Set up for failure. High sedimentation rates, earthquakes and the Tuaheni Landslide Complex.

The Tuaheni Landslide Complex, on the upper slope of the northern Hikurangi Margin, has been studied for over a decade following the interpretation that it is subject to ongoing downslope activity without runaway failure. A number of processes have been implicated as preconditioning and triggering mechanisms, including earthquakes, subsurface gas, static loading and gas hydrates. The most recent investigations have used MeBo and IODP drilling and focused on testing hypotheses related to the role of gas hydrates.

The 145 km² landslide complex debris deposit occurs in 500-900 m water depth. Kinematic indicators from the debris surface indicate failure in three directions, all of which are unsupported on the downslope free face. Surface slope gradients range from 1.5-4° while the interpreted failure surface averages ~2°. The presence of this relatively large debris deposit on the slope with no downslope support (i.e. it has no come to rest in a slope basin) is unusual on the Hikurangi Margin where 99% of landslides have no obvious debris deposit associated with the slide scar. In the context of global understanding this is one of the key elements that makes this landslide intriguing: What enables the deposit to shuffle down slope rather than run away? And, what are the hazard implications of large perched landslide deposits?

IODP site U1517 drilled to 187 m through the Tuaheni Landslide Complex and achieved good recovery. Indicators for gas hydrates were observed from ~100 m below seafloor to the BSR depth of ~165 m. Sediments are within radiocarbon dating age allowing a good age model to be developed in combination with biostratigraphic data. Visual logging, sediment analysis and XRAY-CT image analysis provide insight into the depositional environment of the landslide source material and subsequent deformation of the slide mass. This sedimentary and structural dataset links in nicely with complimentary studies on geochemistry and laboratory testing and modelling to provide significant insight into what is now one of the most studied landslides in the world.