Since 1989 the Ocean Research Institute at the University of Tokyo has started deep sea floor imaging surveys around Japan by using IZANAGI, which was developed by cooperation between the Ocean Research Institute, the University of Tokyo and Seafloor Surveys International, Inc. It is characterized by an interferometric sidescan sonar, towed at a shallow depth, typically 100 m below the sea surface at a speed of up to 10 kn (5 m/s). Two sets of transducer arrays are mounted on each side of the tow fish, transmitting 11 kHz on the starboard side and 12 kHz on the port side. They operate simultaneously at transmitting, but individually at receiving. The range of swath width can be varied from 20- to 1 km. A sidescan image is composed of 1024 pixels per one side of the swath; thus for a 10-km swath each pixel has a constant width of 5 m. Total coverage of our surveys is, so far, approximately 100 000 km[sup 2] from various geotectonic areas in the seas around Japan. The main objective of our surveys is to clarify detailed ocean floor topography which represents slope failure, active faults related to large earthquakes in the deep sea, sedimentary structure related to surface sediments transportation, and submarine volcanoes.