Search

The aim of this paper is to report on fatal landslides resulting from two extraordinary storm events in January and February 2023, in Auckland, New Zealand, including the characteristics of the terrain, the site geology, and the storms. The January 27 Auckland Anniversary storm was from an "atmospheric river," dumping Summer's worth of rain (265 mm) in one day. This was the highest 24-h total on record, estimated a 1 in 200-year event, with a peak 2-min rainfall of 4.2 mm, coinciding with Friday's evening "rush hour." This led to widespread landsliding throughout Auckland, including a fatal landslide in Parnell, central Auckland. Notably, this occurred only meters from a similarly destructive landslide in 1997. With January 2023 Auckland's wettest January on record, and with the region still in a state of emergency, Tropical Cyclone Gabrielle crossed the region on February 13–14. This also caused widespread landsliding, including a fatal landslide at Muriwai on the west Auckland coast. Fatal landslides from the same escarpment had occurred in 1965, close to the February 13, 2023, fatal landslide. Taken together, both storm events caused > 140,000 landslides across the North Island, and flooding, making hundreds of people homeless, with damage estimates of > US$8.6 billion (3.4% of NZ's GDP). For future disaster risk reduction, attention should be paid to revising legislation and local planning, which currently allows people to legally build (and re-build) houses in landslide-prone areas of New Zealand. [ABSTRACT FROM AUTHOR]

New mapping of the Barberton Greenstone Belt in South Africa shows that the central part is a pseudo-stratigraphy made of shallow-water and deep-water siliciclastic and volcanic slide blocks, with individual blocks ranging in size from tens of meters to >10 km in length. The outcrop pattern and scale are remarkably similar to those of large-scale Miocene to recent submarine landslides in New Zealand along the active Hikurangi subduction zone that are periodically triggered by earthquakes on the subduction megathrust, providing evidence for megathrust earthquakes in the Paleoarchean. [ABSTRACT FROM AUTHOR]

Tectonic deformation within fault damage zones can influence slope stability and landslide failure mechanisms due to rock mass strength effects and the presence of tectonic structures. Here, we used detailed site investigations to evaluate controls on deformation within the Half Moon Bay landslide complex, located ~1 km from the surface trace of the Hope fault in the South Island of New Zealand. During the 2016 Mw 7.8 Kaikōura earthquake, the slope experienced up to ~13 m of displacement and partially transitioned into a rock avalanche (with a volume of ~350,000 m³). Deep-seated deformation of the entire slope predated the 2016 earthquake. Results of geomorphological analysis, field mapping, geophysical surveys, slope displacement, and a 60-mdeep borehole in the incipient portion of the landslide indicated the presence of a subvertical tectonic fabric and intense fracturing and weathering of the rock mass, which gradually decrease with depth. Based on these results, we established a conceptual model wherein the landslide failure mechanism is a combination of flexural toppling along the subvertical structures coupled with joint-step-path sliding along preexisting, closely spaced discontinuities within the graywacke rock mass. Coseismic slope displacements revealed a large area of incipient failure behind the headscarp of the 2016 rock avalanche, which will likely result in further avalanching at the site. This case study demonstrates that inherited tectonic structures (combined with seismicity and weathering in an oversteepened coastal slope) play an important role in the evolution of hillslopes near active faults. [ABSTRACT FROM AUTHOR]

Typhoon‐induced landslides are widespread, damaging and deadly. In this study, we elucidated their spatiotemporal features through investigations in Southeast coastal China and proposed a numerical model that involves typhoon wind load, preferential infiltration and root reinforcement. The wind load is calculated through a combination of the autoregressive method and a multi‐degree‐of‐freedom tree swaying mode, transmitting to the slope through the wind–tree interaction. Taproots of trees are modelled as piles, and root–soil interfaces are modelled as preferential infiltration boundaries. Using the numerical model, we quantitatively assessed the impacts of wind load, rainfall infiltration and root reinforcement on the slope stability and compared results with cases in Waipaoa and Wairoa (New Zealand). Results suggest that the impacts of trees depend heavily on meteorological and geological conditions. Trees play a destabilizing role in Southeast China during a typhoon but a stabilizing role in the cases in New Zealand. For slopes in Southeast China with a thick soil layer (>root depth), strong wind load and preferential flow resulting from root–soil interfaces, rather than slope surface infiltration, significantly decrease the slope stability in a short time. Slope failure occurred in all scenarios that account for the preferential infiltration, and its combination with a Force 14 typhoon wind load can fail the slope at 10 h after the typhoon initiation. Differently, tree roots in cases in New Zealand can penetrate through the thin soil layer (<1 m) and provide considerable additional resistance to stabilize the slope. [ABSTRACT FROM AUTHOR]

Landslide deposits preserved in the geological record afford opportunities to better inform hillslope and seismic hazard and risk models, particularly in regions where observational records are short. In the Southern Alps of New Zealand, small coseismic landslides are frequent, but the geological record preserves several instances of more substantial (> 1 km3) but infrequent mass failures. With an estimated volume of 27 km3, the giant Green Lake Landslide represents one of the largest subaerial landslides on Earth. Previous work has suggested this deep-seated mass movement was most likely triggered by high-intensity seismic shaking, but that local structural weakness and/or glacial debuttressing may help to explain the anomalously large failure volume. Resolving the potential contribution of the latter is important given predictions concerning alpine deglaciation in the coming decades to centuries. Key to resolution are secure chronologies of landslide emplacement and past glacier change. Here we present in situ cosmogenic 10Be exposure ages from the Green Lake Landslide that suggest an emplacement age of 15.5 ± 0.7 ka. Recent work shows that glacial retreat in the region was underway by 19 ka, indicating that the Green Lake Landslide was emplaced 3–4 kyr after the onset of glacier retreat. Given the geometry of the former confining valley glacier, we expect that the deglaciation-landslide age gap is closer to the upper end of this estimate. If correct, this conclusion places greater weight on the roles of local geological structure and/or a great earthquake as factors contributing to the exceptionally large volume of this event. [ABSTRACT FROM AUTHOR]

Coastal hillslopes often host higher concentrations of earthquake-induced landslides than those further inland, but few studies have investigated the reasons for this occurrence. As a result, it is unclear if regional earthquake-induced landslide susceptibility models trained primarily on inland hillslopes are effective predictors of coastal susceptibility. The 2016 Mw 7.8 Kaikōura earthquake on the northeastern South Island of New Zealand resulted in ca. 1600 landslides > 50 m 2 on slopes > 15 ∘ within 1 km of the coast, contributing to an order of magnitude greater landslide source area density than inland hillslopes within 1 to 3 km of the coast. In this study, logistic regression modelling is used to investigate how landslide susceptibility differs between coastal and inland hillslopes and to determine the factors that drive the distribution of coastal landslides initiated by the 2016 Kaikōura earthquake. Strong model performance (area under the receiver operator characteristic curve or AUC of ca. 0.80 to 0.92) was observed across eight models, which adopt four simplified geology types. The same landslide susceptibility factors, primarily geology, steep slopes, and ground motion, are strong model predictors for both inland and coastal landslide susceptibility in the Kaikōura region. In three geology types (which account for more than 90 % of landslide source areas), a 0.03 or less drop in model AUC is observed when predicting coastal landslides using inland-trained models. This suggests little difference between the features driving inland and coastal landslide susceptibility in the Kaikōura region. Geology is similarly distributed between inland and coastal hillslopes, and peak ground acceleration (PGA) is generally lower in coastal hillslopes. Slope angle, however, is significantly higher in coastal hillslopes and provides the best explanation for the high density of coastal landslides during the 2016 Kaikōura earthquake. Existing regional earthquake-induced landslide susceptibility models trained on inland hillslopes using common predictive features are likely to capture this signal without additional predictive variables. Interestingly, in the Kaikōura region, most coastal hillslopes are isolated from the ocean by uplifted shore platforms. Enhanced coastal landslide susceptibility from this event appears to be a legacy effect of past erosion from wave action, which preferentially steepened these coastal hillslopes. [ABSTRACT FROM AUTHOR]

Gisborne/Tairāwhiti is particularly susceptible to rainfall‐triggered landslides on account of (1) steep slopes, (2) young, soft geology, (3) landuse change effects. The interplay of these factors led to a particularly damaging rainfall‐induced landsliding event during 3–7 November 2021, following >200 mm of rainfall. This caused a Multiple Occurrence Regional Landslide Event, impacting residential properties and infrastructure in the city and rural areas. We report mainly on landslides in Gisborne city and suburbs, based on field reconnaissance and interferometric synthetic aperture radar monitoring of slopes. This highlights prevailing land instability prior to the more recent ex‐Tropical Cyclone Gabrielle (2023). [ABSTRACT FROM AUTHOR]

