The Earth hums when no earthquake is happening
The planet oscillates continuously, at periods of a few minutes, with amplitudes of nanometres, even in the absence of large quakes.
Seismometers, compared across quiet stations, show a persistent signal between about 2 and 7 millihertz: the Earth expanding and contracting in its free modes, all the time. The amplitudes are nanometres to micrometres at the surface. A 1998 paper established that the signal is not a leakage from storms of earthquakes. It is a background. Later work has tied a good part of it to the ocean, long-period waves pushing on the seafloor, so the hum is weather coupled to the whole planet.
These are the same modes a great earthquake rings up, already an entry here, except the earthquake version is louder and dies away. The hum does not die away. It is what the planet is doing on an ordinary day, under the noise of trucks and surf, at a pitch of several minutes per cycle. Hearing is not the right sense for it. A gravimeter is.
Caveats
- The hum is not a tone in air and not a sound passing through rock to your ear. It is a motion of the whole Earth.
- The ocean explanation accounts for much of the energy. It may not account for every peak. The existence of the background does not depend on the argument about its source.
Sources
- 1Suda N., Nawa K., Fukao Y.. Earth’s background free oscillations. Science 279, 1998.