[1] Despite its importance for plate boundary fault processes, quantitative constraints on pore pressure are rare, especially within fault zones. Here, we combine laboratory permeability measurements from core samples with a model of loading and pore pressure diffusion to investigate pore fluid pressure evolution within underthrust sediment at the Nankai subduction zone. Independent estimates of pore pressure to $20 km from the trench, combined with permeability measurements conducted over a wide range of effective stresses and porosities, allow us to reliably simulate pore pressure development to greater depths than in previous studies and to directly quantify pore pressure within the plate boundary fault zone itself, which acts as the upper boundary of the underthrusting section. Our results suggest that the time-averaged excess pore pressure (P*) along the décollement ranges from 1.7-2.1 MPa at the trench to 30.2-35.9 MPa by 40 km landward, corresponding to pore pressure ratios of l b = 0.68-0.77. For friction coefficients of 0.30-0.40, the resulting shear strength along the décollement remains <12 MPa over this region. When noncohesive critical taper theory is applied using these values, the required pore pressure ratios within the wedge are near hydrostatic (l w = 0.41-0.59), implying either that pore pressure throughout the wedge is low or that the fault slips only during transient pulses of elevated pore pressure. In addition, simulated downward migration of minima in effective stress during drainage provides a quantitative explanation for down stepping of the décollement that is consistent with observations at Nankai.Citation: Skarbek, R. M., and D. M. Saffer (2009), Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering,

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We implement an algorithm to automatically detect migrations of low frequency earthquakes at time scales between 30 min and 32 h during the 2003, 2004 and 2005 slow slip events in Cascadia. We interpret these migrations of seismicity as a passive manifestation of secondary slip fronts (SSFs) that propagate faster than the main front. We identify the dominant features of 383 SSFs, including time, location, duration, area, propagation velocity and estimate: their moment, stress drop, slip, and slip rate. We apply the same algorithm to continuous tremor detection in Cascadia between 2009 and 2015 and characterize 693 SSFs at time scales between 4 h and 32 h. We identify -to our knowledge for the first time -numerous 10-24 h long SSFs that propagate at velocities intermediate between slow slip events and previously reported SSFs. The systematic detection of SSFs fills a gap between seismically and geodetically detectable slow earthquake processes. Analyses of SSF basic features indicates a wide range of stress drops and slip rates (with me-Email address: qbletery@uoregon.edu (Quentin Bletery) dians of 5.8 kPa and 1.1 mm/h) as well as an intriguing relationship between SSF direction and duration that was observed in other contexts and could potentially help discriminate between the different physical models proposed to explain slow slip phenomena.

Eighteen constant-rate-of-strain consolidation tests were performed on whole-round core samples from Integrated Ocean Drilling Program Sites C0002, C0006, and C0007. These sites are located along the Kumano transect of the Nankai Trough offshore south-central Japan; Site C0002 is in the forearc basin and the other two are in the frontal thrust zone. Sample depths range from ~35 to 920 m core depth below seafloor. The objectives of the laboratory tests were to obtain the compression characteristics of the sediments, to estimate the maximum pretest consolidation stress (P′ c ), and to determine values of hydraulic conductivity (K), intrinsic permeability (k), and compression index (C c ). Values of C c at Site C0002 range from 0.391 to 0.780 (average = 0.584); comparable values are 0.236 to 0.418 (average = 0.302) in the trench-wedge facies at Sites C0006 and C0007. Values of in situ intrinsic permeability (k i ) at Site C0002 show no trend with depth, ranging from 2.67 × 10 -17 m 2 to 2.56 × 10 -18 m 2 . Estimates of k i at Sites C0006 and C0007 decrease with depth and range from 1.85 × 10 -16 m 2 to 6.08 × 10 -18 m 2. Test-derived values of P′ c are consistently greater than the estimates of in situ hydrostatic vertical effective stress at equivalent sample depths, which means the specimens are overconsolidated. Environmental scanning electron microscopy was used to assess the degree of preferred particle orientation of the microfabric. The index of orientation averages 0.33 at Sites C0006 and C0007 and 0.38 at Site C0002. Most vertical sections show indexes of orientation greater than those for horizontal sections; exceptions to this pattern include several samples from Sites C0006 and C0007 where bedding dips >40°. Samples from deeper intervals of the forearc basin (Site C0002) and from the accreted upper Shikoku Basin facies at Site C0007 yielded the highest values of orientation index (>0.40), indicating better alignment of platy grains. IntroductionConsolidation characteristics of marine sediments and sedimentary rock are used to evaluate diagenetic, hydrologic, and compaction processes during subduction and accretion (e.g., Lee et al., 1973; Trabant et al., 1975; Carson, 1977; Shepard and Bryant, 1977; Taylor and Bryant, 1985;Morgan and Ask, 2004;Spinelli et al., 2007 Kinoshita, M., Tobin, H., Ashi, J., Kimura, G., Lallemant, S., Screaton, E.J., Curewitz, D., Masago, H., Moe, K.T., and the Expedition 314/315/316 Scientists Proceedings of the Integrated Ocean Drilling Program, Volume 314/315/316 J. Guo et al. Data report: consolidation characteristics of sedimentProc. IODP | Volume 314/315/316 2 from tectonically induced (nonvertical) stress, deformation, cementation, and/or uplift and erosion of overburden, whereas underconsolidation (values less than expected) can be caused by excess pore water pressure (i.e., greater than hydrostatic) and the presence of gas and gas hydrates. Consolidation tests also provide estimates of in situ fluid pressure, permeability, and bulk sediment compressibility (Ka...