In mountainous areas, landslides induced by destructive earthquakes are one of the main causes of human casualties, which is an important link in the chain of earthquake hazards. Earthquake-triggered landslides are mainly controlled by three factors, namely seismic property, topography, and geology. Many studies have been conducted on these controlling factors of earthquake-triggered landslides. However, little is known about the effect of coseismic displacement on the distribution of landslides under different slope aspects and slope angles, hindering our understanding of the mechanism of inducing landslides by the combination of surface displacement and slope geometry at the local scale and leading to controversial opinions about the abnormal number of earthquake-triggered landslides in several cases. Here, we took the 2008 Wenchuan Mw7.9 earthquake in China, the 2015 Gorkha Mw7.8 earthquake in Nepal, and the 2016 Kaikōura Mw7.8 earthquake in New Zealand as examples to investigate the relationship between the distribution of large earthquake-triggered landslides and the three-dimensional (3D) coseismic displacement field. We divided the landslide-prone area around the epicenter into regular grids and calculated the 3D coseismic displacement in each grid according to the radar satellite images and slip distribution model. Then, the 3D coseismic displacement was projected to two coordinate systems related to the slope where the landslides were located for statistical analysis. We determined that the surface uplift perpendicular to the slope is more likely to induce landslides, particularly when combined with large slope angles. Meanwhile, the number of landslides will be significantly reduced where the subsidence occurs. Regardless of uplift or subsidence, landslides are more likely to occur when the direction of coseismic horizontal displacement is far from the slope. The larger the slope angles are, the greater the effects of horizontal displacement and slope aspect. A dominant slope aspect also exists for earthquake-triggered landslides, which is different from the mean slope aspect calculated from the background topography. This dominant aspect angle is related to the focal mechanism and striking angle of surface rupture. These results indicate that we can simulate the 3D coseismic displacement field from known fault location and earthquake mechanism and combine the topographic data for landslide risk assessment in earthquake-prone mountainous areas to minimize the damage caused by possible earthquake-triggered landslides. [ABSTRACT FROM AUTHOR]

We examined the stratigraphy of alluvial fans formed at the steep range front of the Southern Alps at Te Taho, on the north bank of the Whataroa River in central West Coast, South Island, New Zealand. The range front coincides with the Alpine Fault, an Australian‐Pacific plate boundary fault, which produces regular earthquakes. Our study of range front fans revealed aggradation at 100‐ to 300‐year intervals. Radiocarbon ages and soil residence times (SRTs) estimated by a quantitative profile development index allowed us to elucidate the characteristics of four episodes of aggradation since 1000 CE. We postulate a repeating mode of fan behaviour (fan response cycle [FRC]) linked to earthquake cycles via earthquake‐triggered landslides. FRCs are characterised by short response time (aggradation followed by incision) and a long phase when channels are entrenched and fan surfaces are stable (persistence time). Currently, the Te Taho and Whataroa River fans are in the latter phase. The four episodes of fan building we determined from an OxCal sequence model correlate to Alpine Fault earthquakes (or other subsidiary events) and support prior landscape evolution studies indicating ≥M7.5 earthquakes as the main driver of episodic sedimentation. Our findings are consistent with other historic non‐earthquake events on the West Coast but indicate faster responses than other earthquake sites in New Zealand and elsewhere where rainfall and stream gradients (the basis for stream power) are lower. Judging from the thickness of fan deposits and the short response times, we conclude that pastoral farming (current land‐use) on the fans and probably across much of the Whataroa River fan would be impossible for several decades after a major earthquake. The sustainability of regional tourism and agriculture is at risk, more so because of the vulnerability of the single through road in the region (State Highway 6). [ABSTRACT FROM AUTHOR]

The Tuaheni Landslide Complex, located on the upper slope of the northern Hikurangi Margin in New Zealand, is a unique place to research on slow slip creep-like deformation and seabed failure, as well as their possible relationship with the presence of gas hydrates, cold seeps, and fluid migration. Based on the visual interpretation of seismic data, it is sometimes very difficult to identify various subsurface structures and tectonic features. We study certain seismic attributes, namely the reflection strength, instantaneous frequency, instantaneous phase, and the Hilbert transform, in the Tuaheni Landslide Complex and observe that these attributes play a very important role in identifying and interpreting various subsurface geological features and bed boundaries that are not clearly visible in the seismic sections. In general, these seismic attributes are studied to identify hydrocarbons such as oil and gas. However, in this present study these seismic attributes nicely illustrate the fluid migration pathways, the decollement of the sediment slide, the base of the debris flow, the base of the deformed sediment and gas migration, etc., along two perpendicular seismic profiles crossing the Site U1517 of IODP Expedition 372. The instantaneous phase and Hilbert transform attribute depict the bed boundaries and discontinuities, whereas the reflection strength and instantaneous frequency attributes characterize the various strata in terms of whether they are associated with fluid at their bases. The possible role of tectonic activity and seafloor slope failure due to gas hydrate dissociation and vice versa is clearly visible through fluid-filled weak zones in the seismic attribute volumes. Gas hydrates are dissociating and BSRs are abruptly pinching out towards the seafloor due to the movement of hot fluid and free gas, enhancing seafloor sliding and local tectonic activities together. [ABSTRACT FROM AUTHOR]

Earthquake‐triggered slow‐moving landslides are not well studied mainly due to a lack of high‐resolution in‐situ geodetic observations both in time and space. Satellite‐based interferometric synthetic aperture radar (InSAR) has shown potential in landslides applications, however, it is challenging to detect earthquake‐triggered slow‐moving landslides over large areas due to the effects of post‐seismic tectonic deformations, atmospheric delays, and other spatially propagated errors (e.g., unwrapped errors caused by decorrelation noises). Here, we present a novel InSAR phase‐gradient‐based time‐series approach to detect slow‐moving landslides that triggered by the 2016 Mw 7.8 Kaikōura earthquake. Twenty‐one earthquake‐triggered large (>0.1 km2) slow‐moving landslides are detected and studied. Our results reveal decaying characteristics of the temporal evolutions of these landslides, that averagely 3.9 years after the earthquake, their post‐seismic velocity will decay by 90% to reach approximately the pre‐seismic level. Our study opens new perspectives for understanding mass balance of earthquakes and helps reduce associated hazards. Plain Language Summary: Large shallow earthquakes in mountainous regions can trigger widespread landslides that cause major damage to infrastructure. Such landslides are typically identified using aerial imagery, optical satellite images, or fieldwork as such landslides tend to be associated with "fresh" scars and deposits of debris. However, another type of landslides, that are triggered by earthquake but move slowly, are difficult to find and monitor. Satellite‐based interferometric synthetic aperture radar (InSAR) provides an opportunity to monitor slow‐moving landslides, however, given the subtle and localized signals in the InSAR maps, the landslide displacements are easily contaminated by other signals in InSAR. To address this problem, we developed a new InSAR‐phase‐gradient based time‐series method. Using our new method, we find 21 large earthquake‐triggered slow‐moving landslides with average area of 0.84 km2. Through studying the spatio‐temporal displacements of these landslides, we find their movements gradually recover to pre‐seismic level in the years after the earthquake. Our study help to better understand the mechanism of earthquake triggered landslides, and thus contribute to reduce associated hazards. Key Points: A new interferometric synthetic aperture radar phase‐gradient based time‐series approach used to detect earthquake‐triggered slow‐moving landslidesTwenty‐one slow‐moving landslides triggered by the 2016 Mw 7.8 Kaikōura earthquake are detected and monitoredOur results reveal decaying characteristics of post‐seismic velocity of the earthquake‐triggered slow‐moving landslides [ABSTRACT FROM AUTHOR]

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]

Coastal hillslopes often host higher concentrations of earthquake induced landslides than those further inland, but few studies have investigated the reasons for this occurrence. As a result, it remains largely unclear if regional earthquake induced landslide susceptibility models trained primarily on inland hillslopes are effective predictors of coastal susceptibility. The 2016 Mw 7.8 Kaikōura earthquake on the northeast South Island of New Zealand resulted in c. 1,600 landslides > 50 m2 on slopes > 15° within 1 km of the coast. This forms an order of magnitude greater landslide source area density than inland hillslopes within 1 to 3 km of the coast. In this study, the distribution of regionally predictive landslide susceptibility variables, or features, and logistic regression modelling are used to investigate how landslide susceptibility differs between coastal and inland hillslopes and determine the factors that drive the distribution of coastal landslides initiated by the 2016 Kaikōura earthquake. Strong model performance (Area under the Receiver Operator Characteristic Curve or AUC of c. 0.80 to 0.92) was observed across eight models, which adopt four simplified geology types. The same landslide susceptibility factors, primarily geology, steep slopes, and ground motion are strong model predictors for both inland and coastal landslide susceptibility in the Kaikōura region. In three geology types (which account for more than 90 % of landslides source areas) a 0.03 or less drop in model AUC is observed when predicting coastal landslides using inland trained models. This suggests little difference between the features driving inland and coastal landslide susceptibility in the Kaikōura region. Geology is similarly distributed between inland and coastal hillslopes and PGA is generally lower in coastal hillslopes. Slope angle, however, is significantly higher in coastal hillslopes and provides the best explanation for the high density of coastal landslides during the 2016 Kaikōura earthquake. Existing regional earthquake induced landslide susceptibility models trained on inland hillslopes using common predictive features are likely to capture this signal. Interestingly, in the Kaikōura region, most coastal hillslopes are isolated from the ocean by uplifted shore platforms. Enhanced coastal landslide susceptibility from this event appears to be a legacy effect of past active erosion, which preferentially steepened these coastal hillslopes. [ABSTRACT FROM AUTHOR]

