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The Taihape landslide is a large, deep-seated, translational rockslide in central North Island of New Zealand, one of over 7000 large landslides mapped in the late Neogene deposits of this area. This study examined its patterns of movement using high temporal resolution, high-precision monitoring and explored the relationships between the movement and triggering mechanisms. During the 30-year monitoring period, movement was always extremely slow, but included periods of faster and slower motion due to a combination of simultaneous basal sliding, internal plastic deformation and seasonal soil shrinkage and swelling. The data suggested that movement rate was not controlled by pore-water pressure or any other monitored parameter. Intervals of more rapid movement appear to follow toe erosion that debuttress the landslide mass, an example of which occurred following a flood in February 2004 that had a return period of about 100 years. The high-temporal and spatial frequency of the monitoring and the measurement precision of the instruments have allowed new insights into the complex movement patterns of deep-seated, slow moving rockslides. The patterns of movement observed here, and the techniques described to collect and analyse the data, are applicable to many slowly moving landslides in New Zealand, and to those occurring in similar environments worldwide. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]