Abstract
Underwater sound signals in the frequency band below 100 Hz were recorded at several widely spaced SOFAR depth hydrophones in the North Pacific. The signals, which typically lasted from 5 to 30 sec and were spaced about 1 min apart, continued at a high level of activity during the fall and winter of 1973–1974. SOFAR fixes obtained on discrete events in the series plot in the volcano islands near 22°N, 144°E. Some of the signals exhibited spectral banding with fundamental frequencies in the 1–2-Hz region. If one assumes the spectral banding is due to constructional interference of surface- and bottom-reflected sound in the vicinity of the source, the calculated depth of the explosions was between 375 and 750 m. The volcanic noises are therefore believed to have originated from explosive volcanic activity associated with the shallow seamount charted at 21.9°N, 143.4°E with a minimum sounding of 150 m (Navy Hydrographic Office Chart No. 970000).
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