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ABSTRACT:We study the influence of an upper-plate fault on the stress state of accreting sediments under large-scale deformation. We develop drained evolutionary geomechanical models using the Finite Element program Elfen. We simulate sediments as porous-elastoplastic material, and we model the fault as a pre-existing contact surface with a varying frictional strength that is lower than the intact sediment. The weaker fault results in a decrease in sediment differential stress near and especially seaward of the fault. A significant section of the wedge is affected by this stress variation. In contrast, the stress ratio is that of Coulomb failure further away from the fault. We also show that the maximum principal stress the sediments can support decreases with decreasing fault strength. This study offers a significant improvement over previous models of continuum wedge sediments that predict Coulomb failure throughout the wedge. Our results improve our understanding of near-fault stress state, hence improving our understanding of seismic hazards in subduction zones and providing practical insights for reservoir quality and the design of safe and economic well trajectories. 1 INTRODUCTIONAccretionary wedges are geological structures that develop at convergent plate boundaries, particularly at subduction zones. They form as a result of offscraping sediments from the subducting oceanic plate, which then accumulates on the leading edge of the overriding plate (e.g., Moore et al., 2011; Buiter et al., 2016; Gao et al., 2018).Understanding the state of stress in accretionary wedges is crucial for the study of earthquake mechanics in subduction zones (e.g., Brodsky et al., 2017; Huffman and Saffer, 2016; Suppe, 2007). It provides insights for earthquake occurrence, large or slow-slip events (e.g., Kodaira et al., 2012; Tobin et al., 2022; Liu and Rice, 2007) and tsunami potential (e.g., Dean et al., 2010; Riedel et al., 2016). In addition, estimates of mean and differential stress, as well as porosity evolution can help improve the assessment of reservoir quality and the design of safe and economic well trajectories (e.g., Morley et al., 2011).The accumulation of sediments in an accretionary wedge resembles a bulldozer gathering snow as it moves. Hence, wedge sediments are often assumed to be at compressional failure (e.g., Davis et al., 1983; Dahlen et al., 1984; Flemings and Saffer, 2018). Several field observations of faulting, folding and lateral compression as seen in seismic images and drilling measurements (e.g., Flemings and Saffer, 2018; Henry et al., 2003; Moore et al., 1990; Westbrook et al., 1988) provide evidence that the accretionary wedge is at a state of failure in compression. 

Abstract We study stress, pressure, and rock properties in evolving accretionary wedges using analytical formulations and geomechanical models. The evolution of the stress state from that imposed by uniaxial burial seaward of the trench to Coulomb failure within the wedge generates overpressure and drives compaction above the décollement. Changes in both mean and shear stress generate overpressure and shear‐induced pressures play a particularly important role in the trench area. In the transition zone between uniaxial burial and Coulomb failure, shear‐induced overpressures increase more than overburden and are higher than footwall pressures. This rapid increase in overpressure reduces the effective normal stress and weakens the plate interface along a zone that onsets ahead of the trench and persists well into the subduction zone. It also drives dewatering at the trench, which enables compaction of the hanging‐wall sediments and a porosity offset at the décollement. Within the accretionary wedge, sediments are at Coulomb failure and the pore pressure response is proportional to changes in mean stress. Low permeability and high convergence rates promote overpressure generation in the wedge, which limits sediment strength. Our results may provide a hydromechanical explanation for a wide range of observed behaviors, including the development of protothrust zones, widespread occurrence of shallow slow earthquake phenomena, and the propagation of large shallow coseismic slip. 

Abstract Vast amounts of carbon are stored beneath the seafloor in the form of methane hydrate. Hydrate is stable at moderate pressure and low temperature at a depth extending several hundred meters beneath the seafloor to the base of gas hydrate stability (BGHS) often marked by bottom simulating reflections (BSRs) in seismic profiles. However, data from logging‐while‐drilling and coring during Integrated Ocean Discovery Program Expeditions 372 and 375 offshore New Zealand identified hydrate ∼60 m beneath the BSR. This hydrate appears to be dissociating over thousands of years following a gradual temperature increase from sediment burial modulated by changes in bottom‐water temperature and sea‐level fluctuations. Slow hydrate dissociation significantly buffers the release of methane and therefore, carbon through glacial cycles. Dissociating hydrate beneath the BGHS may also increase estimated global budgets of methane stored in hydrate.