京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第19955号 / 理博第4222号 / 新制||理||1607(附属図書館) / 33051 / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 釜井 俊孝, 准教授 王 功輝, 教授 林 愛明 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

A prominent landslide deposit in the Slope Basin seaward of the Megasplay Fault in the Nankai Trough was emplaced by a high-mobility landslide based on analysis of physical properties and seismic geomorphology. Slide acceleration is a critical variable that determines amplitude of slide-generated tsunami but is many times a variable with large uncertainty. In recent controlled laboratory experiments, the ratio of the shear stress to yield strength (defined as the Flow Factor) controls a wide spectrum of mass movement styles from slow, retrogressive failure to rapid, liquefied flows. Here, we apply this laboratory Flow Factor approach to a natural landslide in the Nankai Trough by constraining pre-failure particle size analysis and porosity. Several mass transport deposits (MTDs), were drilled and cored at Site C0021 in the Nankai Trough during International Ocean Discovery Program (IODP) Expedition 338. The largest, MTD B, occurs at 133-176 meters below seafloor and occurred approximately 0.87 Mya. Slide volume is 2 km3, transport distance is 5 km, and average deposit thickness is 50 m (maximum 180 m). Pre-failure water content was estimated from shallow sediments at Site C0018 (82%). The average grain size distribution is 37% clay-sized, 60% silt-sized, and 3% sand-size particles as determined by hydrometer analyses of the MTD. Together, the water content and clay fraction predict a Flow Factor of 3.5, which predicts a relatively high mobility slide. We interpret that the landslide that created MTD B was a single event that transported the slide mass relatively rapidly as opposed to a slow, episodic landslide event. This is supported by the observation of a completely evacuated source area with no remnant blocks or retrogressive headscarp and an internally chaotic seismic facies with large entrained blocks. This approach can be extended to other field settings characterized by fine-grained siliciclastics and where water content and clay percentages are known.