2011 East Japan Earthquake generated unprecedented tsunami damages to Japan. From this severe disaster by mega thrust earthquake, we recognized the importance of off shore real time monitoring. Actually, pressure gauges of Kamaishi ocean floor cable detected large tsunamis at 2011 East Japan earthquake earlier than land tide gauge stations. These facts indicate that off shore real time monitoring systems are quite important and indispensable for tsunami early warning. Around the Nankai trough, as real time monitoring systems, we have already developed and deployed DONET (Dense Oceanfloor Network System for Earthquakes and Tsunamis) and will deploy DONET2 system. Using these ocean floor networks, tsunami early warning system will be improved and become more reliable. DONET and DONET2 are very important and significant for not only contributions to early warning, but also for understandings of crustal activities. Furthermore, offshore real time data is necessary and indispensable for advanced recurrence cycle simulation of mega thrust earthquakes using data assimilation methods. The data assimilation method acts as prediction filters using data and results of simulations. This data assimilation method will be the key research tool for advanced simulation. This data assimilation method requires precise, high quality and stable data from real time monitoring systems. Therefore, DONET, DONET2 have been developed as precise and long term monitoring systems. Finally, integration of science and technology will be the driving force for the ocean science evolutions and natural disaster mitigations.
