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Catastrophic landslides characterized by runaway slope failures remain difficult to predict. Here, we develop a physics‐based framework to prospectively assess slope failure potential. Our method builds upon the physics of extreme events in natural systems: the extremes so‐called "dragon‐kings" (e.g., slope tertiary creeps prior to failure) exhibit statistically different properties than other smaller‐sized events (e.g., slope secondary creeps). We develop statistical tools to detect the emergence of dragon‐kings during landslide evolution, with the secondary‐to‐tertiary creep transition quantitatively captured. We construct a phase diagram characterizing the detectability of dragon‐kings against "black‐swans" and informing on whether the slope evolves toward a catastrophic or slow landslide. We test our method on synthetic and real data sets, demonstrating how it might have been used to forecast three representative historical landslides. Our method can in principle considerably reduce the number of false alarms and identify with high confidence the presence of true hazards of catastrophic landslides. Plain Language Summary: Catastrophic slope failures that pose great threats to life and property remain difficult to predict due to the strong variability of slope behavior. As a result, only a limited number of large rock slope failures have been so far successfully forecasted with associated risks mitigated. Here, we propose a novel predictive framework to prospectively and quantitatively detect slope failure precursors with high confidence. Our research sheds light on one of the most challenging questions in landslide prediction: Would an active landslide slowly move or catastrophically fail in the future? Our method adds a new conceptual framework and operational methodology with a significant potential to support existing early warning systems and hence reduce landslide risks. Key Points: Tertiary creeps of catastrophic landslides accommodate dragon‐kings showing statistically different properties than secondary slope creepsA predictive framework is developed to forecast catastrophic landslides by detecting signatures typical of the emergence of dragon‐kingsA phase diagram characterizes the detectability of dragon‐kings against black‐swans and discriminates catastrophic and slow landslides [ABSTRACT FROM AUTHOR]

Rock slope failures often result from progressive rock mass damage which accumulates over long timescales. In deglaciating environments, rock slopes are affected by stress perturbations driven by mechanical unloading due to ice downwasting and concurrent changes in thermal and hydraulic boundary conditions. Since in‐situ data are rare, the different processes and their relative contribution to slope damage remain poorly understood. Here, we analyze borehole monitoring data from a rock slope adjacent to the retreating Great Aletsch Glacier (Switzerland) and compare it to englacial water levels, climate data, and decreasing ice levels. Rock slope pore pressures show a seasonal signal controlled by infiltration events as well as effects from the connectivity to the englacial hydrological system. We find that reversible and irreversible strains are driven by: (a) hydromechanical effects caused by englacial pressure fluctuations and infiltration events, (b) stress transfer related to changing mechanical glacial loads from short‐term englacial water level fluctuations and longer term ice downwasting, and (c) thermomechanical effects from annual temperature cycles penetrating the shallow subsurface, which primarily result in reversible deformation. We relate most observed irreversible strain (damage) to mechanical unloading from ice downwasting. Damage is strongest directly at the ice margin and moves through the slope at the pace of glacial retreat and advance. Locations with many retreat/advance cycles are very sensitive to landslide formation. The current climate warming impacts very sensitive valley sectors, which is confirmed by landslide distributions and activity in the study area. Plain Language Summary: The formation of rock slope instabilities is a long‐term process related to continuous rock mass weakening. For rock slopes in glaciated valleys, increased rates of rock mass damage are assumed to occur during ice retreat due to a reduction of the glacier weight and concurrent changes in rock temperatures and slope groundwater pressures. However, because direct field measurements are missing, governing processes and their relative importance are still poorly understood. Here, we analyze borehole monitoring data and compare it to englacial water levels, climate data, and decreasing ice levels. We find that the glacial hydrology influences the dynamics of the groundwater pressures in the adjacent rock slope at various timescales. We record micrometer‐scale reversible (elastic) deformation and irreversible rock mass damage and identify the involved drivers. Several processes contribute to progressive rock mass weakening at different timescales and magnitudes, but most damage is related to mechanical unloading from ice downwasting and is strongest directly at the ice margin. Locations with many retreat/advance cycles are sensitive to increased damage accumulation and landslide formation. The current climate warming impacts very sensitive valley sectors, which is confirmed by landslide distributions and activity in the study area. Key Points: We monitored subsurface pore pressures and micrometer scale strain in a rock slope during strong glacial retreat caused by climate warmingWe identified thermo‐hydro‐mechanical drivers for reversible deformation and irreversible rock mass damage acting at various timescalesSlope damage intensity can be related to landslide spatial and temporal distribution [ABSTRACT FROM AUTHOR]