The International Ocean Discovery Program cored Sites U1517 (Tuaheni landslide) and U1519 (Tuaheni Basin) on the Hikurangi slope, North Island, New Zealand. We employed X‐ray diffraction to document clay mineral assemblages within the muddy sediments, with the aim of testing their potential influences on slope stability. Detrital smectite dominates the clays, with average proportions of 52 wt% at Site U1517 and 53 wt% at Site U1519. Bulk sediment from Site U1517 contains up to ∼29 wt% smectite (average = 21 wt%), high enough to reduce the angle of internal friction (on average) to ∼6°. Stratigraphic sections at both sites are homogeneous; compositional excursions are not evident along inferred slip surfaces or weak layers. Smectite decreases in the "downstream" direction of the East Cape Current, and that spatial trend correlates with lower densities of slide scars. The uniformity of compositional preconditioning, however, points to other factors as determinants of slip nucleation. Plain Language Summary: Expandable clay minerals (smectite) are notorious for weakening sediments and contributing to conditions that promote landslides. The Hikurangi margin offshore North Island, New Zealand, is noted for its abundance of submarine landslides. To assess their possible causes, we analyzed clay mineral assemblages from two sites on the upper trench slope that were sampled by the International Ocean Discovery Program. Cores from Sites U1517 and U1519 display unusual degrees of compositional homogeny, with smectite as the dominant clay mineral. We did not find any compositional anomalies at inferred slip surfaces or along weak layers of the Tuaheni landslide. Instead, the entire stratigraphic succession appears to be relatively weak. With inherited preconditioning, dynamic loading during large earthquakes is probably enough to trigger landslides, especially on steeper slopes. Key Points: Hikurangi trench‐slope sediments, including muds within the Tuaheni landslide complex, contain high concentrations of detrital smectiteSmectite is abundant enough to reduce the bulk sediment's coefficient of friction, with little stratigraphic variabilityThe homogeneous mud composition results from strong currents and does not change significantly along inferred slip surfaces or weak layers [ABSTRACT FROM AUTHOR]

Deep‐water sedimentation on active margins often entails complex sediment transport pathways through slope accommodation. Sedimentation in such settings is commonly differentiated into 'fill and spill' vs. 'tortuous corridor' models. To investigate the utility of these models in convergent settings 15,344 km2 of 3D seismic data is used to investigate sedimentation and erosion patterns across the Hikurangi subduction margin. A series of thrust‐bound trench‐slope basins, each tens of kilometres long by kilometres wide, have been diachronously forming, filling and deforming through the Neogene until today. Five primary input points delivered sediment to the basins along the studied part of the margin. Channels display both axial and transverse orientations, the run‐out lengths of which vary temporally. At various times, relatively coarse‐grained sediment was trapped in the interior basins, occasionally then to be cannibalised during landsliding or erosion of growing structures. At other times, coarse‐grained sediment was bypassed to distal basins or the trench. Multiple sediment input points and occasionally tortuous sediment dispersal corridors result in the evolution of convoluted depositional systems, often with similar styles of sedimentation occurring contemporaneously in proximal and distal basins, contrary to simple models of basin fill. A hierarchy of controls on sediment distribution can be distinguished. At the highest level, sediment distribution is controlled by external factors, for example, glacio‐eustacy and tectonics. At basin scale, the interaction of sedimentary systems with local relief (e.g. evolving seafloor structures and landslides) dictates the location and style of deposition. At the lowest level, autocyclic factors (e.g. flow response to earlier deposits) influence the spatiotemporal variation in erosion and sedimentation. The complex interplay of these factors dictates whether basins were filling, spilling or some combination at any point in time, whilst basins that were filled and spilled may subsequently resume filling due to changes in the bounding conditions. Hence simple use of 'fill and spill' or 'tortuous corridor' models to tectonically active margins is not advised. Furthermore, as sedimentation may influence structure growth, constraining the controls on sediment distribution may improve understanding of the broader evolution of convergent margins and their resource distribution. [ABSTRACT FROM AUTHOR]

The East Coast region of New Zealand has some of the highest erosion rates in the world, due to its proximity to an active plate boundary, susceptibility to high-intensity storms and steep terrain underlain by young soft sedimentary rock and soil. In the city of Gisborne, expansion of residential blocks into steeper terrain in peri-urban areas has required improved capacity for the characterisation and monitoring of slope stability. Landslides have affected several properties and have destroyed infrastructure. Slope failure commonly occurs during heavy rainfall events when slow-moving retrogressive slides transition into earthflows and mudflows. In this study, we used in situ sampling and testing methods combined with remote sensing techniques to provide an understanding of the pre-failure and post-failure behaviour of an urban landslide in Gisborne. High-resolution aerial imagery, unmanned aerial vehicle imagery and light detection and ranging data revealed slope morphology and contours of prehistoric failures in the area, and highlighted the more recent impacts of deforestation on slope stability. Furthermore, Sentinel-1 InSAR analysis determined that gradual deformation began in 2017, following two ex-tropical cyclone events. Deformation downslope continued until an initial failure in July 2020. Following that event, some parts of the slope proceeded to accelerate, leading to a further reactivation event in November 2021, following heavy rainfall. During this November 2021 event, average line of sight velocities ranged from −7.9 to −11.2 mm/year, and deformation rates in the vertical direction (related to rotational slumping) averaged −11.2 to −11.9 mm/year, consistent with field observations. [ABSTRACT FROM AUTHOR]

We present a quantitative risk analysis (QRA) case study from the Kā Roimata o Hine Hukatere / Franz Josef Glacier and Te Moeka o Tuawe / Fox Glacier valleys, on the west coast of the South Island, Aotearoa / New Zealand. The glacier valleys are important tourist destinations that are subject to landslide hazards. Both valleys contain actively retreating glaciers; experience high rainfall; and are proximal to the Alpine Fault, which is a major source of seismic hazard on the west coast. We considered the life safety risk from rockfalls, soil/rock avalanches, and flows that either are seismically triggered or occur aseismically. To determine the range in risk values and dominant contributing variables to the risk, we modelled nine different risk scenarios where we incrementally changed the variables used in the risk model to account for the underlying uncertainty. The scenarios represent our central estimate of the risk, e.g. neither optimistic nor conservative, through to our upper estimate of the risk. We include in these estimates the impact time-variable factors, such as a recently reactivated landslide, have had on locally increasing risk and the time-elapsed since the last major earthquake on the nearby Alpine Fault. We disaggregated our risk results to determine the dominant drivers in landslide risk, which highlighted the importance of considering dynamic time-variable risk scenarios and the changing contributions to risk from aseismic versus seismic landslides. A detailed understanding of the drivers of landslide risk in each valley is important to determine the most efficient and appropriate risk management decisions. [ABSTRACT FROM AUTHOR]

Nature-based solutions (NbS) are actions to address societal problems. They are similar to soil and water bioengineering (SWB), where trees and forests are used to mitigate natural hazards. However, NbS can have unintended consequences. Forest-based NbS may involve the enhancement, rehabilitation or restoration of natural forests or planting of trees and forests to provide a range of services including the production of timber and wood. In New Zealand, planted steepland forests have been widely used as NbS for erosion control. While intact, these forests provide various beneficial ecosystem services. However, if these forests are harvested, there is a period of up to 6–‍8 years following clear-fell harvesting, known as the 'window of vulnerability' (WoV) when the landscape is susceptible to rainfall-induced landslides. During this time, the combination of declining root strength and changes in soil hydrology can lead to shallow landslides, especially during heavy storms. This study focuses on three questions: determining whether there is a time within the WoV when susceptibility to rainfall-induced landslides, expressed as landslide density, reaches a maximum; are such landslides related to forestry infrastructure; and are those landslides connected to the stream network. We examined three areas in New Zealand (Tolaga Bay, Marlborough, and Tasman) where exceptional rain events triggered thousands of landslides on forest land harvested in the years immediately preceding those events. Using a range of high-resolution satellite imagery, we manually mapped rainfall-induced landslides and identified those due to the rain events. The maximum landslide number and density occurred on land harvested 1–4 years (and on average 2–3 years) before the event and varied slightly for each study area. Landslides also occurred in areas with trees up to harvest age of about 30 years and on areas with different vegetation covers, i.e., mature indigenous forests, pasture, scrub, etc. There were fewer landslides associated with forest infrastructure such as roads and landings than triggered on clearcut slopes. On average across the three study regions, about half the landslides were connected to streamlines, and so were able to deliver sediment and woody debris. Better information on susceptibility to rainfall-induced landslides following forest removal may help forest managers and regulators understand this hazard and what can (and cannot) be done to mitigate events which often result in 'disastrous' off-forest impacts as observed in New Zealand in recent years. • Maximum landslide density occurred on forest land harvested 1–4 years before a rainfall triggering event • Fewer landslides were associated with forest infrastructure such as roads and landings • On average, half the triggered landslides reached a streamline and so were able to deliver sediment and woody debris • Results were consistent across three regional study areas • In the context of NbS, consideration needs to be given to potential unintended consequences before implementation. [ABSTRACT FROM AUTHOR]

