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Mega thrust earthquakes such as the 2004 Sumatra earthquake and the 2010 Chile earthquake generate severe damages by strong motions and large tsunamis. In Japan, the recurrences of mega thrust earthquakes around the Nankai trough off southwestern Japan, is one of the most severe problems in Japanese natural disasters. Especially, around the Nankai trough, mega thrust earthquakes over magnitude8 class such as the Tokai, Tonankai and Nankai earthquake are occurring with a interval of 100-200 years. According to historical earthquakes, these mega thrust earthquakes occur as seismic linkages with variations. Based on previous research, in 1944 Tonankai /1946 Nankai earthquakes and in the 1854 Ansei earthquake around the Nankai trough, first ruptures were starting from the Tonankai seismogenic zone. These facts are very significant to understand and elucidate the recurrences of mega thrust earthquakes. Therefore, ocean floor real time monitoring systems for the Tonankai seismogenic zone are quite useful and important. We are starting to develop the Dense Ocean Floor Network System (DONET) as a Ministry of Education, Culture, Sports, Science and Technology (MEXT) project during FY2006-2010. In the DONET system, backbone cable, brunching units, nodes, precise sensors etc. are installed. We have already deployed a few observatories and have obtained earthquake data. In preparation of the next mega thrust earthquakes around the Nankai trough, DONET2 with a high voltage system will be deployed in the western part of the Kii peninsula which corresponds to the Nankai earthquake seismogenic zone. DONET2, which is more extendable, rather than DONET will be scheduled as a 10 years project. The dense array data from DONET and DONET2 will be applied to the early warning of earthquakes and tsunamis. Furthermore, the precise data of crustal activities from DONET and DONET2 will improve the recurrence cycle simulation model using data assimilations to estimate seismic linkage around t- e Nankai trough. Finally, we would like to enhance the importance of international collaborations. International ocean floor observatory projects, such as ESONET, NEPTUNE, etc are already starting, therefore we think that the international collaboration including DONET/DONET2 is quite important to extend more ocean floor observatories, to develop technologies, to exchange researcher and scientific results.