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  Geotimes - June 2008 - Earth hums a new tune
NEWS NOTES

Earth hums a new tune

Even between earthquakes, Earth sends out a steady stream of seismic signals. Those barely detectable signals, known as Earth’s “hum,” are only measurable at very seismically quiet stations, and are thought to occur because the planet’s surface is continuously vibrating (it oscillates both horizontally and vertically). Part of the hum — accounting for much of the vertical oscillation — is due to turbulence in the atmosphere or in the oceans, which can create changes in air or water pressure that impact Earth’s surface. For example, surf zone breakers in the coastal ocean can generate certain long-period waves (called “infragravity” waves) that propagate down and pound periodically on the seafloor.

Now, data from seismometers at four “exceptionally” quiet seismic stations show for the first time that Earth’s horizontal oscillations — once thought to be a lesser effect of the same forces producing vertical oscillations — may make an equal contribution to the overall din. Given how noisy they are, the horizontal oscillations must have their own source, according to geophysicists Dieter Kurrle and Rudolf Widmer-Schnidrig of the University of Stuttgart in Germany. One possible explanation for that horizontal hum, the authors wrote April 10 in Geophysical Research Letters, could be small amounts of stretching and distortion of Earth’s surface — perhaps due to winds shearing across the continents or currents along the seafloor.

Carolyn Gramling

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