You can't hear it, but the Earth is constantly humming. And some parts of the world sing louder than others.

After discovering the mysterious low-frequency buzz in 1998, scientists figured out that the Earth's hum is caused not by earthquakes or atmospheric turbulence, but by ocean waves colliding with the seafloor. Now, researchers have pinpointed the source of the Earth's "background noise," and it looks like it's coming primarily from the Pacific coast of North America.

When two waves of opposite direction but similar frequency collide, they create a special kind of pressure wave that carries energy to the ocean bottom. As these waves pound against the sea floor, they generate a constant vibration with a frequency of about 10 millihertz, much too low for humans to hear but easily detectable with seismometers. By comparing the intensity of the hum with the height of waves around the world, scientists can track where the buzz is coming from.

Previous studies suggested that waves from both shallow continental shelves and the deep ocean contribute to the Earth's hum, but new data indicates otherwise. Based on measurements from a seismic observatory called the USArray EarthScope, most of the hum appears to originate from the Pacific coast of North America, with a smaller contribution from the west coast of Europe. Waves from the deep ocean don't seem to make much hum at all.

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The data for this study, published Thursday in Geophysical Research Letters, was gathered from November 2006 to June 2007, so humming patterns might be different during the summer months.

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*Image: Satellite photo of the US West Coast/NASA

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