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In many researches focusing on the mega-thrust earthquakes around the Nankai trough, the structural researches using refractions and reflections seismic has succeeded to image the key structures to understand recurrences of mega-thrust earthquakes around the Nankai Trough. Moreover, results of earthquake recurrence cycle simulation show that the first ruptures seems to occurred around the Tonankai earthquake rupture zone in each recurrence cycle, and the clear segment boundary between the Tonankai and Nankai earthquake rupture zones off the Kii peninsula was analyzed using tsunami data. In 1854, 1944/46, the initial rupture were starting from the Tonankai seismogenic zone ahead of the Nankai seismogenic zone with intervals of 32 hours and 2years in each event. The simulation results of recurrence cycle is consisted with recent historical events in 1854,1944/46. Therefore, based on these researches, we will deploy the advanced ocean floor network around the Tonankai seismogenic zone to monitor the crustal activity, improve the recurrence cycle model and provide the high quality early warning information for the next mega-thrust earthquake.