HR: 16:00h
AN: S34B-01    [Abstracts]
TI: The excitation of Earth Normal Modes by Ocean Waves and Atmospheric Turbulence
AU: * Webb, S C
EM: scw@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AB: The interaction through the quadratic linearity of the ocean surface boundary conditions of low frequency waves within the ocean wavefield (infragravity waves) creates high phase velocity components of the near surface pressure field that excite the seismic normal modes of the Earth. It is shown that the typical spectrum of infragravity waves over the continental shelves is sufficiently energetic to explain the acceleration spectrum of Earth free oscillations observed during seismically quiet days. For a model spectrum of the infragravity wave field over the shelf which is invariant in frequency, the theory predicts a roughly flat spectrum for the normal mode vertical acceleration spectrum, except for a small "hump" between 5 and 15 mHz, in good agreement with observations. Infragravity waves interacting over the deep ocean basins also contribute to the excitation of planetary modes but at level that is small compared to the contribution from the shelves, because the infragravity wave spectrum is typically less energetic and the mechanism less efficient in deep water. Previous work has shown that the frequency of small earthquakes is not sufficient to explain the background spectrum of planetary free oscillations observed, and other work precludes excitation by "slow" earthquakes. Previous work invoking pressure fluctuations beneath atmospheric turbulence is shown to have greatly overestimated the expected level of excitation of normal modes from this source. The new results suggest atmospheric turbulence is unlikely to excite planetary normal modes on Mars at levels sufficient to be detected with current sensors.
DE: 7200 SEISMOLOGY
DE: 7255 Surface waves and free oscillations
SC: Seismology [S]
MN: Fall Meeting 2005