On November 14, 2016, the northeastern South Island of New Zealand was hit by the magnitude Mw 7.8 Kaikōura earthquake, which is characterized by the most complex rupturing mechanism ever recorded. The widespread landslides triggered by the earthquake make this event a great case study to revisit our current knowledge of earthquake-triggered landslides in terms of factors controlling the spatial distribution of landslides and the rapid assessment of geographic areas affected by widespread landsliding. Although the spatial and size distributions of landslides have already been investigated in the literature, a polygon-based co-seismic landslide inventory with landslide size information is still not available as of June 2021. To address this issue and leverage this large landslide event, we mapped 14,233 landslides over a total area of approximately 14,000 km2. We also identified 101 landslide dams and shared them all via an open-access repository. We examined the spatial distribution of co-seismic landslides in relation to lithologic units and seismic and morphometric characteristics. We analyzed the size statistics of these landslides in a comparative manner, by using the five largest co-seismic landslide inventories ever mapped (i.e., Chi-Chi, Denali, Wenchuan, Haiti, and Gorkha). We compared our inventory with respect to these five ones to answer the question of whether the landslides triggered by the 2016 Kaikōura earthquake are less numerous and/or share size characteristics similar to those of other strong co-seismic landslide events. Our findings show that the spatial distribution of the Kaikōura landslide event is not significantly different from those belonging to other extreme landslide events, but the average landslide size generated by the Kaikōura earthquake is relatively larger compared to some other large earthquakes (i.e., Wenchuan and Gorkha). [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]

Flooding is New Zealand's most frequent natural hazard, responsible for the highest number of declared civil defence emergencies and greatest regular economic loss. Stopbanks (levees) perform a critical flood protection function in all regions of the country. However, there have been limited and inconsistent records on the location and physical attributes of these stopbanks at a national level. Through collaboration with regional government representatives, we provide the first standardised national inventory of stopbanks, the New Zealand Inventory of Stopbanks (NZIS). The NZIS provides a standardised geospatial overview of New Zealand's 5284 km long stopbank network, enabling spatial reviews and comparisons of flood protection characteristics at national and regional scales. Around 64% of New Zealand's total stopbank length is situated within five regions (Canterbury, Waikato, Southland, Manawatu-Whanganui and Bay of Plenty), while the nation's most densely population region, Auckland, accounts for just 1.9% of the total national stopbank length. Our research uses the NZIS geospatial dataset to conduct a preliminary exposure assessment of New Zealand's stopbank network to seismic and co-seismic hazards; including proximity to known active earthquake faults, ground shaking intensities and susceptibility to liquefaction and landslides. Preliminary exposure assessments provide an example of how the NZIS can be adopted and applied. [ABSTRACT FROM AUTHOR]

A recently discovered upland paleolake (Redpath Lake) provides evidence of the expansion of lowland podocarp temperate forest to higher elevations within the tectonically active Waipaoa Sedimentary System (WSS), eastern North Island, New Zealand, during the Last Glacial-Holocene Transition. Interpreted as a localised landslide-dammed lake, lithofacies analyses indicate deposition and preservation in a low energy, sub-aqueous, lacustrine environment. Robust Bayesian-derived age-depth models constrained by key tephra fall beds and independent 14C dating (n = 7) indicate storm-bed frequency of c. 226 yrs on average. The absence of extensive soft-sediment deformation or lake-bed tilting implies that seismic shaking was insignificant during the Lake's c. 12 ka yr (17.3–5.5 cal ka BP) existence. Pollen spectra extracted from lacustrine deposits include a mix of montane shrubland and lowland podocarp species spanning a well-dated c. 2200-year (c. 16.3–14.1 cal ka BP) period coeval with a phase of significant climate amelioration (late NZCe–5/NZce-4 mild episode), based on biotic proxies during the latter part of the climatically variable Last Glacial-Interglacial Transition (LGIT) (c. 18.2–11.8 cal ka BP). These pollen spectra represent the first New Zealand record of replacement of montane shrublands in terrestrial uplands of the WSS were replaced by lowland podocarp forest. [ABSTRACT FROM AUTHOR]

Coseismic landslides are observed in higher concentrations around surface-rupturing faults. This observation has been attributed to a combination of stronger ground motions and increased rock mass damage closer to faults. Past work has shown it is difficult to separate the influences of rock mass damage from strong ground motions on landslide occurrence. We measured coseismic off-fault deformation (OFD) zone widths (treating them as a proxy for areas of more intense rock mass damage) using high-resolution, three-dimensional surface displacements from the 2016 Mw 7.8 Kaikōura earthquake in New Zealand. OFD zones vary in width from ~50 m to 1500 m over the ~180 km length of ruptures analyzed. Using landslide densities from a database of 29,557 Kaikōura landslides, we demonstrate that our OFD zone captures a higher density of coseismic landslide incidence than generic "distance to fault rupture" within ~650 m of surface fault ruptures. This result suggests that the effects of rock mass damage within OFD zones (including ground motions from trapped and amplified seismic waves) may contribute to near-fault coseismic landslide occurrence in addition to the influence of regional ground motions, which attenuate with distance from the fault. The OFD zone represents a new path toward understanding, and planning for, the distribution of coseismic landslides around surface fault ruptures. Inclusion of estimates of fault zone width may improve landslide susceptibility models and decrease landslide risk. [ABSTRACT FROM AUTHOR]

The effects of strong ground shaking on hillslope stability can persist for many years after a large earthquake, leading to an increase in the rates of post earthquake land sliding. The factors that control the rate of post‐earthquake land sliding are poorly constrained, hindering our ability to reliably forecast how landscapes and landslide hazards and risk evolve. To address this, we use a unique data set comprising high‐resolution terrestrial laser scans and airborne lidar captured during and after the 2010–2011 Canterbury Earthquake Sequence, New Zealand. This earthquake sequence triggered thousands of rock falls, and rock and debris avalanches (collectively referred to as "rockfall"), resulting in loss‐of‐life and damage to residential dwellings, commercial buildings and other infrastructure in the Port Hills of Christchurch, New Zealand. This unique data set spans 5 years and includes five significant earthquakes. We used these data to (a) quantify the regional‐scale "rockfall" rates in response to these earthquakes and the postearthquake decay in rockfall rates with time; and (b) investigate the site‐specific factors controlling the location of seismic and nonseismic rockfalls using frequency ratios and logistic regression techniques. We found that rockfall rates increased significantly in response to the initial earthquake that generated the strongest shaking in the sequence—The MW 6.2 22 February 2011 event—Compared to the long‐term background rates derived from the dating of pre‐2010 talus piles at the toe of the slopes. Non seismic rockfall rates also increased immediately after the 22 February 2011 earthquake and decayed with time following a power‐law trend. About 50% of the decay back to the pre‐earthquake rockfall rates occurred within 1–5 years after the 22 February 2011 earthquake. Our results show that the short‐term decay in rockfall rates over time, after the initial earthquake, was attributed to the subsequent erosion of seismically damaged rock mass materials caused by environmental processes such as rain. For earthquake‐induced rockfall at the regional‐scale, the peak ground accelerations is the most significant variable in forecasting rockfall volume, followed by the relative height above the base of the slope. For both earthquake and non‐seismic conditions at the site‐specific scale, the probability of rockfall increases when the adjacent areas have failed previously, indicating that accrued damage preconditions localized areas of the slope for subsequent failure. Such preconditioning is a crucial factor driving subsequent rockfalls; that is, future rockfalls are likely to cluster near areas that failed in the past. Plain Language Summary: Evidence from previous earthquakes suggests that the frequency of land sliding after a large earthquake is significantly higher than before it. Strong earthquakes cause slope cracking and generate landslide debris, which can be more readily remobilized post earthquake, creating new hazards, including further landslides and landslide dams. These hazards may persist for years and decades and represent a prolonged risk that the impacted communities must consider. Currently, the relative increase in land sliding and rate of decay during and after a major earthquake is rarely quantified, thus posing a knowledge gap for those rebuilding after a major earthquake. This paper explores high‐resolution terrestrial laser scan models of slope surfaces and how these surfaces changed during and after strong earthquake shaking during the 2010–2011 Canterbury Earthquake Sequence (CES) in New Zealand. These surface "change" models were used to quantify the volumes of debris–rock and soil–that fell from these slopes during and after the CES. These data were used to establish a regional‐scale, physical relationship between the volume of debris falling from the cliffs per earthquake or unit of time, per unit area of slope. Using the change models, we investigated the factors that control the temporal and spatial distribution of the rockfalls at the regional‐ and site‐specific scales. At the regional scale, we found that the size of the slope and the relative increase in rockfall rates above pre‐CES rates controlled the subsequent non‐seismic rockfall decay time. At the site‐specific scale, the main conclusions from this study are: (a) for earthquake triggers, the peak ground acceleration (a measure of earthquake ground shaking) was the most important variable in forecasting the probability of failure, followed by the relative elevation or height above the base of the slope; and (b) for both earthquake and nonseismic triggers, the probability of failure increases when the adjacent areas have failed previously, indicating that preconditioning of the slope to failure is a key factor driving subsequent rockfalls. Key Points: We quantify rock slope changes during and after a major earthquake sequence through repeat high‐resolution terrestrial laser scan surveysWe analyze landslide rates over 5 years, during and after the earthquakes at the regional‐ and site‐specific scalesNon seismic landslide rates are heightened immediately after the initial earthquake and decayed with time (1–5 years) following a power‐law trend [ABSTRACT FROM AUTHOR]

