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Changes in water pressure at the beds of glaciers greatly modify their sliding rate, affecting rates of ice mass loss and sea level change. However, there is still no agreement about the physics of subglacial sliding or how water affects it. Here, we present a new simplified physical model for the effect of transient subglacial hydrology on basal ice velocity. This model assumes that a fraction of the glacier bed is connected by an active hydrologic system that, when averaged over an appropriate scale, is governed by two parameters with limited spatial variability. The sliding model is reminiscent of Budd's empirical sliding law but with fundamental differences including a dependence on the fractional area of the active hydrologic system. With periodic surface meltwater forcing, the model displays classic diffusion-wave behavior, with a downstream time lag and decay of subglacial water pressure perturbations. Testing the model against Greenland observations suggests that, despite its simplicity, it captures key features of observed proglacial discharges and ice velocities with reasonable physical parameter values. Given these encouraging findings, including this sliding model in predictive ice-sheet models may improve their ability to predict time-evolving velocities and associated sea level change and reduce the related uncertainties.

Conditions at the base of an ice stream control the ability of basal material to resist ice sliding, which affects ice stream mass balance. Yet, basal properties are notoriously hard to constrain. Tiny magnitude ~-2 to -1 stick-slip basal icequakes occurring near or on the basal sliding surface hold valuable information about this basal environment. In this dissertation, I investigate spatiotemporal patterns in these basal icequakes occurring beneath the Whillans Ice Plain (WIP), in West Antarctica, and interpret these patterns to gain insight into basal material heterogeneity and temporally evolving basal conditions.In Chapter 2, I determine where basal icequakes happen beneath the entire WIP. I find spatially variable seismicity rates, with basal seismicity most common in a ~40 km wide area surrounding a dynamically important region where ice-plain-wide unstable slip nucleates. This result implicates icequake-generating bed conditions in large-scale ice stream stick-slip. Additionally, I propose that basal icequakes occur where basal erosion exposes over-consolidated till to the ice base.In Chapter 3, I use back-projection to detect basal icequakes beneath a small seismic network near the nucleation region. Here, basal icequakes occur in streaks elongated along ice flow and in conjunction with low-amplitude (~2m) undulating basal topography. These patterns suggest the presence of mega-scale glacial lineations (MSGL), elongate bedforms common on paleo-ice stream beds. One icequake streak may occur in a shallow trough beside an MSGL, suggesting that these icequakes occur in erosion-impacted lows between MSGL where over-consolidated till or stiff sediment outcrops contact the ice base.In Chapter 4, I analyze an improved basal icequake catalog generated by cross correlating icequakes detected in Chapter 3. I estimate icequake moment magnitudes of Mw = -2.1 to -1.2 and fault rupture areas of 1-100 m^2 for several large basal icequakes. Families of nearly-identical repeating basal icequakes continue for ice sliding distances of typically

This paper presents a recent rainfall-induced reactivated compound soil slide in Yanyuan County, China, on July 19, 2018. In this paper, according to the elaborate investigation of the soil slide area at field and prefailure slope stability analysis, the corresponding failure mechanisms are comprehensively studied. Unmanned aerial vehicle (UAV) based on remote sensing, three-dimensional modelling, in situ and laboratory experimental testing, borehole exploration, and numerical simulation were carried out in this work. According to the results, two different sliding processes were identified involving the main sliding and lateral collapse. The main sliding occurred as multiple stages along with two different types of slip zones, which involve a preexisting basal zone of the ancient landslide and an interlaminar shear zone within the ancient landslide deposits. The predominance of reactivated surface geomorphology provides a clue to elucidate the dominant compound movement of the event. Unconsolidated sliding mass and the basal sliding surface are ideal for the development of the reactivation motion. Saltatory microrelief of the original slope determined the retrogressive failure type of the soil slide. The well-developed groundwater in the landslide area significantly increased the soil mass saturation, with an average saturation value of 98%, formed dominant seepage in the sliding mass, and promoted landslide reactivation. Furthermore, rainwater infiltration due to 14 days’ antecedent cumulative rainfall of 350.6 mm directly contributed to the occurrence of the landslide by increasing the pore water pressure in the slope. The maximum shear plastic strain and displacement obtained by numerical simulation show that the lateral slope was unstable with the removal of the main sliding. Because the soil slide is located in the west area of the Emeishan Large Igneous Province (ELIP), it is a partial resurrection of the ancient basalt landslide. Therefore, this study has a certain significance for revealing the reactivated mechanism of basalt ancient landslide.

