Listen closely, and you'll hear the Earth humming - in not just one note but two. The source of this second signal is a mystery. For around a decade we've known about Earth's quiet "vertical" hum, probably caused by the steady thumping of deep waves on the ocean floor. Now a team in Germany has discovered a "horizontal" note, and nobody knows what's causing this new signal. Dieter Kurrle and Rudolf Widmer-Schnidrige of the University of Stuttgart in Germany studied 11 years of data from seismometers at four isolated at four isolated locations in Germany, Japan and China. Theseismometers were designed to detect minute horizontal motion in paralell to the Earth's surface. They found evidence of a "horizontal" signal at all four stations. The signal migrates by around one micrometre one way or the other every three minutes or so, and its horizontal orientation distinguishes it from Eart,s "vertical" oscilliation. Though certain events such as earthquakes, volcanoes and large storms seem to amplify this constant signal, the source is a mistery. "Something entirely new to us is causing this hum", says Spahr Webb of the Lamont-Doherty Earth Obesravtory in New York, who studied the original hum. At the American Geophiysical Union annual meeting December 2007, David Thomson of Queens University in Canada and Frank Vernon of the Scripps Institution of Oceanography, California told researchers that the new hum is synchronised with variations in the Sun's magnetic field.
(Source: New Scientist.com, 23 February 2008, http://tiyurl.com/2ojs86)
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