We present a quantitative risk analysis (QRA) case-study from the Franz Josef and Fox Glacier Valleys, on the West Coast of the South Island, New Zealand. The Glacier Valleys are important tourist destinations that are subject to landslide hazards. Both valleys contain actively retreating glaciers, experience high rainfall, and are proximal to the Alpine Fault, which is a major source of seismic hazard on the West Coast. We considered the life safety risk from rockfalls, soil/rock avalanches and flows that are either seismically triggered or occur aseismically. To determine the range in risk values, and dominant contributing variables on the risk, we modelled nine different risk scenarios where we incrementally changed the variables used in the risk model to account for the underlying uncertainty. The scenarios represent our central estimate of the risk, e.g. neither optimistic nor conservative, through to our upper estimate of the risk. We include in these estimates the impact of time-variable factors, such as a recently reactivated landslide has had on locally increasing risk and the time-elapsed since the last major earthquake on the nearby Alpine Fault. We disaggregated our risk results to determine the dominant drivers in landslide risk, which highlighted importance of considering dynamic time variable risk scenarios and the changing contributions to risk from aseismic versus seismic landslides. A detailed understanding of the drivers of landslide risk in each valley is important to determine the most efficient and appropriate risk management decisions. [ABSTRACT FROM AUTHOR]

Although submarine landslides have been studied for decades, a persistent challenge is the integration of diverse geoscientific datasets to characterize failure processes. We present a core‐log‐seismic integration study of the Tuaheni Landslide Complex to investigate intact sediments beneath the undeformed seafloor as well as post‐failure landslide deposits. Beneath the undeformed seafloor are coherent reflections underlain by a weakly‐reflective and chaotic seismic unit. This chaotic unit is characterized by variable shear strength that correlates with density fluctuations. The basal shear zone of the Tuaheni landslide likely exploited one (or more) of the low shear strength intervals. Within the landslide deposits is a widespread "Intra‐debris Reflector", previously interpreted as the landslide's basal shear zone. This reflector is a subtle impedance drop around the boundary between upper and lower landslide units. However, there is no pronounced shear strength change across this horizon. Rather, there is a pronounced reduction in shear strength ∼10–15 m above the Intra‐debris Reflector that presumably represents an induced weak layer that developed during failure. Free gas accumulates beneath some regions of the landslide and is widespread deeper in the sedimentary sequence, suggesting that free gas may have played a role in pre‐conditioning the slope to failure. Additional pre‐conditioning or failure triggers could have been seismic shaking and associated transient fluid pressure. Our study underscores the importance of detailed core‐log‐seismic integration approaches for investigating basal shear zone development in submarine landslides. Plain Language Summary: Submarine landslides move enormous amounts of sediment across the seafloor and have the potential to generate damaging tsunamis. To understand how submarine landslides develop, we need to be able to image and sample beneath the seafloor in regions where landslides have occurred. To image beneath the seafloor we generate sound waves in the ocean and record reflections from those waves, enabling us to produce "seismic images" of sediment layers and structures beneath the seafloor. We then use scientific drilling to sample the sediment layers and measure physical properties. In this study, we combine seismic images and drilling results to investigate a submarine landslide east of New Zealand's North Island. Drilling next to the landslide revealed a ∼25 m‐thick layer of sediment (from ∼75–95 m below the seafloor) that has strong variations in sediment strength and density. We infer that intervals of relatively low strength within this layer developed into the main sliding surface of the landslide. Additionally, results from within the landslide suggest that the process of landslide emplacement has induced a zone of weak sediments closer to the seafloor. Our study demonstrates how combining seismic images and drilling data helps to understand submarine landslide processes. Key Points: We integrate scientific drilling data with seismic reflection data to investigate the submarine Tuaheni Landslide ComplexBasal shear zone of the landslide likely exploited a relatively low shear strength interval within an older (buried) mass transport depositLandslide emplacement seems to have induced an additional weak zone that is shallower than the interpreted base of the landslide deposit [ABSTRACT FROM AUTHOR]

Earthquakes play a fundamental role in the evolution of Earth's topography through co-seismic uplift and subsidence, as well as erosion through widespread landslides induced by ground motion. Earthquake-induced landslides can result in exceptional increases in the transfer of mass from landscapes, supplying sediment to rivers where impacts can last for decades or longer. Landslides can also erode vegetation and soils, in addition to exposing freshly ground rock mass, which can both result in regionally significant carbon transfers. Recent work has sought to quantify the fluxes, rates, and temporal patterns of the impacts on earth surface processes generated by these tectonic events. In this review, we focus on the impacts of earthquake-induced landslides on erosion and sediment flux, river water chemistry, and carbon export through the lens of the well-studied 2008 Wenchuan earthquake. We then discuss these themes in the context of works from the 1999 Taiwan Chi-Chi earthquake, 2015 Nepalese Gorkha earthquake, and New Zealand historical earthquakes, altogether highlighting an important role of earthquake-induced landslides in erosion and weathering along tectonically active mountain ranges. We suggest that more monitoring, geochemical tracing, paleo-records, and/or modelling studies are needed to compare the processes and fluxes of catchment erosion and weathering before and after earthquake events, which will help to deepen our understanding not only of the impacts of earthquake-induced landslides on earth surface processes, but also the linkage among high magnitude events, continental erosion and weathering, and the long-term global carbon cycle. [ABSTRACT FROM AUTHOR]

Efficient and robust landslide mapping and volume estimation is essential to rapidly infer landslide spatial distribution, to quantify the role of triggering events on landscape changes, and to assess direct and secondary landslide-related geomorphic hazards. Many efforts have been made to develop landslide mapping methods, based on 2D satellite or aerial images, and to constrain the empirical volume–area (V – A) relationship which, in turn, would allow for the provision of indirect estimates of landslide volume. Despite these efforts, major issues remain, including the uncertainty in the V – A scaling, landslide amalgamation and the underdetection of landslides. To address these issues, we propose a new semiautomatic 3D point cloud differencing method to detect geomorphic changes, filter out false landslide detections due to lidar elevation errors, obtain robust landslide inventories with an uncertainty metric, and directly measure the volume and geometric properties of landslides. This method is based on the multiscale model-to-model cloud comparison (M3C2) algorithm and was applied to a multitemporal airborne lidar dataset of the Kaikōura region, New Zealand, following the Mw 7.8 earthquake of 14 November 2016. In a 5 km 2 area, the 3D point cloud differencing method detects 1118 potential sources. Manual labeling of 739 potential sources shows the prevalence of false detections in forest-free areas (24.4 %), due to spatially correlated elevation errors, and in forested areas (80 %), related to ground classification errors in the pre-earthquake (pre-EQ) dataset. Combining the distance to the closest deposit and signal-to-noise ratio metrics, the filtering step of our workflow reduces the prevalence of false source detections to below 1 % in terms of total area and volume of the labeled inventory. The final predicted inventory contains 433 landslide sources and 399 deposits with a lower limit of detection size of 20 m 2 and a total volume of 724 297 ± 141 087 m 3 for sources and 954 029 ± 159 188 m 3 for deposits. Geometric properties of the 3D source inventory, including the V – A relationship, are consistent with previous results, except for the lack of the classically observed rollover of the distribution of source area. A manually mapped 2D inventory from aerial image comparison has a better lower limit of detection (6 m 2) but only identifies 258 landslide scars, exhibits a rollover in the distribution of source area of around 20 m 2 , and underestimates the total area and volume of 3D-detected sources by 72 % and 58 %, respectively. Detection and delimitation errors in the 2D inventory occur in areas with limited texture change (bare-rock surfaces, forests) and at the transition between sources and deposits that the 3D method accurately captures. Large rotational/translational landslides and retrogressive scars can be detected using the 3D method irrespective of area's vegetation cover, but they are missed in the 2D inventory owing to the dominant vertical topographic change. The 3D inventory misses shallow (< 0.4 m depth) landslides detected using the 2D method, corresponding to 10 % of the total area and 2 % of the total volume of the 3D inventory. Our data show a systematic size-dependent underdetection in the 2D inventory below 200 m 2 that may explain all or part of the rollover observed in the 2D landslide source area distribution. While the 3D segmentation of complex clustered landslide sources remains challenging, we demonstrate that 3D point cloud differencing offers a greater detection sensitivity to small changes than a classical difference of digital elevation models (DEMs). Our results underline the vast potential of 3D-derived inventories to exhaustively and objectively quantify the impact of extreme events on topographic change in regions prone to landsliding, to detect a variety of hillslope mass movements that cannot be captured by 2D landslide mapping, and to explore the scaling properties of landslides in new ways. [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]