In deglaciating environments, rock mass weakening and potential formation of rock slope instabilities is driven by long-term and seasonal changes in thermal- and hydraulic-boundary conditions, combined with unloading due to ice melting. However, in-situ observations are rare. In this study, we present new monitoring data from three highly instrumented boreholes, and numerical simulations to investigate rock slope temperature evolution and micrometer-scale deformation during deglaciation. Our results show that the subsurface temperatures are adjusting to a new, warmer surface temperature following ice retreat. Heat conduction is identified as the dominant heat transfer process at sites with intact rock. Observed non-conductive processes are related to groundwater exchange with cold subglacial water, snowmelt infiltration, or creek water infiltration. Our strain data shows that annual surface temperature cycles cause thermoelastic deformation that dominate the strain signals in the shallow thermally active layer at our stable rock slope locations. At deeper sensors, reversible strain signals correlating with pore pressure fluctuations dominate. Irreversible deformation, which we relate with progressive rock mass damage, occurs as short-term (hours to weeks) strain events and as slower, continuous strain trends. The majority of the short-term irreversible strain events coincides with precipitation events or pore pressure changes. Longer-term trends in the strain time series and a minority of short-term strain events cannot directly be related to any of the investigated drivers. We propose that the observed increased damage accumulation close to the glacier margin can significantly contribute to the long-term formation of paraglacial rock slope instabilities during multiple glacial cycles. [ABSTRACT FROM AUTHOR]

We present a new inventory of large rock slope instabilities and an extensive structural data set for northwestern Bhutan. Our data set is largely based on satellite observations, such as optical images and high‐resolution digital surface models, complemented with field observations. Kinematic analyses over seven different structural domains recognized in the study region were carried out to investigate structural control over the landslide distribution at a regional scale. In order to account for the sampling heterogeneity in the different data sets, a weighting system to the structural data is proposed. The results show that structural control exists in five out of seven domains, with foliation strongly influencing the sliding mechanisms. A lithological control appears also evident, with sedimentary rocks showing the highest landslide densities in the region. This study demonstrates how methodologies built on remote sensing data are suitable to investigate landslide predisposition at regional scales especially in largely inaccessible areas. Key Points: Rock slope instabilities and structural inventories for NW Bhutan based on optical images and high‐resolution DSMWeighting system applied to the data set improves its reliability and the assessment of limitationsLarge‐scale kinematic analyses for rock slope instabilities reveal structural control over a large part of the study region [ABSTRACT FROM AUTHOR]

Subglacial water pressures influence groundwater conditions in proximal alpine valley rock slopes, varying with glacier advance and retreat in parallel with changing ice thickness. Fluctuating groundwater pressures in turn increase or reduce effective joint normal stresses, affecting the yield strength of discontinuities. Here we extend simplified assumptions of glacial debuttressing to investigate how glacier loading cycles together with changing groundwater pressures generate rock slope damage and prepare future slope instabilities. Using hydromechanical coupled numerical models closely based on the Aletsch Glacier valley in Switzerland, we simulate Late Pleistocene and Holocene glacier loading cycles including long‐term and annual groundwater fluctuations. Measurements of transient subglacial water pressures from ice boreholes in the Aletsch Glacier ablation area, as well as continuous monitoring of bedrock deformation from permanent Global Navigation Satellite Systems stations, help verify our model assumptions. While purely mechanical glacier loading cycles create only limited rock slope damage in our models, introducing a fluctuating groundwater table generates substantial new fracturing. Superposed annual groundwater cycles increase predicted damage. The cumulative effects are capable of destabilizing the eastern valley flank of our model in toppling‐mode failure, similar to field observations of active landslide geometry and kinematics. We find that hydromechanical fatigue is most effective acting in combination with long‐term loading and unloading of the slope during glacial cycles. Our results demonstrate that hydromechanical stresses associated with glacial cycles are capable of generating substantial rock slope damage and represent a key preparatory factor for paraglacial slope instabilities. Plain Language Summary: Glacial melt produces large amounts of water keeping water pressures at the base of alpine glaciers high. This meltwater in turn influences the groundwater table in adjacent valley flanks. Water seeps into the subsurface through joints and faults reducing the strength of these discontinuities. In a glaciated alpine valley, the position of the glacier changes over time and the groundwater table changes in accordance. Meanwhile, seasonal infiltration from snowmelt raises and lowers the water table in valley flanks, creating measureable movements. These changes in groundwater can alter the strength of a rock mass and together with the changing ice load generate damage in the floors and walls of alpine valleys. This damage helps prepare future slope instabilities, creating fractures that may ultimately define a landslide failure surface. We present new data and models from the Great Aletsch Glacier in Switzerland exploring our hypothesis that groundwater‐glacier coupled interactions are an important mechanism generating damage in alpine valley rock slopes. Our results show that higher water pressures reduce the strength of rock discontinuities and can result in failure, especially as glaciers retreat. This mechanism is an important part of the strength and stress changes in valley slopes that accompany glacial cycles. Key Points: Subglacial water pressures in ablation areas are high and vary with ice thickness, affecting the groundwater table in adjacent valley flanksIncreasing groundwater pressure during glacier advance reduces effective stresses in joints, partly offsetting the increased ice loadHydromechanical coupled stresses during glacial cycles create and propagate fractures in rock slopes, preparing future instabilities [ABSTRACT FROM AUTHOR]

