Don,

There has been an argument for a few years whether the background hum comes from action in the atmosphere or the oceans. The oceans seem to be winning the battle, although it is not clear exactly where on the Earth the motion is excited.

The way the humming is observed is that the fraction of the time, about a quarter to a half, when there has been a recent large earthquake whose waves are still reverberating is removed. A very large earthquake can get the Earth going for a week or so. Then they (Suda or Tanimoto at UC Santa Barbara) look for and find the spectral peaks you refer to.

It's not clear what to do with this new observation. There are easier ways to find storms than looking for the humming in the Earth they incite.

Tanimoto has explored wind-excited normal modes as a way that one might measure the internal structure of another planet, like Mars, from just landing one or two seismometers, without the terrestrial luxury of earthquakes to provide the seismic sources.

John