Science & Light

Joanie April 26th, 2008

The sound of the earth is changing…  Joanie
EARTH’S HUM SOUNDS MORE MYSTERIOUS THAN EVER
By Charles Q. Choi
LiveScience
April 16, 2008

http://news.yahoo.com/s/livescience/20080416/sc_livescience/earthshumsoundsm
oremysteriousthanever;_ylt=AjOQz3ZUepDUSnQMFMHdRL4DW7oF

Earth gives off a relentless hum of countless notes completely imperceptible
to the human ear, like a giant, exceptionally quiet symphony, but the origin
of this sound remains a mystery.

Now unexpected powerful tunes have been discovered in this hum. These new
findings could shed light on the source of this enigma.

The planet emanates a constant rumble far below the limits of human hearing,
even when the ground isn’t shaking from an earthquake. (It does not cause
the ringing in the ear linked with tinnitus.) This sound, first discovered a
decade ago, is one that only scientific instruments — seismometers — can
detect. Researchers call it Earth’s hum.

Investigators suspect this murmur could originate from the churning ocean,
or perhaps the roiling atmosphere. To find out more, scientists analyzed
readings from an exceptionally quiet Earth-listening research station at the
Black Forest Observatory in Germany, with supporting data from Japan and
China.

Different types

In the past, the oscillations that researchers found made up this hum were
“spheroidal” — they basically involved patches of rock moving up and down,
albeit near undetectably.

Now oscillations have been discovered making up the hum that, oddly, are
shaped roughly like rings. Imagine, if you will, rumbles that twist in
circles in rock across the upper echelons of the planet, almost like dozens
of lazy hurricanes.

Scientists had actually expected to find these kinds of oscillations, but
these new ring-like waves are surprisingly about as powerful as the
spheroidal ones are. The expectation was they would be relatively
insignificant.

New thinking

This discovery should force researchers to significantly rethink what causes
Earth’s hum. While the spheroidal oscillations might be caused by forces
squeezing down on the planet — say, pressure from ocean or atmospheric
waves — the twisting ring-like phenomena might be caused by forces shearing
across the world’s surface, from the oceans, atmosphere or possibly even the
sun.

Future investigations of this part of the hum will prove challenging, as
“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 researcher
Rudolf Widmer-Schnidrig, a geoscientist at the University of Stuttgart,
Germany.

Still, a better understanding of this sound will shed light on how the land,
sea and air all interact, he added.

Researcher Dieter Kurrle and Widmer-Schnidrig detailed their findings March
20 in the journal Geophysical Research Letters.

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