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The primary objective of Integrated Ocean Drilling Program (IODP) Expedition 348 was to characterize the variations of lithology and structure at intermediate depths of the Nankai Trough accretionary prism (offshore southwest Japan). Site C0002 (Fig. F1) is located on the upper plate of the subduction zone above the seismogenic (presumably locked) portion of the plate boundary thrust system (see the “Expedition 348 summary” chapter [Tobin et al., 2015a]). The long-term goal of deeper drilling at this site is to cross the megasplay at seismogenic depths where coseismic slip occurred during the 1944 Tonankai earthquake (Ichinose et al., 2003). During Expedition 348, IODP collected a full suite of logging-while-drilling data, cuttings at ~5 m intervals, and cores over a 55.5 m interval from 2163 to 2218.5 meters below seafloor (mbsf). The cores are the deepest ever recovered from an active accretionary prism.

Intrinsic permeability (k) and hydraulic conductivity (K) values for natural clay–rich sediment and sedimentary rock (e.g., Neuzil, 1994; Aplin et al., 2006; Gamage et al., 2011) typically span several orders of magnitude. That natural variability exists because their hydrogeological properties depend on many factors, some of which are inherited from the time of mud deposition. Grain size and shape, sorting, particle orientation, surface charges on clay particles, and microfabric are important, as are the superimposed effects of burial diagenesis (e.g., Moon and Hurst, 1984; Bennett et al., 1989).

The cores from Hole C0002P contain numerous layers and irregular lenses of fine sand- and siltstone, with thicknesses ranging from laminae (millimeter scale) to ~20 cm. Those coarser textures should increase values of permeability. Furthermore, the strata dip steeply over the cored interval, typically at angles >80° (see the “Expedition 348 summary” chapter [Tobin et al., 2015a]). Those orientations should alter the anisotropy of permeability (i.e., horizontal versus vertical) relative to a normally compacted sedimentary basin. Our study contributes new data to the transect-wide Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), thereby allowing for hydrogeological comparisons among samples from the subduction inputs, Kumano forearc basin, the frontal accretionary prism, and the deeper (inner) accretionary prism (Dugan and Daigle, 2011; Ekinci et al., 2011; Guo et al., 2011; Saffer et al., 2011; Hüpers and Kopf, 2012; Rowe et al., 2012; Yue et al., 2012; Dugan and Zhao, 2013; Screaton et al., 2013; Daigle and Dugan, 2014; Guo and Underwood, 2014).