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02.15.2007 03:12 PM |
Earth's hum linked to coastal waves
* 17:28 15 February 2007
* NewScientist.com news service
* Catherine Brahic
The Earth's hum comes from the bottom of the sea and not from turbulence in the atmosphere, says a US researcher, backing a novel theory put forward in 2004.
The hum is a low rumble continually present in the ground even when there are no earthquakes happening, but is detectable only by very sensitive seismometers. Its frequency is near 10 millihertz, below the range of human hearing.
The Japanese researchers who first described the hum in 1998 suggested it might be caused by turbulent air in the atmosphere pounding on land.
Six years later, a group of researchers led by Barbara Romanowicz from the University of California at Berkeley, US, said the hum was in fact caused by the pounding of waves, not air. The theory was based on observations from several groups of seismometers deployed to locate the hum's source. The rumble appeared to be linked to large ocean storms along certain coastlines, but the Californian team could not explain how waves created it.
The following year, in 2005, Goran Ekstrom at Columbia University in the US, showed that the hum's amplitude correlated with wave energy averaged along coastlines around the world. Now Ekstrom's Columbia colleague, Spahr Webb, says he can show how waves drive the hum.
Webb has applied old work on ocean waves to predict what sort of background noise would be made by waves moving over the shallow ocean floor. He found his prediction closely matched the spectrum of the Earth's hum.
He says the hum is caused by the combination of two waves of the same frequency travelling in opposite directions. The waves alternately cancel out and amplify each other so that the sea surface goes from wavy to flat to wavy. This creates a standing wave that "goes thump, thump, thump on the ocean floor at twice the frequency of the waves you started off with, driving the hum", says Webb.
The waves that run along shallow continental shelves are much larger than those over the deep ocean, and so the force applied by the standing waves is also larger in shallow water. This, says Webb, correlates nicely with Romanowicz's observation that, of all the areas she looked at in the Pacific, the Earth's hum matched up best with waves near the Vancouver coastline, off Canada, where the shelf is shallow and the ocean waves can be enormous.
Webb also says his findings dash claims that Mars might have a similar hum to Earth, as the Red Planet has no oceans. However, he adds that it could have a different kind of hum generated by large "Marsquakes".
Journal reference: Nature (vol 445, p 754)
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