites from 2 to 40 mHz including the small "hump" between 5 and 15 mHz and the observed rise at higher frequencies. The model diverges above 40 mHz because the ‘single frequency' microseism peak is generated by a different mechanism. The shape of the hum spectrum is controlled primarily by the elastic properties of the Earth with the attenuation structure controlling mode amplitudes.
DE: 7200 SEISMOLOGY
DE: 7255 Surface waves and free oscillations
SC: Seismology [S]
MN: 2007 Fall Meeting