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.

ABSTRACT Understanding the factors controlling earthquake rupture areas in subduction zones is a fundamental question in geodynamic research. In the Nankai Trough, Japan, three major controlling factors have been proposed so far: surface roughness, such as seamounts, on the subducting Philippine Sea plate, a locally dense and rigid upper plate, and stable or unstable frictional properties of the plate-boundary megathrust with abnormal pore-fluid pressure. The most prominent rupture segment boundary in the Nankai Trough is located off the Kii Peninsula. When the rupture starts and propagates beyond this boundary, the entire megathrust breaks, as in the 1707 Hoei earthquake. When the rupture does not propagate beyond this portion, the rupture area is segmented, as in the 1854 Ansei, 1944 Tonankai, and 1946 Nankai earthquakes. In this case, the boundary works as a barrier. The asperity or barrier depends on the frictional behavior along the megathrust in this area. Thick and dense plutonic rocks are the main component of the upper plate of the Kii Peninsula. Magmatic activity associated with the proto–Izu-Bonin arc collision is speculated to have occurred in the middle Miocene. Thus, we propose that the long-term tectonic hysteresis of the upper plate is the main factor controlling the rupture area segmentation in the Nankai Trough.

Abstract In this study, earthquake rupture models for future mega-thrust earthquakes in the Nankai–Tonankai subduction zone are developed by incorporating the main characteristics of inverted source models of the 2011 Tohoku earthquake. These scenario ruptures also account for key features of the national tsunami source model for the Nankai–Tonankai earthquake by the Central Disaster Management Council of the Japanese Government. The source models capture a wide range of realistic slip distributions and kinematic rupture processes, reflecting the current best understanding of what may happen due to a future mega-earthquake in the Nankai–Tonankai Trough, and therefore are useful for conducting probabilistic tsunami hazard and risk analysis. A large suite of scenario rupture models is then used to investigate the variability of tsunami effects in coastal areas, such as offshore tsunami wave heights and onshore inundation depths, due to realistic variations in source characteristics. Such investigations are particularly valuable for tsunami hazard mapping and evacuation planning in municipalities along the Nankai–Tonankai coast.