Hydrophones from a surface vessel in 1,300 fathoms of water off Juan de Fuca Strait detected, in the course of the “Hardtack” series of tests in the Marshall Islands, three acoustic signals which had peaks in their energy spectra at frequencies less than 20 cps. Two of these appear to have originated from nuclear explosions; the third, though having a similar energy spectrum, was apparently a T-phase from an earthquake near Cape Mendocino with its epicenter at 40° 16′ N, 124° 12′ W, and an origin time of 23:04:46 on May 24, 1958.

Travel-time measurements and signal spectra indicate that the nuclear explosions originated within Eniwetok Atoll. The coupling of their signals to the water path apparently was similar in nature to that of the earthquake T-phase, but the duration of the signals from the nuclear explo sions was considerably less.

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