The Earth rings after a great earthquake
A very large earthquake sets the whole planet into free oscillation. The tones can be recorded for days.
Established2 min read
Any elastic body has normal modes. Earth's are slow: periods of minutes to about an hour, frequencies of millihertz. The 1960 Valdivia earthquake, the largest ever measured, excited them strongly enough that several groups recorded the spectrum and matched it to theory.
The modes split and shift with rotation, and they sample the deep interior the way a bell's note samples the bronze. Modern seismometers see them after smaller great earthquakes too. They are a tool, not a metaphor that got out of hand.
Caveats
- Nothing in this is audible. 'Rings like a bell' describes the mathematics of normal modes.
- The motion at a given place is tiny, far below what a person feels as aftershock.
Sources
- 1Benioff H., Press F., Smith S.. Excitation of the free oscillations of the Earth by earthquakes. Journal of Geophysical Research 66, 1961.