UC Santa Barbara

Continuous oscillations of the Earth are observed for frequencies between 2 and 7 mHz at almost every seismically quiet site in the world. These oscillations ride on a broad noise peak which spans the frequency band from 3 to 15 mHz and reaches its maximum at 7-9 mHz. We propose an oceanic excitation hypothesis which explains both the modal oscillations and the broad noise peak, specifically the action of oceanic infragravity waves on the solid Earth. Using the estimated amplitudes of oceanic infragravity waves from observation, although they are limited in number at the moment, we show that there is sufficient energy in these waves to excite observed seismic signals; for a given time window, the contribution from a small area (which may be as small as 100 km x 100 km) is all that is required to explain the seismic observations. The advantage of this oceanic mechanism over the previously proposed atmospheric mechanism is in the simultaneous explanation of the above two features in seismograms, whereas the atmospheric hypothesis has only explained the modal oscillations. The oceanic mechanism naturally explains the predominant 6-month periodicity as a result of semi-hemispheric ocean-wave activities in the Northern and Southern hemispheres, showing a good match between seismic data and satellite ocean-wave data both in the amplitude and phase of seasonal variation. Our Earth seems to be filled with ubiquitous propagating Rayleigh waves, generated directly by oceanic infragravity waves, for the frequency band 3-15 mHz.