Most of the destructive earthquakes around Japan have occurred beneath the sea floor. We carry out structural exploration and seismic observation by use of the ocean bottom seismographs in offshore areas in order to clarify the relationship between earthquake occurrence and subsurface structure. Seismic tomographic analysis using data from a dense network of OBSs and land stations enables us to estimate the subsurface structure of both onshore and offshore areas in the southern Hokkaido. Here we present an arc-arc collision structure in Figure: P wave velocity structure of the Hidaka collision zone. Moreover various geophysical studies have been conducted in the Atlantic area mainly in Mid-Atlantic Ridges (Figure: The Atlantic transect from East Greenland to the Norwegian mainland).