Dewatering structure and soft-sediment deformation controlled by slope instability: examples from the late Miocene to Pliocene Miura-Boso accretionary prism and trench-slope basin, central Japan

The foliations from both the upper part of the accretionary prism and the slope basin correspond to plausible “pseudo-hydraulic fracture” planes, parallel to σ1 and perpendicular to σ3, and are associated with slope instability. Pore fluids or liquidized silt escaped along the foliations until pore fluid pressures along the uppermost and lowermost levels of the foliated zone were balanced. Consequently, pore fluid pressure increased just below the relatively impermeable shear planes. This pore fluid migration and increase in pore fluid pressure would have been critical for slope instability and could have triggered failures, especially within sediments close to the critical state. The findings indicate that pore-fluid behavior associated with shear stress controls the failure patterns and their locations.