A new perspective of the subduction zone derived from the Ocean Drilling Program for the Nankai Trough Seismogenic Zone Experiments (NanTroSEIZE)

The NanTroSEIZE project has been one of the most complex and challenging scientific ocean drilling projects in history, representing a milestone for the Integrated Ocean Drilling Program (2005–2013) and the current International Ocean Discovery Program (2013–013) and the current International Ocean Discovery Program (2013fic ocseismogenesis of the Nankai Trough is now approaching the final stage; i.e., directly sampling, analyzing, and monitoring the plate boundary fault system responsible for historically recurring megaearthquakes and associated tsunamis. The study area is located southeast of Kii Peninsula and comprises a transect of drill sites extending from the Kumano Basin across the Nankai Trough to the incoming Philippine Sea Plate. The drilling of the Nankai seismogenic subduction zone, initiated in 2007, has resulted in the re-evaluation of previously accepted geological models. The main findings are as follows: 1) The Nankai forearc grew intermittently between ~6 and ~2 Ma due to rapid terrestrial sediment supply, resulting in the formation of a hanging wall wedge as a result of the occurrence of great earthquakes. 2) Slip along the plate boundary megathrust and along the associated splay fault has previously reached as far as the Nankai trough and ocean floor. 3) The fault, composed of clay-rich gouge, is weak in both static and dynamic cases. 4) The in situ stress conditions of the accretionary wedge and incoming Philippine Sea Plate are well constrained, and the horizontal compressional stress, parallel to the direction of plate convergence, suggests tectonic loading of accretionary sediments, implying a possible stress buildup that could result in the next great Nankai earthquake. 5) Borehole observatories and an ocean floor network recorded the earthquake, tsunami, and slow slips along the megathrust on 1 April 2016, and represent a new and innovative technology for application in the field of ocean floor science.