ALBERT

Description: Subaqueous slopes are susceptible to a broad range of failure mechanisms and deformation styles, many of which are not well characterised. We undertook novel laboratory-based testing using a Dynamic Back-Pressured Shearbox on samples collected from an area subject to ongoing slope failures, situated on the upper slope of New Zealand's Hikurangi Margin, to determine how increases in pore water and gas pressures generate shallow mass movement. Using both water and nitrogen gas we observed similar responses in both cases, indicating that behaviour is dominated by the normal effective stress state regardless of pore-fluid phase. Shear-strain accumulation, representing landslide movement, shows a slow episodic pattern, in common with many shallow terrestrial landslides. Our results are relevant for landslides occurring in shallow near surface sedimentary sequences but have implications for deep-seated landslide behaviour. They suggest that once movement initiates at a critical effective stress, its rate is regulated through dilation and pore expansion within the shear zone, temporarily increasing effective stress within a narrow shear band and suppressing rapid shear. Consequently, under certain conditions, shallow submarine landslides (e.g. spreading failures) can undergo slow episodic movement which allows them to accumulate large shear strains without accelerating to catastrophic movement even when they are unconstrained.

Description: Highlights • We report on methane seeps found at shallow depths on New Zealand's Hikurangi Margin. • Tools have been applied to measure bubble characteristics from video footage and to estimate the gas flow rate using acoustic data. • We estimate that the entire Tuaheni seep field produces somewhere in the range of 30–2415 t of methane per year. • The density of seeps at this location is far greater than anything else observed on the Hikurangi Margin. Abstract We analyse an area of high density submarine methane gas seeps situated on the shelf to slope transition (130–420 m water depth) on the northern region of New Zealand's Hikurangi margin, off Poverty Bay. Multibeam and singlebeam echo sounder data collected in 2014 and 2015 revealed 〉600 seeps, at much greater density than any previously mapped areas of seepage on the Hikurangi margin. To broadly constrain the output of methane from these seeps, we have estimated the flow of methane at individual seeps, utilising perspective-measurements applied to still frames from a deep towed camera system to measure the dimensions of rising bubbles. We combine bubble size and rise-rate distributions with singlebeam acoustic data to estimate gas flow rates at six selected seeps sites. Our results predict a wide range (3.0–2249 mL/min) of methane release into the water column. If we assume that the six seeps we analysed are representative of the entire seep population, and that gas flow is constant, we can extrapolate across the seep field and infer a gas release of 30 to 2415 t of methane per year into the ocean.

Description: Highlights • Seismic depth imaging gives insight into the southern Hikurangi subduction zone. • Velocities reveal regional variations in compaction and drainage of input sediments. • Dewatering of subducted sediments might influence décollement strength. • Thrusts at the leading edge of deformation are upper-plate dewatering pathways. • Stratigraphic host of the décollement changes at the southern end of the margin. Abstract The southern end of New Zealand's Hikurangi subduction margin accommodates highly oblique convergence between the Pacific and Australian plates. We carry out two-dimensional (2D) seismic reflection tomography and pre-stack depth migrations on two seismic lines to gain insight into the nature of subducted sediments and upper plate faulting and dewatering at the toe of the wedge. We also investigate the NE to SW evolution of emergent upper plate thrust faulting using 47 seismic lines spanning an along-strike distance of ∼270 km. The upper sequence of sediments that ultimately gets subducted (the MES sequence) has an anomalously-low seismic velocity character. At the southwestern end of the margin, ∼150 km east of Kaikōura, the MES sequence has experienced greater compaction (for an equivalent effective vertical stress) than it has some 200 km further to the northeast. This difference is likely attributable to greater horizontal compression in the southwest caused by impingement of the Chatham Rise on the deformation front. Relationships between velocity and effective vertical stress suggest that the MES sequence is well-drained in the vicinity of frontal thrusts, corroborated by evidence for upper plate dewatering along those thrusts. Effective drainage of the MES sequence likely promotes interplate coupling on the southern Hikurangi margin. The décollement is generally hosted near a seismic reflector known as “Reflector 7”. East of Kaikōura, however, Reflector 7 becomes accreted, indicating that subduction slip at the southwestern end of the margin is no longer hosted at (or above) this reflector. Instead, the décollement steps down to a deeper stratigraphic level further inboard. Further to the SW, approximately in line with the lower Kaikōura Canyon, the offshore manifestation of subduction-driven compression ceases.

Description: Although subaqueous slopes on active continental margins are subject to a variety of failure styles, their movement mechanisms during earthquakes remain poorly constrained. A primary explanation is that few submarine landslides have been directly sampled for detailed investigation. We have conducted a series of dynamic shear experiments on samples recovered from the base of the Tuaheni Landslide Complex, located off the east coast of the North Island of New Zealand, to explore its behaviour during earthquakes. Our experiments suggest that whilst the basal landslide sediments can be prone to liquefaction in certain conditions, this is not a likely failure mechanism at the stress states operating in the low angled shear zone at the base of this landslide system. Instead, episodic landslide movement can occur through basal sliding when pore water pressures increase sufficiently to lower the shear zone effective stress to the material failure envelope. These low effective stress conditions are most likely to be reached during earthquakes that produce large amplitude, long duration ground shaking. The observed behaviour provides a credible mechanism through which subaqueous landslides moving on low angled shear zones in similar materials may be subject to episodic movement during earthquakes without undergoing catastrophic failure.