Ocean survey in Habu Spur.

Abstract

Hydrographic Department of Maritime Safety Agency has surveyed off southeast of Osima, Habu Spur, on Feb. 5, 1987 and on Mar. 3. 1987 by a newly-built survey vessel “Ten-yo”． She is equipped with a narrow multi-beam echo sounder(Hydrochart) for shallow water (up to 1,000 m), so the very precise bathymetric chart of 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. (1) Habu Spur is composed of many lateral volcanoes which are arranged in two parallel lines trending the northwest to the southeast with the space of about 800m. (2) The direction of the lines of lateral volcanoes is coincided with the direction of lateral volcanoes existing on the flank of Osima, Miharayama. (3) Two lines of lateral volcanoes are clearly straight line and are composed of continuous narrower ridges, which suggest something like intrusive dikes. (4) A lateral volcano of 185 m below the sea surface is discovered in the middle of the northeastern line of lateral volcanoes. (5) Some water depths surveyed in 1987 have become deeper than those in 1954 on the northeastern line of the lateral volcanoes (maximum change is 100 m). (6) The change of water depth from 1954 to 1987 on the lateral volcano located in the southern end of the northeastern lateral volcanoes line is divided into two patterns. One, a southeastern part of the volcano,is remained unchanged and the other, a northwestern part, has become deeper. The cause is unknown.