Surface sediment remobilization triggered by earthquakes in the Nankai forearc region

Submarine landslides triggered by earthquakes generate turbidity currents (e.g. Piper et al., 1988; 1999). Recently several studies report that the remobilization of the surface sediment triggered by earthquakes can also generate turbidity currents. However, studies that proposed such process are still limited (e.g. Ikehara et al., 2016; Mchugh et al., 2016; Moernaut et al., 2017). The purpose of this study is to examine those sedimentary processes in the Nankai forearc region, SW Japan using sedimentary records. We collected 46 cm-long multiple core (MC01) and a 6.7 m-long piston core (PC03) from the small basin during the R/V Shinsei Maru KS-14-8 cruise. The small confined basin, which is our study site, block the paths of direct sediment supply from river-submarine canyon system. The sampling site is located at the ENE-WSW elongated basin between the accretionary prism and the forearc basin off Kumano without direct sediment supply from river-submarine canyon system. The basin exhibits a confined basin that captures almost of sediments supplied from outside. Core samples are mainly composed of silty clay or very fine sand. Cs-137 measurement conducted on a MC01 core shows constantly high value at the upper 17 cm section and no detection below it. Moreover, the sedimentary structure is similar to fine-grained turbidite described by Stow and Shanmgam (1980), we interpret the upper 17 cm of MC01 as muddy turbidite. Grain size distribution and magnetic susceptibility also agree to this interpretation. Rapid sediment deposition after 1950 is assumed and the most likely event is the 2004 off Kii peninsula earthquakes (Mw=6.6-7.4). By calculation from extent of provenance area, which are estimated by paleocurrent analysis and bathymetric map, and thickness of turbidite layer we conclude that surface 1 cm of slope sediments may be remobilized by the 2004 earthquakes. Muddy turbidites are also identified in a PC03 core. The radiocarbon age gap of 170 years obtained around 2 mbsf of PC03 core also indicates similar sedimentary process. However, we also obtained large age gap in a thick turbidite layer, indicating remobilization of deeper sediments by landslide. Our results revealed that the studied basin recorded various scales and styles of sediment remobilizations by earthquake shakings.