[H-DS09] Tsunami and tsunami forecast

[HDS09-P05] Bathymetric Survey and Tsunami Modeling of a Submarine Landslide in Tosa-bae, Nankai trough

*Masahiro Kurozumi¹, Toshitaka Baba², Tetsuo Matsuno³, Mamoru Sano⁴, Kentaro Imai⁵ (1.Tokushima University Graduate School of Frontier Sciences, 2.Graduate School of Science and Technology, Tokushima University, 3.Kobe Ocean Bottom Exploration Center, Kobe University, 4.Nippon Marine Enterprises Ltd, 5.Japan Agency for Marine-Earth Science and Technology)

Keywords:submarine landslide, two-flow calculation model, Tsunami calculation

We can expect the risk of tsunamis from strong shaking due to earthquakes. However, submarine landslides would not generate strong shaking. It underestimates the risk of tsunamis. To assess submarine landslides in the Nankai trough, we carried out bathymetric surveys two times in March and August 2019 by a training ship, Fukae-maru, Kobe University. The survey area was the northern slope of Tosa-bae, east of the Cape Muroto, with water depth ranging from 400 to 1200 m. The interval of bathymetric survey lines was at 500 m to acquire a detailed map gridded by 25 m. In the survey area, we observed submarine landslide scars with a wide of about 1.9 km, a length of about 2.3 km, and a thickness of about 60 m. We also found a small gap that may lead to a new collapse upstream of the major scars.
We used a two-flow calculation model in which the upper and lower layers correspond to the seawater and turbidity currents, respectively. For tsunamis caused by submarine landslides in deep water depth, we improved the calculation model at the following two points. The interfacial force between the upper and lower layers uses a depth-dependent friction coefficient assuming Manning's law. Uplift and subsidence at the sea surface by movements of the lower layer consider non-hydrostatic effects by Kajiura's filter at each time step. The Kajiura’s filter at each time step suppresses disturbance at the sea surface due to sudden movement of the lower layer.