The Nankai Trough is well known as the mega thrust earthquake generating tsunamis, with the interval of 100–200 years. The 1944 Tonankai and the 1946 Nankai earthquakes around the Nankai trough, each hypocenter was located off the Kii peninsula. However, according to Prof. Okamura of KOCHI University, super mega thrust earthquakes such as Hoei earthquake (1707) and Hakuho earthquakes (684) are occurring with an interval of 300–400 years or 700 years. Based on structural research, observational research and simulation researches, we proposed and have been starting to deploy the dense ocean floor observatory network system around the Tonankai seismogenic zone, to monitor crustal activities using broadband seismometer, accelerometer and precise pressure gauges. The probability of next Tonankai earthquake recurrence is estimate as 60–70% (The Headquarters for Earthquake Research Promotion; Therefore, the ocean floor network is significant ant important to monitor the crustal activities around mega-thrust earthquakes. In this paper, we explain the recent and detailed developing of this oceanfloor network system (DONET). Especially, the installation of sensors and improvement of ROV for the deploying system in the deep seafloor will be developed. Furthermore, the new project including observation simulation and mitigation researches of mega thrust earthquakes around the Nankai trough is starting as 5 years project (2008–20012). Especially, the Ocean floor network data is very important and powerful to progress the new project.

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