10+ years continuous downhole pore pressure record from the decollement zone at Nankai Trough off Muroto

Since 2001, borehole pore pressure data have been continuously logged near the toe of Nankai accretionary prism off Muroto, by usingtwo 'ACORK'borehole observatories installed during the Ocean Drilling Program (ODP) Leg 196. Pore pressure are recorded at multiple intervals from the seafloor to the plate interface. Data have been retrieved once every year using the Remotely-Operated Vehicle KAIKOor manned submersibleShinkai 6500of JAMSTEC. We now have over 10-year-long continuous pressure records, and present here a preliminary data. The overpressure of up to 100 kPa was observed just above the décollementzone, although the reliability of the pressure data there is yet to be tested in the future. Pressure oscilationsfrom multiple depths,corresponding to the tidal modulations,show systematic decrease with increasing depth.Transient changes were observed at the time of several earthquakes, including a swarm of very-low-frequency events (VLFE) in 2003 and 2004.We believe that continuous pressure data can make an essential reference to the possible pressure anomalies prior to the near-future great subduction zone earthquake.