1988 Volume 33 Issue SPCL Pages S224-S233
Bathymetric, seismic, geomagnetic and gravity surveys around Izu-Osima Volcano were carried out by the Survey Vessel Takuyo of the Hydrographic Department of Japan, after volcanic eruption of Osima volcano in 1986. The survey revealed several geological and geophysical characteristics at the adjacent area of Osima Island as follows : i) NW-SE topographic and geomagnetic elongations due to underwater fissure eruptions were recognized in the area off Tigasaki, northwestern end of Osima Island and off Habu, southeastern end of Osima Island. ii) U-shaped bathymetric high adjacent to the west coast of Osima having high magnetic anomaly, or the Senba Spur, is considered to be an old volcano edifice contiguous to Osima volcano. Subsequently, the Hydrographic Department surveyed in detail off the southeast of Osima, the Habu Spur, on February 5, March 3 and April 16 in 1987 by the newly-built Survey Vessel Tenyo. She is equipped with a narrow multibeam echo sounder (Hydrochart) for shallow water (up to 1, 000 m), so the very precise bathymetric chart of the Habu Spur (scale 1/20, 000) has been made and some specific topography has become clear. The results from the analysis of Hydrochart records are as follows : i) The Habu Spur is composed of many knolls which are arranged in two parallel lines trending the southeast to the northwest. ii) The direction of the lines of lateral volcanoes is coincided with the direction of knolls existing on the flank of Osima volcano. iii) Two lines of lateral volcanoes are just like the straight line and are composed of continuous narrow ridges, which suggest something like intrusive dikes. iv) A 185 m knoll below the sea surface is discovered in the middle of the northeastern line of lateral volcanoes. x) Some water depths surveyed in 1987 have become deeper than the water depths surveyed in 1954 on the northeastern line of the lateral volcanoes (maximum change is 100 m). vi) The change of water depths from 1954 to 1987 on the 223 m knoll in the southern end of the northeastern lateral volcano line is divided into two patterns. One, the southeastern part of the lateral volcanoes line, is remained unchanged and the other, the northwestern part, have become deeper. The cause is unknown.
