Review Article: Potential geomorphic consequences of a future great (Mw = 8.0+) Alpine Fault earthquake, South Island, New Zealand

The Alpine Fault in New Zealand's South Island has not sustained a large magnitude earthquake since ca. AD 1717. The time since this rupture is close to the average inferred recurrence interval of the fault (~300 yr). The Alpine Fault is therefore expected to generate a large magnitude earthquake in the near future. Previous ruptures of this fault are inferred to have generated Mw = 8.0 or greater earthquakes and to have resulted in, amongst other geomorphic hazards, large-scale landslides and landslide dams throughout the Southern Alps. There is currently 85% probability that the Alpine Fault will cause a Mw = 8.0+ earthquake within the next 100 yr. While the seismic hazard is fairly well understood, that of the consequential geomorphic activity is less well studied, and these consequences are explored herein. They are expected to include landsliding, landslide damming, dam-break flooding, debris flows, river aggradation, liquefaction, and landslide-generated lake/fiord tsunami. Using evidence from previous events within New Zealand as well as analogous international examples, we develop first-order estimates of the likely magnitude and possible locations of the geomorphic effects associated with earthquakes. Landsliding is expected to affect an area > 30 000 km2 and involve > 1billion m3 of material. Some tens of landslide dams are expected to occur in narrow, steep-sided gorges in the affected region. Debris flows will be generated in the first long-duration rainfall after the earthquake and will continue to occur for several years as rainfall (re)mobilises landslide material. In total more than 1000 debris flows are likely to be generated at some time after the earthquake. Aggradation of up to 3 m will cover an area > 125 km2 and is likely to occur on many West Coast alluvial fans and floodplains. The impact of these effects will be felt across the entire South Island and is likely to continue for several decades

The recognition and identification of debris flow hazards for proposed development sites in New Zealand

This paper discusses how debris flows, a commonly occurring natural hazard in mountainous regions of New Zealand, may be recognised and identified from field and desk studies for sites that are being considered for development. Use is made of a particular case study from the Southern Alps in which the previous occurrence of debris flows was seen to be a possibility from an initial site inspection. The paper highlights that a combination of simple tools and techniques, from digging trial pits to examination of historical photos, may enable a reasonably detailed picture to be drawn regarding the potential debris flow hazard at a site, despite the relatively poor records that are available in many remote areas of New Zealand

Very large convergent multi-fluted glacigenic deposits in the NW Highlands, Scotland

We describe two large convergent multi-fluted glacigenic deposits in the NW Highlands, Scotland, and point out their resemblance to a number of landforms emerging from presently deglaciating areas of Greenland and Antarctica. We suggest that they all result from locally sourced sediment being deposited by local ice-flow, which was laterally confined by the margins of much larger adjacent glaciers or ice-streams. The NW Highlands features thus seem likely to be the result of processes active during the latter part of the Devensian Glaciation. One of these deposits, on the peninsula between Loch Broom and Little Loch Broom, is evidently sourced from the west-facing Coire Dearg of Beinn Ghobhlach, but was emplaced in a WNW direction rather than along the WSW fall-line. This suggests that the ice that emplaced it was confined by the margins of large glaciers then occupying the adjacent valleys of Loch Broom and Little Loch Broom. The second much larger and more prominent deposit, in Applecross, is composed of bouldery Torridonian sandstone till emplaced on to glacially scoured bedrock; the only feasible source location for this material is about 12 km distant, which requires that the deposit was carried by ice across the trough of Strath Maol Chalum and emplaced while active ice-streams confined it laterally to its present-day location. This, in turn, requires that ice lay in the Inner Sound between Applecross and Skye to an elevation 400–500 m above present-day sea-level. The Wester Ross Re-advance of 15–14 ka left a fragment of lateral moraine against the most easterly flute and buried the distal end of the flutes with hummocky moraine. We hypothesize that the fluted deposits reflect the locations of the ice-stream margins that constrained deposition of locally derived ice-transported sediment, rather than the flow-lines of the ice-stream itself

Regional coseismic landslide hazard assessment without historical landslide inventories: a new approach

