Japan Agency for Marine-Earth Science and Technology, Japan

Integrated Ocean Drilling Program Expedition 315 is one of three Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Stage 1 expeditions. The NanTroSEIZE project is a multistage, multiplatform drilling project designed to investigate fault mechanics and seismogenesis along subduction decollement and megathrusts through direct sampling, in situ measurements, and long-term monitoring in conjunction with allied seafloor laboratory and numerical modeling studies. One of the challenging aspects of NanTroSEIZE is the first use of risers in scientific ocean drilling. Expedition 315, entitled "Megasplay Riser Pilot", is a pilot study for the future deep riser drilling of the megasplay fault in Stage 2. The primary engineering and scientific objectives of this expedition are to obtain geotechnical information needed for well planning of future riser drilling to 3500 meters below seafloor. This site's location is also critical for understanding the nature of the shallow portions of splay faults. The scientific targets of this expedition are deformation mechanics, fault-related fluid source and migration pathways, and correlations between fault activity and slump deposits on the trench slope. In this Scientific Prospectus we present the scientific background and objectives, the drilling operations designed to achieve them, and contingency plan strategies.