Assessing post-failure mobility of submarine landslides from seismic geomorphology and physical properties of mass transport deposits: An example from seaward of the Kumano Basin, Nankai Trough, offshore Japan

Abstract

A prominent paleo-landslide in the Slope Basin seaward of the Megasplay Fault in the Nankai Trough was emplaced by a relatively high-mobility mudflow based on analysis of physical properties and seismic geomorphology. Post-failure behavior of submarine landslides is a critical variable that determines amplitude of slide-generated tsunami but is historically difficult to constrain due to the wide variety of post-failure styles of submarine landslides. 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, was encountered at 133–176m below seafloor and occurred approximately 0.87Ma. Slide volume is 2km3, transport distance is 5km, and average deposit thickness is 50m (maximum 180m). Pre-failure porosity was estimated from shallow sediments at Site C0018 (82%). Grain size distribution was measured directly from samples with the hydrometer method with the following averages: 37% clay-sized, 60% silt-sized, and 3% sand-size particles. Together, the porosity and clay fraction define the Flow Factor to be 3.5, which equates to 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 relatively slow-moving cohesive style, landslide. This is supported by the observation in three-dimensional seismic data of a completely evacuated source area with no remnant blocks or retrogressive head scarp and an internally chaotic seismic facies with large entrained blocks. This back-analysis approach can be extended to other field settings characterized by fine-grained siliciclastics and where water content and clay percentages are known.