This paper presents a detailed analysis of a dramatic rock slope acceleration that occurred in fall 2016 at the Moosfluh Landslide, located at the glacier tongue of the Great Aletsch Glacier (Switzerland). The acceleration that occurred in 2016 was unanticipated and exposed the valley bottom and an adjacent damned lake to high risk. This acceleration occurred in an active deep‐seated gravitational slope deformation (DSGSD) controlled primarily by deep block‐flexural toppling. In 2013, a highly accurate displacement monitoring system was developed and installed in the surroundings of the Great Aletsch Glacier, including a time‐lapse camera, GNSS, and robotic total stations. This monitoring system provided unique data during the 2016 slope acceleration which are used in this study to assess failure mechanisms, landslide volumes, and subsurface displacement geometry. Based on a novel displacement vector analysis, we find that three retrogressive secondary rockslides developed during the first six weeks of the slope acceleration, with rupture surface depths of 30 to 40 m, and estimated volumes between 1 and 4 Mm3. These rockslides display complex deformation features, including head and lateral scarps, which developed during the slope acceleration. The kinematics of these secondary rockslides changed through time, from primarily toppling to combined toppling and sliding. Our results provide a uniquely detailed understanding of the spatial and temporal evolution of deformation features and movement kinematics that occur when several sectors of a slowly moving DSGSD transitions into rapid rockslides. Plain Language Summary: The slow moving (~30 cm per year) deep‐seated (up to 170‐m depth) landslide of Moosfluh experienced a substantial acceleration of slope movement in September 2016. A comprehensive monitoring system recorded that several compartments in the center of the Moosfluh Landslide suddenly started to move rapidly with velocities of up to several meters per week. This caused a strong modification of the slope surface morphology and structure which is documented in detail in this study. Based on monitored changes in the directions of slope movement over time, a new method was developed to interpret subsurface movement types and geometries. With this method we found that on top of the Moosfluh deep‐seated landslide several shallow (30–40 m) rockslides developed during the Moosfluh slope acceleration in 2016. We are able to assign depths and inclinations of the failure surfaces and failure mechanisms. In this case the deep‐seated landslide is formed by rotation (toppling) of steep rock columns, whereas the shallow secondary rockslides within the deep‐seated landslide show sliding movements along planar rupture planes and mixtures of both sliding and toppling. Key Points: The rapid formation of 3 secondary rockslides within a toppling mode DSGSD (Moosfluh, CH) is monitored with DIC from webcam images, aerial photogrammetry and total station monitoringThis transition corresponds to an acceleration of the landslide from a few dm/a to 50 m/a in the lowermost sectors of the instabilityA novel method developed to interpret total station reflector data revealed a change in landslide kinematics (toppling, sliding) through time [ABSTRACT FROM AUTHOR]

