Natural and experimental faulting of rocks, Nankai Accretionary Prism. Follow-up work to lODP Expeditions 315 and 316.

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Presently the Nankai Accretionary Prism offshore Japan is the stage of a most ambitious research venture, the NanTroSeize Project, geared at understanding fundamental processes of faulting and earthquake generation in a transect of shallow non-riser and deep riser drillholes. IODP Expeditions 315 and 316 investigated the shallow frontal thrusts, and the hangingwall to a major active splay fault, to document near-surface active deformation in front of, and above the seismogenic zone. Here we advance the hypothesis that the partitioning of deformation in the Nankai accretionary wedge reflects the capability of the rocks involved to generate seismic faulting and related tsunamis. Is brittle faulting in the accretionary wedge in response to a large seismic event capable of producing dramatic surface breaks, or are the rocks more amenable to slow, stable slip, strain weakening, and distribution of deformation within large volumes? We investigate this by conducting a series of deformation experiments that will provide data on strength, stress paths, and pore pressure evolution in muds and mudstones from the drill cores. To cover those parts that were, and possibly will not be reached by the drilling, we have supplemented the Nankai data base by studying samples from two important analogue cases on land: the Shimanto Belt and the Bozo accretionary wedge. Comparison of natural and experimental deformation structures and fabrics will guide extrapolation of the laboratory data to the natural case.