Japan Trench earthquake and tsunami monitoring network of cable-linked 150 ocean bottom observatories and its impact to earth disaster science

The big project is an undertaking to construct a large-scale ocean-bottom network of cable-linked 150 observatories along the Japan Trench. It is currently in progress in Japan. NIED (National Research Institute for Earth Science and Disaster Prevention) takes in charge of the project which is supported by MEXT (the Ministry of Education, Culture, Sports, Science and Technology) financially. The network is for earthquake, tsunami and vertical crustal deformation. The major purpose of the network is to provide the in-situ and real-time geophysical data which will be used for disaster prevention. Such real-time data from the ocean-bottom observations make it possible to forecast the next-generation early tsunami warning which could precisely predict coastal tsunami height. Also the data may make it possible to forecast an earthquake warning much earlier than the present system. The project started in November, 2011 with an area of the 2011 off the Pacific coast of Tohoku earthquake (M _w 9.0) as the catalyst to move this project forward. The 2011 off the Pacific coast of Tohoku earthquake occurred off the northeastern Japan coast along the Japan Trench on 11 ^th of March in 2011 and a devastating tsunami over 10 m in height hit the Pacific coastal area of the northeastern Japan and severely damaged the communities and infrastructures in and around this area. There were several offshore tsunami observatories such as cabled seafloor hydro-pressure gauges and GPS tsunami buoys in the sea at the time of the occurrence of the 2011 off the Pacific coast of Tohoku earthquake. The offshore tsunami observatories caught the tsunami registering at 5 meters high about 10 minutes before the tsunami arrival at the coast. The tsunami warning, which was emitted by the JMA based on the land seismic observatories data, under-estimated the tsunami height to hit the coast. In the case of the 2011 off the Pacific coast of Tohoku earthquake, the offshore tsunami observed data were not effectively utilized for the tsunami height estimation and the under-estimated tsunami height caused severe casualties (about 20,000 people). We urge this development of the next-generation tsunami warning utilizing the real-time data from the NIED seafloor network along the Japan Trench. 150 seafloor observatories are connected by fiber optic cables of 5,800km in total length. Each observatory is an earthquake and tsunami observatory. Seismometers of several types are equipped to cover a dynamic range of 4G in acceleration and a frequency range of 0.05 Hz to 30 Hz. As a tsunami meter two hydro-pressure gauges of the same type are equipped for redundancy. The resolution of the pressure gauge is a few millimeters in a water column. The NIED seafloor network estimate completion in FY 2014.