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Landslides are widespread geomorphological features on the North Island of New Zealand, where they represent one of the primary landscape-forming processes. This study focuses on the steepland terrain surrounding Gisborne, a city located on the east coast of the North Island, at the Hikurangi subduction margin. This terrain consists of young, weak, sedimentary rocks and soils; the most common modes of slope failures are soil creep, slides and flows in shallow, clay-rich soil and regolith, triggered by heavy rainfall. Based on observational data from Sentinel-1 imagery, this study leverages results from interferometric synthetic aperture radar (InSAR) processing to reveal the distribution of deformation across Gisborne's steepland periphery from January 2016 to December 2021. Velocities in the line of sight were obtained from the stack of interferograms and projected along the direction of maximum slope, to extract the true displacement on the slopes. The ascending and descending data sets were combined to reveal the vertical and horizontal components of the deformation. The results were combined with a regional LiDAR data set, aerial imagery and field observations to delineate areas of slope deformation. Finally, slope deformation time series data was compared with rainfall records to identify seasonal changes, due to shrinking and swelling of expansive soils. Although the InSAR displacement data contains some noise, results could be used to identify 132 unstable slopes within the study area, caused by soil creep and earthflows. Also, the shrink-swell of expansive soils causes a seasonal pattern of displacements, which varied by 10–20 mm/year between Austral winter and summer, strongly correlated to rainfall. [ABSTRACT FROM AUTHOR]

Gisborne (North Island, NZ) is affected by rainfall-induced landslides, earthquakes and tsunami, as well as mud volcanoes (MVs). The latter form via upward mobilization of Eocene–age sediments, and have not been studied from an engineering geological standpoint, so the 15 December 2018 Waimata Valley MV eruption provided a unique opportunity. The event erupted c. 16 900 m3 of mud, forming an elevated vent area, and three mudflows propagating north, east and south. Scanning electron microscopy (SEM) identified smectite as the dominant clay in the Sr-rich mud, and Atterberg limits indicate high plasticity. In-situ testing using dynamic cone penetrometer and shear vane (3–168 kPa) revealed wide variability in strength properties with depth, while ring shear values are 11.3–13.5°. A fault extends NW beneath the Waimata Valley MV, coinciding with the pre-existing Arakihi Road MV. The Waimata Valley MV area was subject to uplift and cracking during the September 2016 Te Araroa earthquake (Mw7.1), which caused increased activity at pre-existing mud volcanoes at that time. Geodetic data for the Gisborne district shows an uplift phase culminating around the December 15 2018 MV eruption, followed by the commencement of a Hikurangi subduction zone 'slow slip event'. Nevertheless, relationships between tectonics and MV eruptions remain equivocal. [ABSTRACT FROM AUTHOR]

Overseas, relict deep-seated coastal landslides have attracted significant analysis because of potential hazards caused by sea level rise and climate change. Despite the rapid growth in the Auckland region, regional coastal landslide hazards are poorly understood. We investigated the geomorphology and material properties of Ohuka coastal landslide on the southwestern coastline of the Auckland region, to better understand the failure mechanism. The site is formed on weak Neogene sediments of varying lithological properties, with very low angles of dip, exposed on both the seaward (west) and landward (east) flanks of Ohuka Hill. Displacement appears to have been rotational, along a low-angled basal shear surface, the Koheroa Clay Seam. The surface geomorphology consists of discontinuous uphill-facing scarplets indictive of rotational slump blocks, along the basal shear surface. However, the preservation of slump block surface topography is hindered due to the lack of a thick, strong caprock, and high annual rainfall (1400 mm), and erosion. The proposed failure mechanism and general geometry is consistent with correlations between graben width and depth to failure plane reported from bedding-controlled coastal landslides formed on low dip angles in southern England. This provides important context, given the paucity of New Zealand bedding-controlled coastal landslide case studies. [ABSTRACT FROM AUTHOR]

Landslide dams can be very dangerous, with inundation occurring via rising waters upstream, and flooding downstream via dam breaching. Here, we report on a landslide that dammed the Mangapoike River in eastern North Island, New Zealand. The landslide is a low-angle wedge failure in the Miocene weak rock sandstones and mudstones of the Tolaga Group, forming a landslide dam (volume c. 8 million m3) and a lake 50 m deep with a surface area of 0.35 km2, before explosives were used to form a dam spillway to decrease lake level. The landslide formed along an escarpment in northwest-dipping sandstones, and is characterised by a linear lateral scarp, a headscarp, and a bedding-plane rupture surface, which controlled the landslide block geometry. The headscarp and lateral scarp have developed along propagating vertical fractures. The slide surface is a smoothed, northwest-dipping bedding plane, and intersects the vertical fractures in the lateral scarp, forming a wedge. While the principal failure mechanism was sliding involving a single large wedge-shaped block, the rapid movement led to disintegration of most of the block. Part of the detached slide block remained intact, but most of the displaced mass forming the landslide dam is disaggregated blocks in a sandy-silty matrix. Rainfall and meteoric groundwater probably did not initiate failure. Instead, river incision of the dip slope toe, and overpressurisation of fluids that are known to accumulate in sandstones overlain by impermeable mudstones in the region, probably decreased the effective stress along the existing bedding plane, initiating failure. [ABSTRACT FROM AUTHOR]

Mountain glaciers are useful climatic indicators in alpine regions where instrumental climate observations are sparse. Here we review the state of understanding of past and present glaciation in North Island, New Zealand. During the last glacial, mountain glaciers descended to ∼1200 m on both Ruapehu and Tongariro. Glacier modelling indicates that air temperatures during the Last Glacial Maximum were 5–7°C lower than present. Under such conditions, favourable topoclimatic mass balance processes (e.g. snowblow, shading) may have promoted marginal glaciation in lower-elevation North Island mountain ranges. The timing and magnitude of North Island glacier changes are broadly consistent with moraine chronologies and climatic reconstructions from glaciers in the Southern Alps. As of 2016, several glaciers and multi-year snow patches persist in North Island (totalling 3.0 km2 – a 25% reduction in area since 1988), all situated on the summit region of Ruapehu volcano. Mass balance of these glaciers is modulated by topoclimatic factors and debris cover, which may reduce rates of mass loss in the near term. However, given the proximity of the present equilibrium line altitude to the summit elevation of Ruapehu, complete loss of the present glaciers is probable under even moderate warming scenarios predicted for the coming decades. [ABSTRACT FROM AUTHOR]

The article presents a study regarding the tilting of active folds and drainage asymmetry on the Manawatu Anticlines, New Zealand. It explores the southward tilting of five anticlines in the North Island, Manawatu, and New Zealand. It utilizes two drainage basin analysis techniques including the transverse topographic symmetry factor (T) and the asymmetry factor (AF). Techniques were utilized in order to ascertain whether westward flowing trunk streams on the gentle western limbs of the five anticlines are migrating southward across their drainage basins, accompanying with the down-tilting of the fold axes, wherein results were inclusive.