A new approach to real time measurements at Izu-Bonin trench using TPC-1, demission telecommunication cable

Real-time earthquake monitoring in the deep ocean is one of the urgent demands in Japan because Japan has been badly damaged by large earthquakes occurring in the surrounding area of the Japanese archipelago. To obtain real-time seismic data from the deep ocean bottom, a disused submarine coaxial cable (TPC-1) deployed in 1964 has been used. The Ninomiya, Japan, and Guam section is currently called the GeO-TOC cable whose length is 2,659 km. In this project, several technological questions arising from reuse of disused submarine cables for scientific purposes, have been solved. For example, there was a question on the remaining life of cables because the cable system uses vacuum tubes in its repeaters. In January, 1997, an ocean bottom seismometer unit was installed at 2,708 m depth on the Izu-Bonin trench slope by adding extra cables with a different diameter. The “IZU OBS” has several sensors including a three-axes accelerometer. Data from accelerometers are digitized by 24 bit A/D converters and are transmitted from the bottom unit to the shore station in Ninomiya through the GeO-TOC analog cable. The electrical power needed for the ocean bottom package is fed from the cable DC supply in Guam. Continuous data are currently processed jointly with land seismic data