NanTroCORK: Development of hydrologic long-term CORK observatories in the seismogenic Nankai Trough subduction zone, Japan

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IODP has started the complex drilling project "NanTroSEIZE" (Nankai Trough Seismogenic Zone Experiment), a multi-stage effort to drill a transect of 7 sites into the the subduction megathrust off Japan. One fundamental goal of NanTroSEIZE is the establishment of a distributed long-term observatory network spanning the incoming plate down to up-dip limit of the seismogenic zone. We here propose installations at two drillsites which are governed by high strains and rapid dewatering in the frontal forearc of the subduction system. At these sites, pore pressure (as a proxy for strain) and temperature will be monitored in sediments of the accretionary wedge and underthrust succession as well as in their equivalent further up-dip. The selected sites are especially critical for monitoring because of their location at the overriding Eurasian Plate (NT 1-03) and the downgoing Phillipine Sea Plate (NT 1-07). In the former location, the frontal thrust at the prism toe will be penetrated, and the installed instruments will be able to eventually "communicate" with similar ones down-dip once Stage 3 drilling intersects the plate boundary there. At the latter site, the entire subduction input will be characterized in situ, which is particularly vital since sands in the lower portion are permeable and have been found charged with deep-seated fluids. Instrument development will be carried out in parallel with an US CORK to be installed in spring 2008, which is already underway.