The magnitude (Mw) 7.8 earthquake that struck North Canterbury on New Zealand's South Island on the evening of 14 November 2016 caused widespread damage to infrastructure, in particular to road and rail within the South Kaikōura Transport Corridor (SKTC). This is the main transport corridor along the east coast of the South Island, and the coseismic landslides covered and destroyed parts of the State Highway 1 (SH1) road, as well as the railway lines. Post-earthquake aftershocks and significant rainfall events (e.g. Cyclones Debbie, Cook and Gita), further mobilised the de-stabilised greywacke rock mass and paleo-landslide debris that mantled the slopes. Indeed, the SKTC was affected by > 80 separate landslides that have impacted either the rail corridor, the road corridor, or both. A number of different landslide mechanisms occurred, including shallow translation slides of existing paleo-landslide debris and rotational slumping, as well as rock mass failure via wedge, planar or toppling. Here, we report on monitoring of three case study landslides, using multi-temporal analyses of digital elevation data (from LiDAR, TLS and UAV surveys), real-time extensometer and land deformation monitoring using GPS receivers, as well as survey prisms. These datasets provided important monitoring of near-surface movement, important for risk management along the transport corridor during the reinstatement works. [ABSTRACT FROM AUTHOR]

The historically unstable Sunkist Bay cliff in Auckland, New Zealand, is formed of weak Pleistocene alluvial sediments, infilling a paleovalley eroded into Miocene sandstone. Weathered tephra layers dip at low angles out of the cliff, and part-control stability. The last major failure prior to 2017 occurred in 2011, immediately to the west of the 2017 failure. Despite the long-term history of instability, over the last two decades, residential property construction has continued to < 10 m of the cliff edge. On 5 April 2017, ex-tropical cyclone Debbie delivered 157 mm of rainfall in 24 h, the 3rd highest 24-h total in 40 years. This was preceded 1 month earlier by 210 mm of rainfall on 8 March (the 'Tasman Tempest' storm), locally a 1 in 100-year event. A seawall constructed at the base of the cliff may have acted to inhibit pore-water pressure reduction during March–April 2017, because horizontal drains were absent in the seawall directly in front of the 2017 slope failure. Irrespective of the exact process-mechanisms of the 2017 failure, this case study highlights the importance of careful land use planning and imposition of adequate setback distances in coastal zones in New Zealand, and globally. [ABSTRACT FROM AUTHOR]

In this study, we use version 2.0 of the digital inventory of landslides triggered by the MW 7.8 Kaikōura, New Zealand, earthquake of 14 November 2016 to revaluate some of the controls on their spatial distribution. Version 2.0 contains 29,557 hand-digitised landslides, whilst version 1.0 included only 10,195 mainly larger (> 10,000 m2) landslides. The main results from the re-analysis using version 2.0 of the inventory are the following: (1) the landslide frequency and density results indicate that the power laws fitted to version 1.0 and version 2.0 landslide inventories are similar; (2) the landslide densities decrease with increasing distance from the nearest fault that ruptured to the surface during the earthquake; (3) the larger landslides in the inventory occur on slopes with higher local slope relief; and (4) the results from logistic regression modelling using the version 2.0 inventory show that (a) the geological materials have a larger relative contribution to the fit of the overall model, when compared with the previous model based on the version 1.0 inventory, and (b) the "distance to fault" predictor variable continues to have more statistical power in predicting landslide probability than the modelled peak ground acceleration or peak ground velocity. These results suggest that future research on the Kaikōura earthquake landslide distribution—and the landslide distributions associated with other similar large earthquakes—might be directed towards investigating the interaction between surface fault rupture, fault damage-zone properties, earthquake-induced ground shaking, and the initiation of slope failures. [ABSTRACT FROM AUTHOR]

The intra‐arc Liquiñe‐Ofqui Fault System (LOFS) is an active transpressive fault zone located in the Patagonian Andes of Chile. In 2007, a seismic sequence occurred in the Aysén Fjord region of Chilean Patagonia along the LOFS, with a Mw 6.2 main earthquake that triggered dozens of landslides, some of which induced tsunami waves that caused severe damage and casualties. Through the analysis of high‐resolution seismic reflection and bathymetric data, we identify six submarine faults cutting the Late Quaternary postglacial sedimentary infill of the fjord. The most conspicuous are the dextral‐normal NE‐SW striking Quitralco fault (QF) and the N‐S striking strike‐slip Río Cuervo (RCF) and Punta Cola faults (PCF). Our paleoseismological analysis reveals at least seven paleolandslide events buried in the fjord sediments that were triggered by local paleoearthquakes, which occurred since local ice sheet retreat, that is, circa 12 kyr. By combining tectonic observations with local seismicity data, we propose a seismotectonic model for the evolution of the 2007 seismic sequence where three structures were progressively activated from the depth toward the upper continental crust, causing surface rupture along the PCF and with earthquakes, suggesting only partial ruptures along other faults. Because the other faults did not rupture to the seafloor they remain important sources of seismic hazard. Thus, the last seismic sequence was a consequence of a stress transfer from the lower‐ductile toward the upper‐brittle continental crust, close to the triple junction of the Nazca, South American, and Antarctica Plates. Our results emphasize on the potential synergies between multiple geological and geophysical methods to assess complex events. Plain Language Summary: When crustal faults rupture, the energy released is the earthquakes we feel at the surface of the Earth. Recent studies along strike‐slip faults demonstrate that these phenomena are often not only related to a single fault but instead take place along several faults like was seen in the 2016 Kaikoura earthquake in New Zealand. Using novel high‐resolution seismic reflection imagery and bathymetric data together with the reanalysis of local seismicity, we show (i) multiple active faults in the Aysén Fjord area along the Liquiñe‐Ofqui Fault System in the Patagonian Andes; (ii) several of these faults were activated during the last 2007 seismic sequence and one of them (the Punta Cola fault), ruptured to the surface causing a Mw 6.2 earthquake that generated massive landslides and local tsunami; and (iii) mapping of similar landslides in the fjord sediments demonstrates similar events occurred at least seven times since the last ice sheet retreat in the area, that is, in the last 12,000 yr. This demonstrates the necessity for including the possibility of complex ruptures involving multiple faults regarding seismic hazard assessment along crustal faults, especially for faults near populated areas. Key Points: High‐resolution seismic reflection and bathymetric data reveal seafloor rupture and multiple faults cutting postglacial fjord sedimentsFour‐dimensional seismotectonic model shows that the 2007 Aysén Seismic Sequence resulted in multiple faults activation and surface ruptureSeven paleoearthquakes similar to 2007 main event caused landslides and conspicuous deformation of the fjord's seafloor, during the last 12 kyr [ABSTRACT FROM AUTHOR]

Overbank fine sediment deposition in the lower Oroua River leads to decreased flood capacity between stopbanks and increased flood risk. Identifying the dominant sources of erosion contributing to this deposited sediment can inform targeting of erosion management strategies most likely to reduce deposition. Sediment fingerprinting quantifies proportional contributions of catchment sources to suspended and deposited sediment transported downstream. Here, it is used to determine source contributions to recent overbank sediment deposits in the lower Oroua River, North Island, New Zealand. The analysis comprised six sources (channel bank; greywacke mountain range; hill subsurface; hill surface; mudstone; and, unconsolidated sand/silt) and two particle size fractions (< 63 µm and 125-300 µm) using bulk geochemical and radionuclide tracers. The dominant contributions to overbank deposition for the < 63 µm size fraction, which represents 42% of the deposits, are estimated to be from unconsolidated sand/silt (36 ± 8%) and hill subsurface (27 ± 8%), followed by channel bank (12 ± 12%), mudstone (11 ± 8%), hill surface (11 ± 6%), and greywacke mountain range (3 ± 4%). The overbank deposits for the 125-300 µm sediment fraction, which represents 28% of the deposits, show a different order of dominance with the largest contributions from hill subsurface (37 ± 7%) and channel bank (27 ± 10%), followed by mountain range (17 ± 10%), unconsolidated sand/silt (10 ± 7%), mudstone (7 ± 6%), and hill surface (2 ± 4%). Combining these two size fractions by their proportions in the overbank sediment provides an overall sediment source contribution of hill subsurface (31 or 38%), unconsolidated sand/silt (26 or 29%), channel bank (18%), mudstone (9 or 10%), mountain range (9 or 15 %), and hill surface (7 or 8%) (including/excluding channel bank, respectively). Hill subsurface, which represents shallow landslides in steep hill country, is a significant sediment source for both particle size ranges. Therefore, targeting erosion from this source is likely to have the most significant impact on sediment supply to overbank deposition. Differences in source contribution estimates for the two particle size fractions may reflect the composition of source material, variation in erosion and deposition processes, and discrimination challenges caused by channel bank sediment. These differences highlight the importance of accounting for particle size distributions when using sediment fingerprinting. Particle size also needs to be considered when implementing erosion management strategies because sources may have disproportionate downstream impacts depending on their particle size distribution and the aim of the mitigation. Specific sediment yield was estimated using sediment fingerprinting results distributed across a slope-weighted model of catchment outlet sediment yield. The highest specific sediment yield is from unconsolidated sand/silt (1,900 t km-2 yr-1), followed by mudstone (1,100 t km-2 yr-1) and hill subsurface (1,000 t km-2 yr-1). The presentation of sediment fingerprinting outputs can be improved by incorporating results into spatial modelling to produce maps of sediment yield. [ABSTRACT FROM AUTHOR]