Abstract Large catastrophic rock slope failures are difficult to predict because the underlying mechanisms causing slope accelerations are difficult to study under in-situ conditions. For slope failures with compound basal rupture planes, not only fracture propagation and slip in the basal rupture surface, but also within the landslide body control the displacement evolution and time-to-failure. The Preonzo instability complex in southern Switzerland failed several times since 2002 and offers a unique opportunity to study these mechanisms under in-situ conditions. The largest failure at Preonzo occurred in May 2012 and was conditioned by large slope parallel (NN E -SSW) fracture zones, which are intersected by NE and SW and E-W striking faults creating diverging lateral boundaries. Whereas in the head scarp region oblique and flexural toppling along pre-existing fracture sets is the prevailing mechanism, sliding of either planar or wedge type is the preferred kinematic mode facilitated by the new discontinuities developed within the basal rupture surfaces. These dominating mechanisms, leading to a classical rock slope collapse, could only be revealed in retrospect. We compare fracture patterns in stable ground (tectonic and unloading fractures) with new fractures related to rock slope failure observed on basal rupture surfaces and in the head scarp. Simplified mechanical and kinematic analyses show that old pre-existing tectonic fractures cannot accommodate substantial deformations under the in-situ stress conditions and propagate/connect through curved wing cracks forming at high angles. This situation favors the development of new nearly slope parallel fractures, which are interpreted as synthetic P-shears along localized shear zones. Fractographic markings on these fractures indicate a continuous and relatively fast formation, which presumably has taken place during a period of about 60 years prior to catastrophic failure. This detailed description of the multi-stage evolution of slope failure at Preonzo represents a unique data set for future numerical studies of progressive failure in crystalline rocks. Highlights • Structural evolution of retrogressive failures with fracture orientation change • Interplay of pre-existing fractures and progressive failure in basal rupture planes • Multi-stage kinematic analyses reveal change in kinematics of a retrogressive slope • Possible analogies of tectonic shear zones with fracture reactivation + propagation [ABSTRACT FROM AUTHOR]

In this paper, we describe the investigations and actions taken to reduce risk and prevent casualties from a catastrophic 210,000 m rockslope failure, which occurred near the village of Preonzo in the Swiss Alps on May 15, 2012. We describe the geological predisposition and displacement history before and during the accelerated creep stage as well as the development and operation of an efficient early warning system. The failure of May 15, 2012, occurred from a large and retrogressive instability in gneisses and amphibolites with a total volume of about 350,000 m, which formed an alpine meadow 1250 m above the valley floor. About 140,000 m of unstable rock mass remained in place and might collapse partially or completely in the future. The instability showed clearly visible signs of movements along a tension crack since 1989 and accelerated creep with significant hydromechanical forcing since about 2006. Because the active rockslide at Preonzo threatened a large industrial facility and important transport routes located directly at the toe of the slope, an early warning system was installed in 2010. The thresholds for prealarm, general public alarm, and evacuation were derived from crack meter and total station monitoring data covering a period of about 10 years, supplemented with information from past failure events with similar predisposition. These thresholds were successfully applied to evacuate the industrial facility and to close important roads a few days before the catastrophic slope failure of May 15, 2012. The rock slope failure occurred in two events, exposing a compound rupture plane dipping 42° and generating deposits in the midslope portion with a travel angle of 39°. Three hours after the second rockslide, the fresh deposits became reactivated in a devastating debris avalanche that reached the foot of the slope but did not destroy any infrastructure. The final run-out distance of this combined rock collapse-debris avalanche corresponded to the predictions made in the year 2004. [ABSTRACT FROM AUTHOR]

We evaluate the dynamic response of the large (around five million cubic meters) unstable rock slope at Randa, Switzerland, based on ground-motion recordings during small earthquakes. Seismic measurements revealed strong polarization of the wavefield in the direction of maximum slope displacement with spectral amplification factors ∼5 within the unstable area. Results further highlighted resonant frequencies of both the large unstable rock mass and individual blocks within it. Block vibration was confirmed by phase analysis of in situ displacement measurements from two bounding tension fractures during a small earthquake. Numerical simulation using a discontinuous elastic model showed that the presence of steeply-dipping compliant fractures is crucial for recreating the observed site response. Such fractures are common in rock slope instabilities and are here shown to contribute to polarization and amplification of seismic shaking, site effects that are rarely considered for hard rock slopes and which may influence the potential for earthquake-triggered failure. [ABSTRACT FROM AUTHOR]

Investigating surface displacements in high alpine environments is often subject to challenges due to the difficult accessibility or harsh climatic conditions. Measurement systems have improved greatly in recent years regarding accuracy, range, or energy consumption. Continuously receiving high-precision, real-time monitoring data from a remote location can still support a better understanding of slope dynamics and risk. We present the design, construction, operation, and performance of a complex surface displacement monitoring system installed in the surroundings of the Great Aletsch Glacier in the Swiss Alps, based on two robotic total stations to continuously measure 3D displacements with high accuracies. In addition, GNSS stations are also considered in order to pass from a local to a geographic reference system, as well as to improve the measurement accuracy. The monitoring network is aimed at studying several types of deformation processes, i.e., (i) gravitationally driven and irreversible rockslide movements around the tongue of the Great Aletsch Glacier, (ii) reversible rock slope deformations caused by annual cycles of groundwater recharge and depletion, and (iii) small irreversible deformations of stable rock slopes resulting from progressive rock damage driven by glacier retreat and cyclic hydraulic and thermal loading. We describe the technical details of the monitoring system, which has been in operation successfully for 6 years, and discuss the system performance in terms of its robustness and accuracy. [ABSTRACT FROM AUTHOR]

