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No. 122: Mar-Apr 1999

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The Earth Hums More Loudly In The Afternoons

It has been known for almost a century that large earthquakes set the earth to ringing like a bell. In SF#118, we reported that the planet also "hums" when there have been no earthquakes. Just what forces stimulate this seismic humming of the earth-as-a-whole is still a matter of conjecture.

Actually, "hum" is a poor choice of words. The period of these vibrations ranges from 3 to 8 minutes, which puts them in the range of infrasound.

Recently, N. Suda of Nagoya University has found a clue suggesting that thunderstorms may excite these very-lowfrequency vibrations. Suda and his colleagues analyzed the seismic records at four seismically quiet locations around the globe and discovered that the hum is loudest between noon and 8 PM local time. The quietest period is from midnight to 6 AM. These are the same time frames when thunderstorms are most active and quiet. It's circumstantial evidence, but it makes sense.

(Kerr, Richard A.; "Earth Seems to Hum along with the Wind," Science, 283:321, 1999.)

Comment. Infrasound in the atmosphere may originate from storms thousands of miles away and from strong winds blowing across mountain crests. It appears that the earth is an immense, spherical aeolian harp!

From Science Frontiers #122, MAR-APR 1999. � 1999-2000 William R. Corliss