Earth's Relentless Hum

Ten years ago, researchers discovered that Earth gives off a constant humming sound, basically imperceptible to the human ear which cannot hear sounds with a frequency below 16 Hertz, and called it the Earth's hum. The sound continues to make itself heard to seismometers even when there is no seismic activity in the Earth's crust, however the source of this sound hasn't been identified even today.

Some researchers proposed that it is determined by the motion of the planet's atmosphere or that of the oceans. New observations made in one of the quietest location on the surface of the planet, a research station in Germany's Black Forest, may provide some insight in the sources of this mysterious sound.

Change in oscillation pattern

Previous observations revealed that the oscillations determining the humming sound have a 'spheroidal' shape, and were probably determined by patches of rock moving in a up and down manner. However, recent observations have revealed that the oscillation pattern has changed to a circular shape, as if vibrations in rock in the upper layers of the Earth's interior are twisted in circles, much like low-speed hurricanes.

Although this change in the oscillation pattern was predicted by scientists, they were not expecting circular waves to have the same strength as the spheroidal ones.

New theory

This change in the oscillation pattern forced scientists to throw out the window basically everything they previously knew about the Earth's hum. The spheroidal oscillation waves could have been triggered by pressure waves inside the planet's core (oceanic or atmospheric waves), while the circular oscillation waves are probably caused by shearing forces in the Earth's crust, and determined by the oceans, atmosphere or maybe the Sun.

"This is a very small signal that is hard to measure, and the excitation is probably due to multiple interactions in a complex system," said Rudolf Widmer-Schnidrig, from the University of Stuttgart. Further investigation will probably provide a better understanding on how the Earth's crust, oceans and atmosphere interact with each other.