Most natural rock slope failures result from long-term strength degradation. It is often hypothesized that glacial retreat leads to enhanced progressive damage accumulation in adjacent rock slopes, due to mechanical unloading and changes to the thermal and hydraulic boundary conditions. However, direct observations of subsurface processes in a rock slope subject to glacial retreat are rare. In this paper, we present the design, implementation, and performance of a new borehole monitoring system installed on a rock slope in fractured crystalline rock located beside the glacier tongue of the retreating Great Aletsch Glacier (Valais, Switzerland). The three vertical, 50 m deep boreholes were instrumented to continuously monitor groundwater pressure, temperature and deformation at high resolution, in order to investigate thermo-hydro-mechanical coupled slope processes that drive progressive rock mass damage. We show that the system is capable of measuring both reversible and irreversible displacements along single fractures at magnitudes ranging between 0.001 mm to >2 mm in the studied rock slope, and that it is often possible to identify drivers of these deformation signals. The transient subsurface temperature field shows clear indications of former ice occupation in form of cold temperatures preserved at depth and superimposed annual temperature cycles penetrating down to a depth of about 17 m. The variability of the pressure head in the slope is driven by annual snowmelt infiltration cycles, rainfall events, and the assumed connection to englacial water of the temperate glacier. The new and continuously growing dataset presented here will enable us to relate the changing boundary conditions caused by glacial retreat and fatigue from daily to annual thermal and hydraulic loading cycles to progressive rock mass weakening, which may ultimately result in rock slope failure. • We installed a borehole monitoring system in a glacier adjacent rock slope. • We measure subsurface temperature, pore pressure and strain during deglaciation. • This data is used to analyze thermo-hydro-mechanical coupled rock slope processes. • We document rock mass damage at the micrometer-scale linked to deglaciation. [ABSTRACT FROM AUTHOR]

Rock slopes next to the tongue of the Great Aletsch Glacier, Switzerland are characterized by rapid environmental adjustment to non-glacial conditions. This study investigates and describes in detail the historic development of the largest rock slope instability in this area, called Moosfluh Landslide. We study in detail the structure, evolution and stability since the end of the Little Ice Age (LIA) until September 2016 and discuss their relationships with the evolution of the Great Aletsch Glacier since the Lateglacial period. In 2016 around 50 m of glacial ice thickness were left at the Moosfluh Landslide toe, where in 1850 glacial ice was >400 m thick. The changing stability conditions at the interface with the melting valley glacier are studied based on novel balanced cross sections and kinematic model of the Moosfluh Landslide dominated by toppling phenomena in metamorphic rock. The morphology and evolution of this landslide since the LIA are investigated with multi-temporal landslide maps based on aerial digital photogrammetry (ADP) applied to historic images since 1961. Internal deformation at Moosfluh is accommodated by shear slip along uphill-facing foliation and fault planes, and by extensional faulting forming tension cracks and graben-structures at the landslide head. Together with Digital Image Correlation (DIC) and total station monitoring (TPS) the Moosfluh Landslide displacement history was reconstructed, evidencing post-Egesen landslide displacements and an acceleration of movements since the LIA and especially since 2007. The displacement rates increase from few mm per year until the nineties to several meter per day in September 2016. Different kinematic models have been tested and changes in the Moosfluh rock slope stability in response to retreating glacial ice and changing groundwater conditions was explored with limit-equilibrium analysis of the stepped planar block toppling model of Goodman and Bray (1976). For the observed conditions and a toppling joint friction angle of 19° the simulated factor of safety drops non-linearly from the LIA maximum (1.12) to the year 2007 (1.02), when the height of ice above the valley bottom melted down to 100 m. This study illustrates with unprecedented detail the time scales, displacement magnitudes and structural evolutions of a large toppling mode slope instability in a paraglacial setting. The long-term cumulative slope displacements between the Egesen stadial and the LIA are of the same magnitude as the cumulative displacements between the LIA and the year 2016. As large portions of the studied slope underwent multiple retreats and advances of the Great Aletsch Glacier during the Lateglacial and Postglacial period, the observed onset of large slope displacements should be related to incremental damage (slip weakening and weathering) occurring along steeply dipping toppling fractures during the LIA. [ABSTRACT FROM AUTHOR]

