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New, high-resolution multi-beam bathymetric data from RV Sonne cruise SO251 show a widely variable surface morphology along the southern Kumano Basin and Nankai accretionary prism off SW Japan. Combined with a three-dimensional seismic volume, these data provide insight into the ubiquitous and varied nature of faulting typical of accretionary prism settings, a high number of submarine landslides across the entire study area that vary both spatially and temporally, a pronounced absence of slide deposit bathymetric manifestations, widely varied slope angles and a potential subducted seamount scar. We have mapped scars of 442 primary and 184 secondary landslides and have measured the areas evacuated by these slides. Most of the slides are completely disintegrative, so surficial landslide deposits are almost absent. The incidence with which temporally sequential slope failures and fault structures cross-cut themselves and one another provides evidence of potential failure pre-conditioning such as gas hydrates, pore fluid overpressures and bottom current activity. Seismic loading and slope over-steepening are then the most likely final trigger mechanisms to slope failure. The majority of observed landslides (64%) occur seawards of the outer ridge, providing insight into the relationship between surficial landsliding and subsurface tectonic processes along this accretionary prism.

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