Abstract

On 17 September 1952 a fishing boat, the MYOJIN MARU, discovered a submarine volcanic eruption 200 nautical miles south of Tokyo. The volcano appears to be the central cone of a caldera lying along the Fuji volcanic zone. The near-by Bayonnaise Rocks mark the highest, and the only subaerially exposed, portion of the caldera rim. A series of great explosions accompanied the extrusion of augite-hypersthene dacite. Since 26 September, when the main series of explosions ended, there has been only sporadic activity.

Because of its remoteness, only a few visual observations of Myojin were made. Additional data were obtained from a tsunami recorder on Hachijo Island, 130 kilometers north of Myojin, which detected several tsunamis initiated by the volcanic eruptions, and by an atmospheric electricity recorder at Tokyo which detected atmospherics associated with the eruption. In contrast, a rather complete record of the Myojin activity was recorded by the U. S. Navy Sofar stations at Point Sur and Point Arena, California, about 8600 kilometers from the scene. More than 100 explosions were detected, one of which lasted for more than an hour. Explosions recorded on Sofar equipment agree in time with those observed visually or inferred from tsunami and atmospherics data.

From the tsunami data and the Sofar records, it was independently concluded that the explosion which destroyed the Japanese Hydrographic vessel, No. 5 KAIYO MARU, occurred at about 1220 JST on September 24. All 31 people aboard the vessel were lost.

This is believed to be the first time any signals on Sofar records have definitely been identified as of volcanic origin, and, since some of the signals were distinctive, installations similar to those of Sofar stations may prove of value for monitoring oceanic volcanic activity.

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