Fractured rock-masses deform in response to changes in pore pressure. In alpine catchments, groundwater flow dynamics is generally dominated by strong recharge signal from snowmelt infiltration in spring / early summer and in early winter, and punctuated by large rainstorms in snow-free periods. In crystalline rock slopes, infiltration occurs along discontinuities, such as faults and joints. Under constant normal stress joint apertures increase non-linearly during pore pressure increase, increasing the volume of saturated rock masses and leading to surface deformation. The latter can be measured with high-resolution geodetic monitoring systems and remote sensing methods such as DInSAR.In this work, we discuss temporal and spatial variations of reversible ground surface deformation measured in the Aletsch region, Switzerland. The study area is situated in the vicinity of the tongue of the Great Aletsch Glacier, central Swiss Alps, in the gneissic and granitic bedrock of the Aar massif. The valley (1550 to 2300 m.a.s.l.), is roughly oriented parallel to the main Alpine foliation (NE-SW) and to major fault zones. Total annual precipitation is less than 1 m/y, but snow depth frequently reaches 2 m during winter, with strong spatial variability. We use a network of 4 continuous Global Positioning System (cGPS) stations and 90 reflectors linked to 2 total stations (TPS), operating since 2014, to track deformation at millimetric to centimetric scale. We use weather and hydrological data from surrounding stations to estimate the timing of infiltration.At some locations in our study area we observe peak-to-peak magnitude of seasonal deformation over 2 cm, whereas other areas show no significant seasonal deformation. Measured displacement vectors are generally oriented towards the valley center in spring, when deformation rates are the highest (>0.5 mm/d). The start of the high-rate of deformation period also varies spatially. The South-facing slope starts deforming around 20 days earlier than the North-facing slope. The vertical component of motion is generally smaller than the horizontal one, and often below the noise level. Finally, we discuss possible factors (e.g. topography and slope orientation, discontinuities' orientation and density, lithology, groundwater recharge magnitudes and durations) that might influence the spatial and temporal variations of reversible deformation on the slopes adjacent to the tongue of the Great Aletsch Glacier. [ABSTRACT FROM AUTHOR]

This study is based on detailed monitoring data from a large DSGSD called Moosfluh instability, located at the current glacier tongue of the Great Aletsch Glacier (Switzerland). This about 100 Mm³ large, and up to 170 m deep toppling mode instability started to accelerate around 1990, presumably caused by increased rates of glacier ice downwasting at the landslide toe. In 2013 and 2014, when average velocities reached about 500-700 mm/a, we installed a high accuracy displacement monitoring system, composed of 2 robotized total stations measuring displacements at about 15 reflectors positioned within the active landslide, 4 GNSS stations, and 2 Time-Lapse cameras. In the fall of 2016 a dramatic increase in acceleration occurred, leading to velocities of individual landslide compartments of up to 12 m in 6 weeks. Optical imagery and digital image correlation revealed that this acceleration was caused by the rapid development of three secondary rockslide scarps, and that movement initiated at the toe of the slope and evolved upslope. To investigate the depths of movements and the subsurface kinematic mechanism in detail, we developed a new analytical model, which considers the time series of three-dimensional displacements and plunge angles of discrete points within the landslide. The application of this model leads to the conclusion that a transition from primarily toppling to sliding mode displacements occurred during fall 2016. We can trace the detailed evolution of individual compartments, showing that toppling/sliding ratios varied in both space and time. The secondary rockslides forming during this period have a depth of about 30-40 meters and volumes ranging between 1 and 5 Mm³. Planar sliding surfaces are expected to have developed at a major kink zone within the large toppling rock mass.This kinematic evolution observed at Moosfluh significantly impacts the corresponding hazard and risk assessment, as flexural toppling tends to be a self-stabilizing mechanism, whereas planar rockslides can potentially transition into catastrophic failures. The estimated potential runout length probabilities show that the main hazard would be created by the formation and subsequent breach of a rock avalanche dam in the Massa gorge below the tongue of the Great Aletsch Glacier. [ABSTRACT FROM AUTHOR]

