The Earth keeps ringing between earthquakes
Even when no major earthquake is occurring, Earth oscillates continuously in very low-frequency normal modes with amplitudes far below ordinary sensation. Ocean waves are a major source of excitation, with atmospheric and coupled processes contributing across frequency bands.
The background hum was extracted in the late 1990s by stacking long gravimeter and seismometer records and removing known earthquakes. Later studies linked seasonal source regions to storm-driven ocean activity and pressure on the seafloor. The signal is not one simple global note: different modes and frequency bands can receive energy through different atmosphere–ocean–solid-Earth couplings.
The supposed “quiet” baseline was only an instrumental limitation. Once measurement and averaging improved, that apparent background noise became a persistent planetary signal and later a tool for imaging Earth.
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.
Saying that the ocean alone “strikes the Earth” is too clean. Infragravity waves are central to much of the hum, but atmospheric forcing and coupled mechanisms also contribute. The observation survives; the single-cause metaphor does not.
A hum component that persists with the correct amplitude in a period of globally suppressed infragravity wave activity.
What have you been treating as background because your instrument could not resolve it?
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