for National Geographic News
Earth hums, and it's the west coasts of Europe and parts of the Americas that are the main sources of the sound, a new study says.
Since 1998 researchers have known that Earth emits a low-frequency hum inaudible to humans. The sound waves register on instruments used to detect earthquakes even when no quakes are occurring.
The team that first discovered Earth's hum suggested the sound could be caused by turbulent air hitting the land.
Later studies, though, concluded that the hum is generated by waves along coastlines, said Peter Bromirski of the Scripps Institution of Oceanography in La Jolla, California.
Those studies had recorded an increase in the hum's intensity when storm-generated waves hit coasts, but no one had pinpointed the exact coasts involved.
West Coast Vibrations
For their new study, Bromirski and fellow Scripps scientist Peter Gerstoft collected data between November 2006 and June 2007 from a U.S.-based array of sensors that can read global seismic activity.
The pair found that the hum is most intense along the Pacific coasts of North and Central America and the west coasts of Europe.
This fits with the theory that the hum is linked to stormy conditions, since big storm waves frequently batter the coasts of Europe and North America, Bromirski said.
"The surprising area was the Pacific coast of Central America," which is not known to be exceptionally stormy, he said.
Still, analysis of the data showed hum intensity increasing when that region was hit by storm waves generated in the South Pacific Ocean.
"Whenever [South Pacific storms] come up, they illuminate a large stretch of [Central American] coastline kind of simultaneously," Bromirski said.
Sound waves from the low-frequency hum can penetrate deep inside the Earth, so Bromirski hopes that researchers will eventually be able to use the hum as a natural tool to study the planet's interior.
"But first," he said, "you have to understand what [the sound waves'] source mechanism is, where they are being generated."
Findings published in the August 6 issue of Geophysical Research Letters.
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