We leverage on optical and radar remote sensing data acquired from the European Space Agency (ESA) Sentinels to monitor the surface deformation evolution on a large and very active instability located in the Swiss Alps, i.e., the Moosfluh rock slope. In the late summer 2016, a sudden acceleration was reported at this location, with surface velocity rates passing from maximum values of 0.2 cm/day to 80 cm/day. A dense pattern of uphill-facing scarps and tension cracks formed within the instability and rock fall activity started to become very pronounced. This evolution of the rock mass may suggest that the most active portion of the slope could fail catastrophically. Here we discuss advantages and limitations of the use of spaceborne methods for hazard analyses and early warning by using the ESA Sentinels, and show that in critical scenarios they are often not sufficient to reliably interpret the evolution of surface deformation. The insights obtained from this case study are relevant for similar scenarios in the Alps and elsewhere. [ABSTRACT FROM AUTHOR]

In this work we present a methodology based on remote sensing data to map and classify unstable slopes in high alpine areas and apply this methodology to the Himalaya of northwestern Bhutan. We leverage on radar images acquired by the ESA Envisat and the JAXA ALOS-1 satellites between 2007 and 2011. Synthetic aperture radar differential interferometry (DInSAR) is used to identify and quantify recent ground surface displacements associated with potentially unstable slopes. Optical satellite images and a high-resolution digital surface model is used for the geomorphological classification of individual landforms and deposits associated with displacements. The analysis of the geomorphological characteristics reveals that displacements are associated with: rock slides, mountain slope deformation and rock slope deformation (77.5%), soil creep (4%), soil slides (1.9%) and rock glaciers (12.3%); 4.3% of the detected displacements remain of unknown origin. The method applied here for the assessment of activity likelihood uses a very large number of interferograms obtained with images acquired by different satellites. The method, which is new and replicable, takes advantage of the combination of different acquisition geometries and then combines indices of vegetation cover, layover, velocity estimate and temporal sampling associated with each detected active area to assign a weight. The analysis shows that a cluster of unstable slopes exists in the northwest, potentially related to the presence of an important regional lineament and the distribution of permafrost. Comparatively little ongoing activity is detected in the south, but field validation documents substantial landslide activity in the recent geological past. The dataset here generated, containing 700 potentially unstable areas with an associated activity likelihood, is a novel dataset that makes an essential basis of future analysis to investigate rock slope instability distribution and predisposing factors. Moreover, this dataset represents a new important step towards future landslide hazard assessment in the Himalaya of Bhutan. • 693 previously unknown unstable slopes in NW Bhutan were found with DInSAR • Geomorphological classification revealed that almost 78% are rock slope instabilities • The spatial distribution of potentially unstable slopes in the region is clearly non uniform • Activity of unstable slopes is likely influenced by regional tectonic structures • Unstable slopes seem to be less abundant in the region in recent times than in the past [ABSTRACT FROM AUTHOR]

Recently, advances in the evaluation of river channels have shown that transient erosional signals can preserve detailed information on the formation of topography and morphology of mountain ranges (e.g., Perron and Royden, 2012). Records of temporal variations in erosional activity within a landscape have also been demonstrated to be retained in the morphology of affected hillslopes (e.g., Colman and Watson, 1983). Here, we present a new method bringing together both the record of erosional activity stored in the hillslopes and the transient erosional signals stored along the river channel.The limit equilibrium relationship between topography and erosional processes has been described in the work of both Selby (1982) and Blöthe et al. (2015), the former showing a direct dependence of stable slope angle to rock mass strength, while the latter study discusses the erosional potential of slopes exceeding the critical angle of the material (excess topography) through landsliding. We use these mechanical concepts in addition to geometrical criteria to identify hillslope surfaces related to former erosional stages in two large (~5000 km2) catchments in the Bhutan Himalaya. These can then be tied to knickpoints extracted along the river profiles, thus allowing for a side by side analysis of hillslope and fluvial processes related to the recent tectonic and erosional activity of the region within and across catchments.ReferencesBlöthe, J. H., Korup, O., & Schwanghart, W. (2015). Large landslides lie low : Excess topography in the Himalaya- Karakoram ranges, 43(6), 523–526. https://doi.org/10.1130/G36527.1Colman, S. M., & Watson, K. (1983). Ages Estimated from a Diffusion Equation Model for Scarp Degradation. Science, 221(4607), 263 LP-265. https://doi.org/10.1126/science.221.4607.263Perron, J. T., & Royden, L. (2012). An integral approach to bedrock river profile analysis. Earth Surface Processes and Landforms, 3302. https://doi.org/10.1002/esp.3302Selby, M. J. (1982). Controls on the stability and inclinations of hillslopes formed on hard rock. Earth Surface Processes and Landforms, 7, 449–467. https://doi.org/10.1002/esp.3290070506 [ABSTRACT FROM AUTHOR]

