An extremely low velocity layer was detected in a deep water area, whose velocity was estimated to be 1500 m/s by velocity analysis of the seismic data. Its thickness is approximately 500 m in average and exceeds 700 m in maximum. Although obtained data are limited to reveal its origin, the layer was interpreted to be a mass transport deposit based on the observations: a distribution feature spreading toward the deep water, a chaotic-to-transparent reflection pattern, basal erosions of the underlying sediments and a pressure ridge developed at the tip region. However, mass transport deposit is normally characterized by high seismic velocity according to previous studies, whereas such a mass transport deposit with low velocity is seldom reported. From detailed seismic interpretation, active normal faults cutting the seafloor were recognized in overlying sediments. The faults appear also to cut the uppermost part of the mass transport deposit. Moreover, pore pressure gradient within the deposit was estimated to be from 12.0 to 14.4 MPa/km, being comparable with the fracture gradient from previous study. As a result, the presently-detected low velocity layer can be concluded most likely as a slow-moving earthflow type of mass transport deposit in deep water.
