MEGASPLAY FAULT AND SUBMARINE LANDSLIDE HISTORY IN THE NANKAI TROUGH, SW JAPAN

Data and results from the ongoing Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), which eventually attempts to drill, sample, and instrument the seismogenic zone to yield insights into processes responsible for earthquakes and tsunamis offshore SW Japan, provide an extensive base for evaluating both long-term and short-term tectonic processes controlling evolution and current state of this subduction margin, as it relates to the present-day earthquake and tsunami activity.

Here, I review recent NanTroSEIZE achievements and present own results from 3D seismic data interpretation and IODP coring of slope-apron and slope-basin stratigraphic successions in the shallow megasplay fault zone area to document the tectono-stratigraphic development of the Quaternary Nankai accretionary wedge, the origin and evolution of the margin-dominating megasplay fault system and its relation to earthquakes, submarine landslides and tsunamis.

The stratigraphic succession of the slope basin spans ~ 2 Myrs and comprises intercalated intervals with evidence for significant sediment remobilization periods, which are in phase with enhanced activity along the megasplay fault. A remarkable 1 Myrs old lithological transition between a sandy turbidite sequence below and ash-bearing hemipelagites intercalated with mass-transport deposits (MTDs) above, documents a prominent change in sediment delivery and routing pattern in the study area. This correlates to a significant shift in the sediment´s depocentre within the Kumano Basin following ~300 kyr of extensive landward tilting of the outer forarc basin sediments, which has been interpreted to represent a major period of motion along the megasplay that formed the modern fault geometry.

Submarine mass movement is a dominant ongoing process, as evidenced by surficial slump scars and sub-recent MTDs recovered in IODP cores. This, as well as the occurrence of a thin mud-breccia layer related to the 1944 Tonakai earthquake indicates that slope sediments deformation structures and MTDs not only provide long-term records of the structural evolution of the megasplay fault system but also may record the recent seismic activity of large megathrust rupture events.