Slope instabilities and gravity processes in fluid migration and tectonically active environment in the eastern Nankai accretionary wedge (KAIKO-Tokai’96 cruise)

One main feature imaged on the Nankai slope by using the SAR/PASISAR system (deep-tow single-channel seismic streamer) during the Franco-Japanese KAIKO-Tokai cruise in April 1996 is the gravitational displacement of large volumes of the sediment. This slope failure is assumed to be mainly related to an increase of the slope angle beyond its threshold value, tentatively linked to the uplift effect of the subduction of a paleo-Zenisu ridge and to the existence of a basal shear plane possibly controlled by altered properties of the sediments at the base of the gas hydrate stability field. The role of pore fluid (or free gas) is discussed but remains unknown without any in situ data, this role may be major if considering the gassy environment in the sedimentary cover where earthquake loading can cause excess pore pressure and act as a major triggering mechanism for sliding.