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.

Fundamental mode Rayleigh waves generally show retrograde particle motion at the surface of the Earth. If there exists a thick sedimentary layer, however, reversal of the sign of vertical eigenfunction occurs near the surface, resulting in prograde Rayleigh-wave particle motion at the surface. We show that, for structures similar to those found in the Los Angeles basin (with thickness up to 8 km), surface prograde motion may occur within the frequency band 0.05-0.3 Hz. Although it has been suggested that the effect of gravity on waves in unconsolidated surface layer may be important, partitioning of energy between the elatic and gravitational energy shows that the gravitational energy is less than I per cent and thus is not important. The phenomenon is caused by elastic effects, mainly caused by extremely slow shallow seismic velocities. Observation of prograde elliptical particle motion may be difficult, however, because particle motion is largely horizontal and high microseismic noise exists in the same frequency band.