Basal sliding and plastic deformation of a slow, reactivated landslide in New Zealand

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 100years. 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. •Displacements of the Taihape landslide, New Zealand, were measured•The landslide represents one of 7,000 mapped of this type in sedimentary rocks•Landslide displacement comprised basal sliding and plastic deformation•Landslide displacement appears to be controlled by erosion of the toe by a stream•Seasonal-cyclic movements were also identified and linked to variations in soil moisture