Geophysical observations at the ocean bottom

Abstract

Trench systems surrounding the circum Pacific Ocean are the subduction zones of the oceanic plates according to plate tectonics. These subduction zones are the source regions generating large earthquakes which caused huge damage in Japan and Taiwan. Seismological studies during three decades suggest that these earthquakes are caused by frictional slip between the subducting oceanic plate and the continental plate. If we can monitor any precursory slips at the plate boundary prior to the large earthquakes, it may be possible to reduce the earthquake damage. The precursory seismic activities and/or slip can be observed by seismometers, and/or geodetic instruments at the ocean bottom. In order to monitor the seismic activity at deep ocean, we developed free-fall-pop-up digital ocean bottom seismometers (OBSs). In addition to non-real time seismic observation using free-fall-pop-up OBSs, submarine cable OBSs enabling real-time observation have been used. There are many scientific submarine cable systems in Japan. Each submarine cable system is nearly 100 km long. Some use optical fibers and others use coaxial cables. We also examined the possibility of using decommissioned telecommunication submarine cables. The GeO-TOC cable OBS located at the 2800 m depth of ocean bottom along the Izu-Bonin Trench was operated for 5 years and brought important seismic and hydrophone data to the Japanese seismic network continuously. The VENUS cable system using the GOGC submarine cables was the test system deployed off Okinawa Island and operated for two months. It was an interdisciplinary ocean bottom observatory. In USA and Canada, the NEPTUNE (North East Pacific Time-series Undersea Networked Experiment) project is intending to install scientific submarine cables systems near Vancouver Island, Monterey Bay and off Cascadia-Juan de Fuca. In Japan, the ARENA project has been proposed. To make such a project economical and successful, international cooperation is requested because the branching from the main submarine cable needs a technological breakthrough. The development of power supply systems, undersea mateable connectors and a fail safe system are key points. This development of new technologies may greatly contribute to the ocean engineering field.