We use a mapped landslide inventory coupled with a 2‐m resolution vertical difference model covering an area of 6,875 km2 to accurately constrain landslide volume‐area relationships. We use the difference model to calculate the source volumes for landslides triggered by the MW 7.8 Kaikōura, New Zealand, earthquake of 14 November 2016. Of the 29,519 mapped landslides in the inventory, 28,394 are within the analysis area, and of these, we have calculated the volume of 17,256 source areas that are ≥90% free of debris. Of the 28,394 landslides, about 80% are classified as soil or rock avalanches and the remainder as mainly translational slides. Our results show that both the soil avalanches and the rock avalanches, ignoring their source geology, have area to volume power‐law scaling exponents (γ) of 0.921 to 1.060 and 1.040 to 1.138, respectively. These are lower than the γ values of 1.1–1.3 (for soil) and 1.3–1.6 (for rock) reported in the literature for undifferentiated landslide types. They are, however, similar to those γ values estimated from other coseismic landslide inventories. In contrast, for 50 selected rotational, translational (planar slide surfaces), or compound slides, where much of the debris remains in the source area, we found γ values range between 1.46 and 1.47, indicating that their slide surfaces were considerably deeper than those landslides classified as avalanches. This study, like previous studies on coseismic landslides, shows that soil and rock avalanches (disrupted landslides) are the dominant landslide type triggered by earthquakes and that they tend to be shallow. Key Points: We use a 2‐m resolution vertical difference model to estimate source volumes for 17,256 landslides with sources ≥90% free of debris triggered by the MW7.8 2016 Kaikōura EarthquakeThe model was derived by subtracting a tectonically adjusted pre‐EQ surface model from a post‐EQ model, covering an area of 6,875 km2Landslide trigger mechanism, type/failure mode, and source material are critical for accurate estimation of landslide volumes from source‐area geometries [ABSTRACT FROM AUTHOR]

The Tuaheni Landslide Complex (TLC) on the Hikurangi Margin is subject to reactivation, yet the timing of slide emplacement remains unknown. Here we modeled pore‐water data collected from the TLC during the International Ocean Discovery Program (IODP) Expedition 372 in 2017/2018 using a transient‐state reaction‐transport modeling approach. Our simulations reveal that the TLC was formed by two separate depositional events and that the most recent one occurred 12.5 ± 2.5 ka as a coherent ~40 m sediment block that carried its initial pore‐water signature. In addition, we show that the rapid burial of pore‐water SO42− in the pre‐slide sediments consumed an additional ~5.6 × 109 mole of CH4 over the entire Tuaheni South following the most recent depositional event. These findings provide significant insights into the nature and timing of the TLC and highlight the role of slope failure on subsurface methane cycling on millennial timescales. Plain Language Summary: One remarkable feature of the Tuaheni Landslide Complex (TLC) is the geomorphic signature of ongoing post‐slide deformation of the slide debris, analogous to slow‐moving terrestrial earthflows and rock glaciers. To reconstruct the temporal history of the TLC and evaluate its potential effect on subsurface methane cycling, we modeled the pore‐water data collected from the TLC using a nonsteady‐state model. We demonstrate that this landslide complex was formed by two separate depositional events and that the most recent one occurred 12.5 ± 2.5 ka as a coherent ~40 m sediment block. In addition, modeled response of the methane cycling rates to the slide mass emplacement shows that the rapid burial of pore‐water SO42− in the pre‐slide sediments into the newly developed methanic zone consumed an additional ~70 mol m−2 of CH4 over the 12.5 kyr that followed the most recent depositional event. This is the first study that provides the age constraint of the TLC and highlights the role of slope failure on subsurface methane cycling on millennial timescales. Key Points: The Tuaheni Landslide Complex was formed by two separate depositional eventsThe most recent depositional event occurred 12.5 ± 2.5 ka as a coherent ~40 m sediment blockBurial of sulfate in the pre‐slide sediments consumed an additional ~5.6 × 109 mol of methane over the entire Tuaheni South [ABSTRACT FROM AUTHOR]

Geological structures precondition hillslope stability as well as the processes and landslide mechanisms which develop in response to deglaciation. In areas experiencing glacier retreat and debuttressing, identifying landslide preconditions is fundamental for anticipating landslide development. Herein, the ~ 150 M m3 Mueller Rockslide in Aoraki/Mount Cook National Park, New Zealand, is described; and we document how preconditions have controlled its morphology and development in response to thinning of the adjacent Mueller Glacier. A combination of geomorphological and geotechnical mapping—based on field, geophysical and remote sensing data—was used to characterise the rock mass and morphology of the rockslide and surrounding hillslope. Mueller Rockslide is identified as a rock compound slide, undergoing dominantly translational failure on a dip slope. The crown of the rockslide is bounded by several discontinuous, stepped scarps whose orientation is controlled by joint sets; these scarps form a zone of toppling that is delivering rock debris to the main rockslide body. Surface and subsurface discontinuity mapping above the crown identified numerous joints, fractures and several scarps that may facilitate continued retrogressive enlargement of the rockslide. The presence of lateral release structures, debuttressing of the rockslide toe and steeply dipping bedding suggest that the rockslide may be capable of evolving to a rapid failure. [ABSTRACT FROM AUTHOR]

The aim of this research is to investigate the influence of stratigraphy and structure on the development of present-day topography in the Rangitikei hill country. This is achieved by (1) comparing a new inventory of deep-seated Late Quaternary relict landslide zones to the hydraulic, physical and structural properties of the Whanganui Basin fill, (2) analysing the proximity of landslides to structures and rivers, and (3) exploring the relationships between geology and slope morphology. Analysis suggests postglacial incision and daylighting of the Whanganui Basin fill is a primary control on landslide distribution, providing the necessary conditions upon which a range of triggering mechanisms operate. Faults display a clear relationship to landslide frequency, suggesting earthquakes are possibly an important triggering mechanism. Landscape evolution involves inversion and denudation of alternating coarse and fine-grained sedimentary units. Frequent, shallow landslides dominate on steep, northern scarp slopes, and deep-seated rock planar slides dominate on gentle, southern dip slopes. [ABSTRACT FROM AUTHOR]

Rainfall is one of the most important triggers of slope failure. Weathered pyroclastic (tephra) deposits are especially vulnerable to slope failure because they commonly form slopes of high porosity and high clay content. Empirically derived thresholds for the triggering of landslides are commonly based on rainfall conditions and have been widely applied in volcanic soils. However, so far only few researchers utilized pore water pressure in the slope as additional variable for the threshold calibration. Here, we derived a new rainfall threshold for initiating the decrease in effective stress in the slope by analyzing a long-term record of rainfall and piezometer data from a slide-prone coastal area in northern New Zealand that consists of clayey, halloysitic tephra deposits. The level of effective stress decrease increased with rainfall intensity and duration. We observed highest effective stress decrease of up to 36% during rainfall events that triggered landslides in our study area. The effective stress threshold exhibits a satisfactory predictive capability. The probability of correctly predicting a decrease in effective stress is 53%. The effective stress threshold contributes towards the implementation of the decrease in effective stress into rainfall thresholds for the occurrence of landslides. [ABSTRACT FROM AUTHOR]

The infrequent and unpredictable nature of earthquakes means that their landslide‐generated impacts on forests are rarely investigated. In montane forests, landslides are the main cause of tree death and injury during earthquakes. Landslides range from soil movements that uproot and bury trees over extensive areas to rock falls that strike individual trees. We examined unexplored relationships between tree survival and distance from an epicentre, soil‐available phosphorus (P) as an indicator of soil development and tree diameter. We expected decreased tree growth in damaged forests because of tree injury.We used a plot network, established in 1974 and resurveyed regularly ever since, to quantify survival and growth responses 6–30 km from the epicentre of a 1994 earthquake in New Zealand's Southern Alps. Our Bayesian analysis used 8,518 trees from 250 plots that representatively sampled a naturally monospecific Nothofagus forest. As the time‐scales over which responses could emerge were unknown, we compared relationships for a pre‐earthquake period with 0–5 years post‐earthquake, and with 5+ years post‐earthquake.Not all plots were affected by the earthquake. We found that 0–5 years post‐earthquake survival increased logarithmically with distance from the epicentre with lowered survival up to 20 km from the epicentre. Survival was low on plots with high soil‐available P. An inverted U‐shaped relationship between survival and diameter pre‐earthquake was not found 0–5 years post‐earthquake. This was because of surprisingly high survival by large trees. The earthquake most often suppressed 0–5 years post‐earthquake growth up to 15 km from the epicentre, but this was only apparent after accounting for more general growth differences among periods. The positive relationship between growth and soil‐available P pre‐earthquake and 5+ years post‐earthquake reflected enhanced growth on young soils. This contrasted with a negative effect of soil‐available P on growth 0–5 years post‐earthquake.Synthesis. Soil‐available P, tree diameter and distance from the epicentre independently determined how tree survival and growth responded to an earthquake. The impacts on survival and growth largely occurred 0–5 years post‐earthquake and suggests a level of resilience in mountain beech forests. [ABSTRACT FROM AUTHOR]