This study explores the ice flow acceleration (21.1%) of Pedersenbreen during 2016–2017 after the extremely warm winter throughout the whole Arctic in 2015/2016 using in situ data and quantitatively analyses the factors contributing to this acceleration. Several data sets, including 2008–2018 air temperature data from Ny-Alesund, ten-year in situ GPS measurements and Elmer/Ice ice flow modelling under different ice temperature scenarios, suggest that the following factors contributed to the ice flow acceleration: the softened glacier ice caused by an increase in the air temperature (1.5°C) contributed 2.7%–30.5%, while basal lubrication contributed 69.5%–97.3%. The enhanced basal sliding was mostly due to the increased surface meltwater penetrating to the bedrock under the rising air temperature conditions; consequently, the glacier ice flow acceleration was caused mainly by an increase in subglacial water. For Pedersenbreen, there was an approximately one-year time lag between the change in air temperature and the change in glacier ice flow velocity.

Giant landslides can cause significant damage to their dynamic runout processes. Understanding the mechanics of the runout process is essential for landslide risk assessment and mitigation design. This paper investigates the runout process of the 1.16 × 109 m3 giant Daguangbao landslide triggered by the 2008 Ms 8.0 Wenchuan earthquake. The Daguangbao landslide is a typical bedding-plane slide and the properties of basal sliding surface play a key role in the runout behaviors of the landslide. The material point method (MPM) is adopted as a numerical tool of the simulations. A thermal-based velocity-weakening friction law is used to simulate the contact between the sliding body and the basal sliding surface. A strain-softening constitutive model is used to evaluate the damages inside the landslide body and their effects on the runout behaviors of the landslide. Numerical results reveal that landslide mass slides along the bedding-plane as a whole body initially and then disintegrates during the runout process. The calculated duration of sliding is 64 s and the maximum velocity reaches 60 m/s. The friction coefficient of the slip surface decreases sharply as the landslide body starts to move, and a steady-state friction coefficient μ ≈ 0.06 is reached when the velocity exceeds approximately 20 m/s. Friction degradation of the slip surface shows a sensible influence on the final runout distance and the depth of the deposit zone. The dynamic fragmentation of the landslide body, the final runout distance, and deposition topography are also significantly affected by material softening.

We reconstruct patterns of subglacial processes on a hard bedrock and a soft bed under the southern sector of Scandinavian Ice Sheet (SIS) occurring in the basal till of the Late Saalian Glaciation at the Wapienno, Barcin and Młodocin sites (north-central Poland). Based on detailed sedimentological studies, two phases of SIS transgression were recognised. In the initial phase of the transgression, the SIS advanced onto a frozen substrate (continuous permafrost). The low permeability of the substratum led to a high subglacial water pressure (SWP) and increased basal sliding. The local increase of SWP led to the development of different types of structures and sediments. On a hard bedrock, with low SWP, abrasion predominated and linear structures were developing, while in the case of high SWP, the ice was decoupled from the hard substrate, pressurised liquefied sediment flowed, and structures of the p-form and s-form type developed. On a soft bed, the ice-bed contact was of a mosaic type and the ice movement had an ice-stream character. The ice-stream developed towards the east in the marginal zone of the SIS and used a W-E oriented valley filled by the Wapienno Formation fluvial complex. During a later phase, the ice movement was slower and did not have a stream character. Its direction changed to SE. The deposition of the main part of the diamicton occurred mainly as a result of the lodgement process.