Currently, regional coseismic landslide hazard analyses require comprehensive historical landslide inventories as well as detailed geotechnical data. Consequently, such analyses have not been possible where these data are not available. A new approach is proposed herein to assess coseismic landslide hazard at regional scale for specific earthquake scenarios in areas without historical landslide inventories. The proposed model employs fuzzy logic and geographic information systems to establish relationships between causative factors and coseismic slope failures in regions with well-documented and substantially complete coseismic landslide inventories. These relationships are then utilized to estimate the relative probability of landslide occurrence in regions with neither historical landslide inventories nor detailed geotechnical data. Statistical analyses of inventories from the 1994 Northridge and 2008 Wenchuan earthquakes reveal that shaking intensity, topography, and distance from active faults and streams are the main controls on the spatial distribution of coseismic landslides. Average fuzzy memberships for each factor are developed and aggregated to model the relative coseismic landslide hazard for both earthquakes. The predictive capabilities of the models are assessed and show good-to-excellent model performance for both events. These memberships are then applied to the 1999 Chi-Chi earthquake, using only a digital elevation model, active fault map, and isoseismal data, replicating prediction of a future event in a region lacking historic inventories and/or geotechnical data. This similarly results in excellent model performance, demonstrating the model's predictive potential and confirming it can be meaningfully applied in regions where previous methods could not. For such regions, this method may enable a greater ability to analyze coseismic landslide hazard from specific earthquake scenarios, allowing for mitigation measures and emergency response plans to be better informed of earthquake-related hazards

Coseismic landsliding estimates for an Alpine Fault earthquake and the consequences for erosion of the Southern Alps, New Zealand

Landsliding resulting from large earthquakes in mountainous terrain presents a substantial hazard and plays an important role in the evolution of mountain ranges. However estimating the scale and effect of landsliding from an individual earthquake prior to its occurrence is difficult. This study presents first order estimates of the scale and effects of coseismic landsliding resulting from a plate boundary earthquake in the South Island of New Zealand. We model an Mw 8.0 earthquake on the Alpine Fault, which has produced large (M 7.8–8.2) earthquakes every 329 ± 68 years over the last 8 ka, with the last earthquake ~ 300 years ago. We suggest that such an earthquake could produce ~ 50,000 ± 20,000 landslides at average densities of 2–9 landslides km− 2 in the area of most intense landsliding. Between 50% and 90% are expected to occur in a 7000 km2 zone between the fault and the main divide of the Southern Alps. Total landslide volume is estimated to be 0.81 + 0.87/− 0.55 km3. In major northern and southern river catchments, total landslide volume is equivalent to up to a century of present-day aseismic denudation measured from suspended sediment yields. This suggests that earthquakes occurring at century-timescales are a major driver of erosion in these regions. In the central Southern Alps, coseismic denudation is equivalent to less than a decade of aseismic denudation, suggesting precipitation and uplift dominate denudation processes. Nevertheless, the estimated scale of coseismic landsliding is considered to be a substantial hazard throughout the entire Southern Alps and is likely to present a substantial issue for post-earthquake response and recovery

A new technique for identifying rock avalanche–sourced sediment inmoraines and some paleoclimatic implications.

Moraine chronologies are widely used to infer local climate change events and to correlate these events globally, based on the assumption that moraines always reflect climatic drivers. However, this assumption is unreliable in tectonically active terrain because moraines can also be formed by large landslide (rock avalanche) deposits on glaciers. These can affect glacier motion and cause moraines to form while requiring no climate variation, and can thus cause significant errors in climatic signals extracted from moraine chronologies. To eliminate such errors requires a method for identifying moraines that have been influenced by rock avalanches. Herein we present and test a new diagnostic technique that unambiguously identifies rock avalanche sediments using newly discovered fine-sediment signatures characteristic of rapid, high-stress comminution. We test this technique on Holocene moraines in the Southern Alps, New Zealand, which have previously been interpreted as climatic indicators, and demonstrate that some of them unambiguously contain rock avalanche material; so their climatic significance is questionable

The extremely long-runout Komansu rock avalanche in the Trans Alai Range, Pamir Mountains, southern Kyrgyzstan

