In subduction zones where large sediment prisms develop, incipient thrusting at the deformation front defines the plate boundary décollement, which partitions accreted and underthrust sediment. Most of the plate convergence is accommodated by this feature, which eventually becomes the location of megathrust earthquakes. However, although large magnitude earthquakes cannot nucleate at shallow depths, coseismic slip can propagate all the way to the trench as evidenced by the 2011 Mw = 9.0 Tohoku earthquake [Ito et al., 2011; Kodaira et al., 2012]. Large slip in the proximity of the trench is also known to generate very large tsunamis, even if the rupture is slower than usual [Polet and Kanamori, 2000]. Identification of the lithologic unit that hosts the plate boundary fault zone is of primary importance, because the geomechanical properties of the constituent sediments control the slip behavior of major faults.
 The Nankai Trough subduction zone offshore Japan is known for a long history of great Mw ≥ 8 earthquakes [Ando, 1975; Rikitake, 1976] and has been the subject of extensive scientific drilling, primarily on three borehole transects arranged from southwest to northeast known as the Ashizuri, Muroto, and Kumano transects (Figure 1). These three transects are located within 300 km of each other but comparing structurally similar regions of the wedge along each transect reveals that the taper angle can be significantly different, implying corresponding differences in strength both within the wedge and on the basal décollement [e.g., Kimura et al., 2007]. The décollement was penetrated during drilling at the prism toe on the Muroto transect, but was not reached on the Ashizuri transect. Drilling on the Kumano transect is currently active as part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), but depths where the décollement was expected to be located were not reached during expeditions to the toe region [Kinoshita et al., 2009]. The exact stratigraphic location of the plate boundary fault near the trench on the Kumano transect, and how its mechanical properties evolve with progressive subduction, therefore remain open questions.
 Because major faults form as planes of weakness, they are expected to develop in strata that is either intrinsically weak, e.g., due to the presence of clays, or weak due to elevated pore pressures that result in low-effective shear stresses. This is supported by observations from ocean drilling, for example in the Barbados subduction zone where the décollement localizes in a smectite-rich package [e.g., Vrolijk, 1990; Deng and Underwood, 2001]. For the Kumano transect, most friction studies have focused on the behavior of major fault zones such as the megasplay and frontal thrust [e.g., Ikari et al., 2009a; Ikari and Saffer, 2011; Tsutsumi et al., 2011; Ujiie et al., 2011], however, these studies utilize residual friction values using remolded samples. In the case of incipient plate boundary fault formation at the toe, the peak strength of previously undeformed, intact sediment is the key parameter, which is sensitive to lithologic variation but also other factors such as consolidation history and diagenesis progression. Here we measure the peak (maximum) as well as residual shear strength of intact core samples from each major lithologic unit of presubduction sediments at in situ conditions. The entire column of sediment approaching the Nankai Trough was successfully recovered from two Kumano transect reference sites (Sites C0011 and C0012, Figure 1) that were drilled seaward of the trench, providing a complete set of possible lithologic units within which the plate boundary fault may form. We incorporate these measurements with observations of forearc geometry from seismic reflection data in critical taper models (or “Coulomb wedge models”) in order to constrain the location of the plate boundary fault and its strength relative to the prism [Davis et al., 1983; Dahlen, 1990]. Using residual friction measurements at elevated effective stresses, we discuss implications for the plate boundary décollement further landward.