In this clip (an excerpt from a CD), we hear three weeks of continuous recordings of earth vibrations, as recorded simultaneously by four seismometers in North America and Asia. Time has been compressed here 10,000-fold; the entire 3-minute recording spans 3 weeks of real Earth time.
You can hear dozens of earthquakes, each of which triggers a frog-like “chirp” of Rayleigh (surface) waves that can travel great distances along the Earth’s thin crust. An extraterrestrial familiar with the physics of wave propagation would correctly deduce from these ascending chirps that the density of rock in the Earth’s crust increases with depth.
The remarkable similarity of these chirping surface waves to the sounds of a dawn chorus of bubbling kassina frogs is purely coincidental. Or is it?