No. 118: Jul-Aug 1998
After a major earthquake, the entire earth quivers like a sphere of jelly suspended in space. Already a slightly oblate sphere, the planet becomes successively a bit more oblate, then a bit more prolate, as illustrated. The amplitudes of these deformations are small, just a centimeter or so after big quakes. The tone or frequency of the quivers is just a few millihertz, which translates to periods ranging from 3-54 minutes. We doubt that the telestomping elephants mentioned under BIOLOGY can detect these quiverings.
That the earth does indeed "ring" is old news. Geophysicist A.E.H. Love mentioned the possibility in 1911. It is also recognized that large earthquakes can set the earth to ringing ("quivering" is better). What is news is the discovery by N. Suda et al that our planet rings even when no major quakes are occurring, and no one yet knows why. Suda et al write:
"The observed "background" free oscillations represent some unknown dynamic process of Earth."
Suda and his colleagues detected these oscillations using a superconducting gravimeter, which they installed in a seismically-quiet place: Antarctica.
The favorite explanation for the background oscillations is turbulence in the earth's atmosphere. Ocean tides and currents are also on the list as potential "bell-ringers." [El Ninos were not mentioned!]
(Suda, Naoki, et al; "Earth's Background Free Oscillations," Science, 279: 2089, 1998. Also: Kanamori, Hiroo; "Shaking without Quaking," Science, 279:2063, 1998.)