Massive rock avalanches form some of the largest landslide deposits on Earth and are major geohazards in high-relief mountains. This work reinterprets a previously reported glacial deposit in the Alai Valley of Kyrgyzstan as the result of an extremely long-runout, probably coseismic, rock avalanche from the Komansu River catchment. Total runout of the rock avalanche is ~28 km, making it one of the longest-runout subaerial non-volcanic rock avalanches thus far identified on Earth. This runout length appears to require a rock volume of ~20 km3; however, the likely source zone in the Trans Alai range likely contained just ~4 km3 of rock, and presently, the deposit has a volume of only 3–5 km3; a pure rock avalanche volume of >10 km3 is therefore impossible, so the event was much more mobile than most non-volcanic rock avalanches. Explaining this exceptional mobility is crucial for present-day hazard analysis. There is unequivocal sedimentary evidence for intense basal fragmentation, and the deposit in the Alai Valley has prominent hummocks; these indicate a rock avalanche rather than a rock-ice avalanche origin. The event occurred 5,000–11,000 yr B.P., after the region’s glaciers had begun retreating, implying that supraglacial runout was limited. Current volume—runout relationships suggest a maximum runout of ~10 km for a 4-km3 rock avalanche. Volcanic debris avalanches, however, are more mobile than non-volcanic rock avalanches due to their much higher source water content; a rock avalanche containing a similarly high water content would require a volume of about 8 km3 to explain the extreme runout of the Komansu event. Rock and debris avalanches can entrain large amounts of material during runout, with some doubling their initial volume. The best current explanation of the Komansu rock avalanche thus involves an initial failure of ~4 km3 of rock debris, with high water content probably deriving from large glaciers on the edifice that subsequently entrained ~4 km3 of valley material together with further glacial ice, resulting in a total runout of 28 km. It is as yet unclear whether glacial retreat has rendered a present-day repetition of such an event impossible

Evaluation of coseismic landslide hazard on the proposed Haast-Hollyford Highway, South Island, New Zealand

Coseismic landsliding presents a major hazard to infrastructure in mountains during large earthquakes. This is particularly true for road networks, as historically coseismic landsliding has resulted in road losses larger than those due to ground shaking. Assessing the exposure of current and planned highway links to coseismic landsliding for future earthquake scenarios is therefore vital for disaster risk reduction. This study presents a method to evaluate the exposure of critical infrastructure to landsliding from scenario earthquakes from an underlying quantitative landslide hazard assessment. The method is applied to a proposed new highway link in South Island, New Zealand, for a scenario Alpine Fault earthquake and compared to the current network. Exposure (the likelihood of a network being affected by one or more landslides) is evaluated from a regional-scale coseismic landslide hazard model and assessed on a relative basis from 0 to 1. The results show that the proposed Haast-Hollyford Highway (HHH) would be highly exposed to coseismic landsliding with at least 30–40 km likely to be badly affected (the Simonin Pass route being the worse affected of the two routes). In the current South Island State Highway network, the HHH would be the link most exposed to landsliding and would increase the total network exposure by 50–70% despite increasing the total road length by just 3%. The present work is intended to provide an effective method to assess coseismic landslide hazard of infrastructure in mountains with seismic hazard, and potentially identify mitigation options and critical network segments

Near-real-time modelling of landslide impacts to inform rapid response: an example from the 2016 Kaikoura, New Zealand, earthquake

Reliable methods for the near‐real‐time modeling of landslide hazard and associated impacts that follow an earthquake are required to provide crucial information to guide emergency responses. After the 2016 Kaikoura earthquake in New Zealand, we undertook such a near‐real‐time modeling campaign in an attempt to pinpoint the location of landslides and identify the locations where roads and rivers had been blocked. The model combined an empirical analysis of landslide hazard (based on previously published global relationships) with a simple runout model (based on landslide reach angles). It was applied manually, with a first iteration completed within 24 hrs of the earthquake and a second iteration (based on updated shaking outputs) within ∼72  hrs . Both models highlighted the expectation that landsliding would be widespread and that impacts to roads likely meant that the township of Kaikoura was cut off from the surroundings. These results were used by responders at the time to formulate aerial reconnaissance flight paths and to identify the risk that landslide dams could cause further damage. Subsequent model verification based on available landslide inventories shows that although these models were able to capture a large percentage of landslides and landslide impacts, the outputs were overpredicted, limiting their use for pinpointing the precise locations of triggered landslides. To make future versions of the model more useful for informing emergency responses, continued work must be done on modification and adaptation of the approach so that this overprediction is minimized. Nevertheless, the results from this study show that the model is a promising initial attempt at near‐real‐time landslide modeling and that efforts to automate the approach would greatly increase the utility and speed of modeling in future earthquakes