The Earth rings continuously, and the ocean is what strikes it
Even with no earthquake anywhere on the planet, the Earth oscillates in its normal modes at a few millihertz — periods of two to seven minutes, amplitudes well below a nanometre. The energy comes from long ocean waves pressing on the seafloor.
The background hum was first extracted in 1998 from a superconducting gravimeter in Antarctica, by stacking years of data and removing every known earthquake. The excitation is now generally attributed to infragravity waves — very long-period ocean swell — coupling into the solid Earth over the continental shelves, with a seasonal shift in source location that follows the storm belts.
It is a signal that only exists once you subtract everything else. It also means the quiet between earthquakes was never quiet: the instrument was never at rest, and the baseline you assumed was a limitation of your own reading.
A great earthquake rings the whole planet in discrete modes, the slowest of which — designated 0S2 — has a period near 54 minutes and alternately stretches the Earth into a rugby-ball shape along two axes. After the 2004 Sumatra earthquake it was still measurable for weeks. The planet is a bell with a very long decay.
Seismologists worked for a century to remove ambient noise. Then it turned out that cross-correlating the noise between two stations recovers the wave that would have travelled between them, which lets you image the crust with no earthquake and no explosion at all. Ambient-noise tomography is now standard. The garbage was the signal.
“The ocean strikes the Earth” is a simplification of an argument that is still live.
Atmospheric forcing and coupled ocean–atmosphere mechanisms both contribute, the partition varies with frequency band, and source regions shift seasonally. The clean single-cause sentence is a summary of a contested literature, not a settled result.
A hum component that persists with the correct amplitude in a period of globally suppressed infragravity wave activity.