No. 115: Jan-Feb 1998
Exactly two years ago, we reported on strange seismic signals detected by German geophysicists near Mount Semeru, in Java. These signals consisted of a fundamental tone and evenly spaced harmonics. Sometimes, the fundamental tone rose and fell. This "natural trombone" was thought to be a gas-filled subterranean cavity capped at the top by rock, with a pool of magma at the bottom. Volcanic vibrations resonated in this chamber. As the magma pool rose and fell, the fundamental tone changed.
More recently, a network of seismic stations in French Polynesia has picked up more mysterious seismic signals. These differ from those in Java in that each fundamental tone is "pure"; that is, there are no harmonics. Dubbed "T-waves," the sounds originated from an active volcanic ridge in the South Pacific. Suspicion fell on one flat-topped volcano that rose to within 130 meters of the ocean surface. But, how could this peak generate such a pure tone?
The theory is that the active volcano spews out a column of steam bubbles bounded at the bottom by the flat volcano and by the ocean at the top. Computer simulations proved that sound could resonate in a column of bubbles just as it does in an organ pipe. Since the height of the column remains fixed, so does the fundamental tone. Certainly harmonics are generated, too, but the bubbles damp out the higher frequencies, leaving a pure tone.
(Schneider, David: "A Blue Note," Sci entific American, 277:18, August 1997.)
Comments. The resonating-bubble-cloud theory was proposed in a 1996 issue of the Bulletin of the Seismological Society of America.