Although slow-moving landslides represent a substantial hazard, their detailed mechanisms are still comparatively poorly understood. We have conducted a suite of innovative laboratory experiments using novel equipment to simulate a range of porewater pressure and dynamic stress scenarios on samples collected from a slow-moving landslide complex in New Zealand. We have sought to understand how changes in porewater pressure and ground acceleration during earthquakes influence the movement patterns of slow-moving landslides. Our experiments show that during periods of elevated porewater pressure, displacement rates are influenced by two components: first an absolute stress state component (normal effective stress state) and second a transient stress state component (the rate of change of normal effective stress). During dynamic shear cycles, displacement rates are controlled by the extent to which the forces operating at the shear surface exceed the stress state at the yield acceleration point. The results indicate that during strong earthquake accelerations, strain will increase rapidly with relatively minor increases in the out-of-balance forces. Similar behaviour is seen for the generation of movement through increased porewater pressures. Our results show how the mechanisms of shear zone deformation control the movement patterns of large slow-moving translational landslides, and how they may be mobilised by strong earthquakes and significant rain events. [ABSTRACT FROM AUTHOR]

This study compares the impact of global and region-specific variables on the prediction of co-seismic landslides, investigating whether the incorporation of higher-resolution data into a global framework can improve the prediction outcome for regional landslide assessments. Based on a statistical landslide model, which was developed through logistic regression involving various global landslide inventories, the likelihood of a landslide occurring during an earthquake is estimated for a regional event. Using the recently updated landslide inventory of the 2016 M W 7.8 Kaikōura earthquake (centroid of landslide source area), the prediction performance of the model is evaluated by individually replacing the global variables for slope, compound topographic index, lithology and land cover with New Zealand specific datasets that offer higher resolution and/or more accurate information. Statistical metrics are used to compare the changes in the prediction performance of each modification, while probability maps and binary prediction maps are evaluated to identify spatial differences. The results suggest that calculating the slope variable from a higher-resolution DEM (e.g., 25 m) improves the model's prediction performance without changing the model equation or replacing the other global variables. Other region-specific variables such as land cover show stronger correlation but require an adjustment of the model coefficients (e.g., regression analysis). As the region-specific datasets increase the landslide estimates, a sensible interpretation of the model results is important, including the identification of a probability threshold which allows for a binary prediction (landslide occurs versus landslide does not occur). Future research should investigate the role of global and region-specific datasets for other earthquake events and consider incorporating other variables (e.g., local slope relief). The findings demonstrate the limitations of global variables for regional landslide predictions and illustrate the impact as well as implications of region-specific datasets on the model outcome, providing a better understanding on how incorporating region-specific datasets into a global approach can improve the landslide prediction in areas where regional models are not available or where inventory data is not sufficient to develop a robust model. • The impact of region-specific data on predicting co-seismic landslides is assessed. • A higher-resolution DEM (e.g., for slope) notably improves the landslide predictions. • The approach could benefit from other DEM-retrieved variables. [ABSTRACT FROM AUTHOR]

Abstract The Hikurangi margin is an active continental margin east of New Zealand's North Island. It is well recognized as a seismically active zone and is known for the occurrence of free gas and gas hydrates within the shallow sediments. A variety of subaqueous landslides can be observed at the margin, including the Tuaheni Landslide Complex off Poverty Bay. This slide complex has been interpreted previously as a slowly creeping landform, as its morphology and internal deformation is comparable to terrestrial earthflows and rock glaciers. In 2014, we acquired a high-resolution 3D seismic volume covering major parts of the Tuaheni South landslide. The 3D data show a variety of fluid migration indicators, free gas accumulations and manifestations of the base of gas hydrate stability in the pre-slide sedimentary units and the lower unit of the landslide system. The data also show that the landslide system is composed of an upper and lower unit that are separated by an intra-debris negative-polarity reflection. Free gas accumulations directly beneath the landslide units suggest that the debris acts as a boundary for rising fluids and only few migration pathways to the intra-debris reflector are observed in the distal parts of the landslide. Deformation within the landslide's debris is focused in the upper landslide unit, and we interpret the intra-debris reflector as a basal shear zone or 'glide plane' upon which the debris has been remobilized. The origin of the intra-debris reflector is unclear, but we suggest it could be a relatively coarse-grained horizon that would be prone to fluid flow focusing and the development of excess fluid pressure. Our seismic study provides one of the most detailed examples of a subaqueous landslide system and reveals insights into the fluid flow system and potential basal shear zone development of the Tuaheni Landslide Complex. Highlights • 3D seismic imaging of an entire landslide complex. • Shallow gas accumulation within and underneath Tuaheni Landslide Complex. • Imaging of a basal shear zone within a subaqueous landslide complex. [ABSTRACT FROM AUTHOR]

The 2016 MW 7.8 Kaikōura earthquake involved complex rupture of multiple faults for > 170 km, generating strong ground shaking throughout the upper South Island leading to widespread landsliding. As a result of surface fault rupture and landslides, State Highway (SH) 1 and SH70 were blocked, isolating Kaikōura and the surrounding communities, and necessitating evacuations by air and sea. In all these respects, the Kaikōura earthquake can be considered an analogue for a future Alpine Fault earthquake, providing lessons for the necessary emergency response. Landslide blockages primarily occurred where surrounding slopes averaged > 17° and where peak ground acceleration was > 0.43 g, peak ground velocity was > 41 cm/s, or the modified Mercalli intensity was > 7.9. Using a potential future Alpine Fault scenario earthquake, this study identifies locations on other key state highway routes that have similar predictive variables that may, therefore, become blocked in a future earthquake. This suggests that SH6 between Hokitika and Haast, SH73 near Arthur’s Pass, and SH94 south of Milford Sound are all likely to be affected. This will necessitate the evacuation of large numbers of spatially distributed tourists as well as the resupply of isolated local populations. The possibility of bad weather along with a lack of sea ports south of Hokitika will likely make such activities challenging. Contingency planning based on experiences from the Kaikōura earthquake is therefore necessary and likely to prove invaluable following an Alpine Fault earthquake. [ABSTRACT FROM AUTHOR]

Understanding how rainfall events influence the pattern and magnitude of landslide response is an important research focus from geomorphological and hazard planning perspectives. Few studies quantitatively relate spatial patterns in rainfall and landslides, largely due to difficulties in acquiring landslide inventories and data on rainfall patterns for individual storm events. However, the increasing availability of frequent, high-resolution satellite imagery and weather radar is overcoming these impediments. Here, we aim to a) identify which factors most influence susceptibility to shallow landslides at the event scale and b) assess how the spatial density of landslides varies in relation to rainfall. We combine landslide inventories spanning study areas located across the upper North Island of New Zealand with rainfall estimates from weather radar to assess the influence of different explanatory variables using a logistic regression model. We found land cover and slope exert the largest influence on landslide susceptibility ahead of intra-event rainfall intensities and pre-event rainfall accumulations. Of the rainfall variables, maximum 12-h rainfall normalised by the 10-y recurrence interval intensity and the 10-d pre-event accumulation normalised by mean annual rainfall had the most influence on susceptibility. Forest cover reduced the sensitivity of landslide spatial density to variations in slope, rainfall, and rock type, in contrast to pasture. We observed a 3.5-fold increase in mean landslide density once the maximum 12-h intensity was ≥25 % above the 10-y recurrence interval intensity for pastoral land on weak sedimentary rocks. This threshold is consistent with the increase in 12-h rainfall by late century under the highest levels of projected warming in New Zealand, which suggests that the landslide response to storm rainfall could be significantly amplified by climate change. Our study demonstrates the insights gained from combining event inventories with spatial rainfall data to better understand factors influencing landslide susceptibility. [ABSTRACT FROM AUTHOR]

This study proposes and evaluates the performance of two image matching algorithms applied to hillshades derived from consecutive high-resolution digital surface models (DSMs) to measure surface displacements on landslides. The method is applied on Te Horo, a slow-moving landslide in the Dart Valley of New Zealand's Southern Alps/Kā Tiritiri o te Moana using pairs of hillshades derived from Airborne and Satellite Photogrammetric Mapping (APM/SPM) of imagery captured in 2018 and 2020. This novel approach uses the consistency of displacement predictions generated from multiple hillshade pairs to gauge match quality and mask unreliable displacement predictions. The study compares a widely used normalised cross-correlation algorithm (NCC) alongside an optical flow approach for image matching. The performances of both algorithms are assessed against manually derived displacements of prominent surface features. We demonstrate the effectiveness of the masking approach as well as the good performance of the optical flow algorithm in delivering dense, accurate displacement measurements efficiently, particularly when high resolution DSMs are available. The results show that the main translational body of the landslide was displaced at rates averaging 25–55 mm day−1 over the 2018–2020 period. • Hillshades derived from high resolution DSMs are used to track landslide movement. • Varying the illumination parameters enables construction of multiple hillshade pairs. • Hillshade pairs are applied to two feature tracking algorithms, optical flow and NCC. • The consistency of displacements is evaluated at each pixel to mask poor predictions. • Optical flow produces a smoother flow field and delivers dense results efficiently. • The masking technique achieves similar results as the NCC coefficient. [ABSTRACT FROM AUTHOR]