Abstract The distribution and characteristics of marine surface sediments are a basic marine geological information. Large river floods are a frequent natural hazard that transport substantial terrigenous sediments into the marine environment. In August 2003, TY ETAU (0310) caused heavy rainfall in the southern coast of Hokkaido, north Japan, where some mountainous rivers in the Hidaka region flooded. Two deposition modes for the 2003 flood sediments can be identified by comparing the pre- and post-flood surface sediment distribution. Shore-normal shallow depressions off the mouth of the Saru and Atsubetsu rivers served as channels for the discharged floodwater preventing dispersion and maintaining the necessary water density to transport the materials as density bottom currents. This action also promoted long-distance transport of flood materials across the continental shelf. Absence of depression on the inner shelf off the mouth of the Niikappu and Shizunai rivers may have dispersed floodwaters near the river mouth and deposited the flood materials close to the shore. Marine geological mapping suggests that the differences in submarine topography (the presence or absence of shallow depressions) are closely related to the regional geological structure. Thus, submarine geology is a controlling factor of the seafloor environments influenced by the river flood.

Abstract Major river flooding is a typical geohazard frequently characterized by the transportation of large quantities of terrigenous material from the land to the sea. However, the types and patterns of flood material deposition and the influence on seafloor environments differ because of the diversity in geological and climatic conditions. In September 2011, the activity of typhoon 201112 produced severe rainfall in the central Kii Peninsula and caused a large flood along the Kumano River. The event was considered a once-in-a-century-scale flood. We examined the occurrence of flood deposits along the slope off the mouth of Kumano River in four surface sediment cores collected around 2 months after the 2011 flood. We found a thin and muddy event bed at the core top of three cores, with two or three event beds corresponding to past historical flood events. The sediment thickness, grain size and sedimentary structures for three historical flood deposits demonstrated significant variation. The thicknesses and the grain sizes of the three flood deposits from the same river showed no relationship with the maximum discharge from each flood. The differences in the types and fate of sediments discharge during the floods probably account for the variations.

Abstract Submarine landslides triggered by earthquakes can generate turbidity currents. Recently, several studies have reported that the remobilization of surface sediment triggered by earthquakes can also generate turbidity currents. Such sedimentary processes may be influenced by sediment characteristics, seafloor morphology and seismic motions. Here, we verify surface sediment remobilization using sedimentary records from the Nankai forearc region, SW Japan. We collected multi-core and piston core samples from a small confined basin, mainly composed of silty clay or very fine sand. Radiocaesium measurements of the multi-core show consistently high values in the upper 17 cm and low values below this depth. Rapid sediment deposition after 1950 is assumed, and the most likely cause is the 2004 off the Kii Peninsula earthquake. Based on calculations using bathymetric maps and palaeocurrent data, settlement of the upper 17 cm can be explained by redeposition of the surface ( c. 1 cm) slope sediment around the basin. Muddy turbidites are also identified in the piston core. The gap in radiocarbon age observed around 2.0 m bsf (metres below seafloor) implies similar sedimentary processes. Our study represents the first examination of surficial remobilization from sedimentary cores in the Nankai forearc region.