Transpacific Detection by Underwater Sound of Myojin Volcanic Explosions

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 nearby Bayonnaise Rocks mark the highest, and the only subaerially ex posed, 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 were made of Myojin. 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 con trast, 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. Well over 100 explosions were detected, one of which lasted for more than one hour. A comparison of the explosions recorded on Sofar equipment shows agreement in time with those observed visually or inferred from tsunami and atmospherics data. From the tsunami data and the Sofar records, it was independently con cluded that the Japanese Hydrographic vessel the Kaiyo Maru No.5 was destroyed by a large eruption at about 1220 JST 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, it is suggested that installations similar to those of Sofar stations may prove of value for monitoring oceanic volcanic activity. CONTENTS TEXT Introduction Acknowledgements Geological setting, and history of eruptions near Myojin since 1896 Myojin eruption, 1952 General Visual observations Tsunami records Atmospherics accompanying the eruption * Fulbright Research Scholar in Oceanography, Tokyo University, Japan. On leave of absence from the U.S. Navy Electronics Laboratory, San Diego, California. ** U .S. Navy Electronics Laboratory, San Diego, California. (1) 54 Journ. Oceanogr. Soc. Japan, Vol.9 No.2 (1953) Sofar detection of explosions Volcanic activity at Myojin subsequent to September, 1952 Destruction of the Kaiyo Maru, No.5 Energy of the explosions Future possibilities References cited ILLUSTRATIONS Figure 1. Submarine topography of the area near Myojin volcano 2. Regional setting of Myojin and the Fuji volcanic zone 3. Myojin explosions according to Pacific Sofar station records 4. Point Sur Sofar station record, 1733 to 2141 JST, 18 September 1952 5. Point Sur Sofar station record, 2142 JST, 18 September to 0148, JST 19 September 1952 6. Explosion which presumably sunk the Kaiyo Maru No.5 Plate 1. Explosion of Myojin volcano at 0834 on 23 September 1952 2. Explosion of 1312 on 23 September 1952 as seen from the Shin'yo Maru one mile away 3. Explosion of 1340 on 23 September 1952 as seen from the Shin'yo Maru one mile away 4. Myojin Islands No.2 and 3 formed subsequent to the September 1952 eruption TABLES Table 1. History of eruptions at or near Myojin prior to 1952 2. Visual observations of 1952 Myojin explosions 3. Tsunami records of 1952 Myojin explosions 4. Correlation of atmospherics with visual and Sofar observations of the 1952 explosions 5. Underwater sound signals received at Pacific Sofar stations from the Myojin explosions