Development of Compact Seafloor Cabled Seismic and Tsunami Observation System Using ICT and Installation Plan to Off-Sanriku Region, Japan

A seismic and tsunami observation system using seafloor optical fiber had been installed off Sanriku, northeastern Japan in 1996. The objectives of the system are to obtain exact seismic activity related to plate subduction and to observe tsunami on seafloor. The continuous real-time observation has been carried out since the installation. In March 2011, the Tohoku earthquake occurred at the plate boundary near the Japan Trench, and the system recorded seismic waves and tsunamis by the mainshock. These data are useful to obtain accurate position of the source faults and source region of tsunami generated by the event. However, the landing station of the system was damaged by huge tsunami, and the observation was suspended. Because the real-time seafloor observation by cabled system is important in this region, we decide to reconstruct a landing station and install newly developed Ocean Bottom Cabled Seismic and Tsunami (OBCST) observation system for additional observation and/or replacement of the existing system.

From 2005, we have been developed the new compact Ocean Bottom Cabled Seismometer (OBCS) system using Information and Communication Technology (ICT). Our system is characterized by securement of reliability by using TCP/IP technology and down-sizing of an observation node using up-to-date electronics technology. In 2010, the first OBCS was installed near Awashima-island in the Japan Sea, and is being operated continuously. The new OBCST system is placed as the second generation of our system, and has two types of observation nodes. Both types have accelerometers as seismic sensors. One type of observation nodes equips a crystal oscillator type pressure gauge as tsunami sensor. Another type has an external port for additional observation sensor by using Power over Ethernet technology. Clocks in observation nodes can be synchronized through TCP/IP protocol with an accuracy of 300 ns (IEEE 1588). A simple canister for tele-communication seafloor cable is adopted for the observation node, and has diameter of 26 cm and length of about 1.3 m. At the present, we are producing a practical OBCST system which has total length of approximately 100 km and three observation nodes. We have a plan to install the practical system in 2015.