Near the Brink: An Example of a Weak Layer in the Tuaheni Landslide Complex, Hikurangi Margin, New Zealand

International audienceWeak layers in submarine margins are often cited to explain landslide occurrence or reactivation, but little is known about the origin of these weak layers, and it is not always clear if the weak layer existed prior to the landslide or resulted from the failure process. IODP Expedition 372 logged, cored, and sampled Site U1517 to gain deeper insight into the interaction of gas hydrate and submarine landslides. Using physical data and core from Site U1517, we identify and characterize a weak layer in Tuaheni Landslide Complex (TLC) in the Hikurangi margin, New Zealand. The TLC is believed to behave in a creeping mode. We hypothesize that this weak layer could have been the result of deformation during failure (e.g., strength weakening) or the result of inherent weakness in the sediments. The identified weak layer in the TLC has an anomalous reduction in shear strength, porosity, and permeability that cannot be attributed to normal consolidation or composition changes. Furthermore, this weak layer could act as a potential slip plane for future creeping (i.e., reactivation) in the presence of a minor change in the stress or pressure field (e.g., earthquake). Based on scanning electron microscopic imaging of core samples, we conclude that the properties in the weak layer could be the result of mechanical rearrangement of clay particles that form “clay bridges” around framework grains. The measured low strength of the weak layer is approximately equal to the downslope gravitational stresses suggesting that the landslide is at or near failure— which may explain the creep-like behavior attributed to this landslide. Based on the physical properties from the TLC, we estimate the factor of safety and run numerical simulations using Plaxis software. We finally create sensitivity analysis to test likely scenarios that could reactivate this landslide. This study and methodology are relevant for many other underwater margins around the world where submarine landslides may be at or near the brink of failing