Music of the Sphere: Why the Earth Hums

Scientists Pinpoint Source of Earth’s "Hum"

It's not just whales that sing, the Earth does too. A pair of physicists at the University of California figured out that the Earth’s hum is caused not by earthquakes or atmospheric turbulence, but by ocean waves colliding with the seafloor.

About ten years ago researchers discovered infrasonic background noise of 3-7 mHz believed to originate from the solid Earth itself. claim to have the most convincing physical evidence yet. Using data from a wide-spread array of seismic recorders, Peter Bromirski and Peter Gerstoft, have pinpointed the Pacific coast of Central America as the dominate source of background noise, with the western coast of Europe acting as the main secondary source.

Hum beam power observations using the USArray EarthScope transportable array, which monitors seismic activity right across the US and its surrounding waters, combined with infragravity wave observations, show that the dominant source area of the Earth’s hum during winter months is the Pacific coast of North America, with the western coast of Europe a secondary source region.

Pbo_obs Within this new view of hum, long-wavelength surface waves - known as ocean swell - act as the mediator between the sky and the sea. As the swell reaches shallower waters, a portion of its energy is then transformed by non-linear processes into infragravity (IG) waves, which have even longer wavelengths than the surface waves. Some of the energy from IG waves affect the Earth, registering as blips at seismic recording stations around the world.

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