Valley glaciers have traditionally been expected to significantly influence the stability and movement rates of adjacent paraglacial landslides. However, detailed studies related to the mechanical and displacement interactions between glacier ice and unstable rock slopes are very rare. Here we present a detailed in-situ investigation of the spatial variations of the displacement field of the Great Aletsch Glacier in the surroundings of a large active instability, i.e., the Moosfluh Landslide, a Deep-Seated Gravitational Slope Deformation, with superimposed large (1–5 million m3) secondary rockslides formed during fall 2016. We performed repeat UAV-based photogrammetric surveys during ~3 days (74 h) and applied Digital Image Correlation techniques to record high-resolution surface displacement vector fields of the landslide, stable slopes and adjacent glacier. Our results show that the secondary rockslide adjacent to the glacier is composed of two parts of 1.5 and 2.8 million m3 volume respectively, showing significant differences in mean displacement velocities (0.4 and 0.9 m in 74 h respectively, excluding rapid movements from detached blocks). Both rockslide compartments induce clear deflections of the glacier flow field, moving with a maximum velocity of about 0.3 to 0.4 m in 74 h. This influence is highest near the ice-contact boundary and decreases within a distance of about 100–200 m from the rock slope instability. We investigate the viscous forces at the landslide/glacier contact using a straightforward analytical model for an incompressible rockslide block sliding along a planar, cohesionless surface into ice. These forces are then applied to a slope stability model based on the limit equilibrium concept, representing the real geometry at the interface boundary to quantitatively explore the true buttressing effects of valley glaciers on an already moving slope instability. We show that the viscous ice deformation plays an important role in mediating the displacement velocities of landslides in unstable conditions, while, on the other hand, the slope support from the valley glacier has very little influence on the stability of the investigated rockslides. As most valley glaciers are currently strongly retreating due to global warming, uncovering significant numbers of pre-LIA slope instabilities, this detailed investigation provides important hints on their potential displacement behavior. Understanding the factors controlling landslide velocity is of great importance for hazard analyses and early warning. Hence, this study has implications beyond academic interest, e.g., for the planning and operation of alpine infrastructure, such as cable cars or hydropower systems. • We monitor glacier displacement fields at contacts to stable slopes and an active rockslide. • Distortions of glacier displacements fields extend over 100-200 m. and correlate with variations in rockslide velocity. • A simplified analytical model with viscous ice forces can reproduce observed rockslide velocities. • Limit equilibrium analyses demonstrate negligible effect of glacial debuttressing on mature rockslide stability. [ABSTRACT FROM AUTHOR]

Slope deformation in mountainous terrain can be driven by different processes, the nature of which is either gravitational and irreversible or seasonal and reversible, the latter induced by permafrost variations or by hydromechanical or thermomechanical effects. The importance of identifying such deformations is not only related to the hazard they can pose, but also to the understanding of changes that permafrost or local hydrological conditions undergo. Here, we carry out a multi-temporal InSAR analysis over a mountainous area 8000 km2 large, straddling north-western Bhutan and southern Tibet. We propose a methodology to separate locally deforming areas from the effects of long-wavelength signals through the analysis of the spatio-temporal behaviour of 4-years long time series of surface displacements. We present the mapping of hundreds of small-scale features that appear to be actively deforming, as well as several examples of reversible deformation rarely detected at this scale in such a challenging and vast region. The analysis of the multi-annual trend of ground deformation shows a relatively small number of irreversible gravitational movements clearly related to rock slides which attests a low level of recent activity of large rock slope instabilities in the region. In the southernmost, lower elevation parts of the study area, we quantify reversible surface displacements with amplitudes ranging between 5 and 17 mm, and showing maximum displacements towards the satellite in summer months, thus compatible with hydro-mechanical effects related to groundwater table variations. In addition, the ground movement induced by the active layer's response to thawing and freezing over the gentler slopes and high-elevation permafrost regions of Bhutan and southern Tibet is on average around 10 mm, with maxima up to 28 mm. The localised displacements appear to be largely associated to braided stream plains, glacier outwash plains or low angle, fine sediment covered slopes. • Inventory of gravitational and reversible surface displacements for NW Bhutan • Classification of slope processes made on the basis of time series trends • Reversible, mostly vertical deformation, observed in the permafrost region • Reversible deformation with horizontal component observed at lower elevations • Rock glaciers appear to have lower creep rates than in the Alps. [ABSTRACT FROM AUTHOR]