In subduction zones, megathrust seismicity depends on the hydrogeological, physical and mechanical properties of sediments before they enter the subduction zone and how these properties evolve through the subduction process. In particular, fluids are progressively released by compaction and/or mineral dehydration reactions as burial increases, resulting in the build-up of pore fluid pressure in low-permeability sediments that strongly affects fault behavior through its control on effective normal stress. Thus, characterizing the compaction state and bound water content of sediments in subduction systems is crucial. During IODP Expedition 372 and 375, Site U1518 was drilled in the frontal wedge of the Northern Hikurangi margin ~6.5 km west of the deformation front. This site penetrated an active thrust fault, the Papaku fault, its hanging wall and uppermost footwall. The Papaku fault, intersected at ~304 meters below sea floor, is a westward-dipping splay fault which is thought to lie in the SSE rupture area, to host SSEs and to have accommodated several kilometers of shortening within the prism. It is composed of a ~18m-thick main fault zone with a mixture of brittle and ductile structures and ductile features locally overprinted by brittle fractures and faults. Below, there are a ~21m-thick zone of gradually decreasing brittle-ductile deformation and a ~10m-thick subsidiary fault zone. The folded, faulted and pervasively fractured hanging wall corresponds to Early-Mid Pleistocene hemipelagic silty-claystone with fine-grained turbidites sequences. The footwall is composed by relatively undeformed Mid-Late Pleistocene bioturbated hemipelagic mudstones with turbidites sequences. We use IODP expeditions 372-375 and post-cruise porosity, logging and chemical data to characterize the porosity of sediments at Site U1518, including interstitial porosity, bound water content and fracture porosity. Interstitial porosity is obtained by correcting total porosity measured onboard accounting for clay-mineral bound water using Cation Exchange Capacity (CEC). Unlike total porosity measured onboard, interstitial porosity is representative of the compaction state of sediments. To better characterize interstitial porosity, we document the evolution of the structure of meso- to macropores with depth using mercury injection capillary pressure and low-field nuclear magnetic resonance. We assess the compaction state of sediments at Site U1518 by comparing the interstitial porosity-effective vertical stress curve with that of equivalent siliciclastic units at Site U1520. This site sampled and logged the undeformed input sedimentary section and the top of the oceanic crust ~95 km from shore. We show that the hanging-wall of the Papaku thrust fault is overconsolidated whereas the footwall is normally consolidated. Finally, we discuss deformation history of sediments during accretion regarding the compaction profile and the deformation structures observed at Site U1518.

Attempts to determine physical property across thrust faults at subduction zones through drilling, logging and core sampling have been limited and restricted to exhumed accretionary prisms or shallow parts of active wedges. However, characterizing porosity evolution across the sedimentary section entering subduction zones and accreted sediments is crucial to understand deformation history at accretionary margins through determination of sediment trajectories, quantification of transported volumes of sediments and fluids with related mechanical responses and understanding deformation processes in and around fault zones. International Ocean Discovery Program Expeditions 372 and 375 drilled, logged and cored the entering basin (Site U1520) and active Pāpaku thrust (Site U1518) few kilometers landward of the northern Hikurangi margin deformation front where tsunami earthquakes and recurrent slow slip events occur. Here, we examine physical properties evolution across the Pāpaku thrust at Site U1518 including geophysical logging data, pore size distribution obtained by combining Nuclear Magnetic Resonance and Mercury Injection Capillary Pressure, and interstitial porosity that is representative of sediment compaction state, and compare with that of Site U1520. Interstitial porosity is determined by correcting total connected porosity from clay-bound water content based on cation exchange capacity. We evidence strong variations of physical properties across the thrust fault, with lower porosity, higher P-wave velocity and resistivity in the hanging-wall than in the footwall. We suggest that the porosity pattern at the Pāpaku thrust evidences differences in maximum burial depth with an overcompacted hanging-wall that has been uplifted, thrusted and concomitantly eroded above a nearly normally consolidated younger footwall.