Simulations of the glacial–interglacial history of the Antarctic Ice Sheet provide insights into dynamic threshold behavior and estimates of the ice sheet's contributions to global sea-level changes for the past, present and future. However, boundary conditions are weakly constrained, in particular at the interface of the ice sheet and the bedrock. Also climatic forcing covering the last glacial cycles is uncertain, as it is based on sparse proxy data. We use the Parallel Ice Sheet Model (PISM) to investigate the dynamic effects of different choices of input data, e.g., for modern basal heat flux or reconstructions of past changes of sea level and surface temperature. As computational resources are limited, glacial-cycle simulations are performed using a comparably coarse model grid of 16 km and various parameterizations, e.g., for basal sliding, iceberg calving, or for past variations in precipitation and ocean temperatures. In this study we evaluate the model's transient sensitivity to corresponding parameter choices and to different boundary conditions over the last two glacial cycles and provide estimates of involved uncertainties. We also discuss isolated and combined effects of climate and sea-level forcing. Hence, this study serves as a “cookbook” for the growing community of PISM users and paleo-ice sheet modelers in general. For each of the different model uncertainties with regard to climatic forcing, ice and Earth dynamics, and basal processes, we select one representative model parameter that captures relevant uncertainties and motivates corresponding parameter ranges that bound the observed ice volume at present. The four selected parameters are systematically varied in a parameter ensemble analysis, which is described in a companion paper.

The Parallel Ice Sheet Model (PISM) is applied to the Antarctic Ice Sheet over the last two glacial cycles (≈210 000 years) with a resolution of 16 km. An ensemble of 256 model runs is analyzed in which four relevant model parameters have been systematically varied using full-factorial parameter sampling. Parameters and plausible parameter ranges have been identified in a companion paper (Albrecht et al., 2020) and are associated with ice dynamics, climatic forcing, basal sliding and bed deformation and represent distinct classes of model uncertainties. The model is scored against both modern and geologic data, including reconstructed grounding-line locations, elevation–age data, ice thickness, surface velocities and uplift rates. An aggregated score is computed for each ensemble member that measures the overall model–data misfit, including measurement uncertainty in terms of a Gaussian error model (Briggs and Tarasov, 2013). The statistical method used to analyze the ensemble simulation results follows closely the simple averaging method described in Pollard et al. (2016). This analysis reveals clusters of best-fit parameter combinations, and hence a likely range of relevant model and boundary parameters, rather than individual best-fit parameters. The ensemble of reconstructed histories of Antarctic Ice Sheet volumes provides a score-weighted likely range of sea-level contributions since the Last Glacial Maximum (LGM) of 9.4±4.1 m (or 6.5±2.0×106km3), which is at the upper range of most previous studies. The last deglaciation occurs in all ensemble simulations after around 12 000 years before present and hence after the meltwater pulse 1A (MWP1a). Our ensemble analysis also provides an estimate of parametric uncertainty bounds for the present-day state that can be used for PISM projections of future sea-level contributions from the Antarctic Ice Sheet.

Glacier-erosion rates range across orders of magnitude, and much of this variation cannot be attributed to basal sliding rates. Subglacial till acts as lubricating ‘fault gouge’ or ‘sawdust’, and must be removed for rapid subglacial bedrock erosion. Such erosion occurs especially where and when moulin-fed streams access the bed and are unconstrained by supercooling or other processes. Streams also may directly erode bedrock, likely with strong time-evolution. Erosion is primarily by quarrying, aided by strong fluctuations in the water system driven by variable surface melt and by subglacial earthquakes. Debris-bed friction significantly affects abrasion, quarrying and general glacier flow. Frost heave drives cirque headwall erosion as winter cold air enters bergschrunds, creating temperature gradients to drive water flow along premelted films to growing ice lenses that fracture rock, and the glacier removes the resulting blocks. Recent subglacial bedrock erosion and sediment flux are in many cases much higher than long-term averages. Over glacial cycles, evolution of glacial-valley form feeds back strongly on erosion and deposition. Most of this is poorly quantified, with parts open to argument. Glacial erosion and interactions are important to tectonic and volcanic processes as well as climate and biogeochemical fluxes, motivating vigorous research.

The full velocity structure of glacial ice is critical to understanding glacier dynamics, but such measurements are rare. We propose a novel data processing method to resolve the horizontal and vertical velocity profile of an ice column using existing autonomous phase-sensitive radio echo sounder (ApRES) systems. The availability and relatively low cost of these systems can enable more widespread measurement of the 3D velocity structure of ice, with important implications for observational constraints on ice dynamics and basal drag.

Basal sliding in the ablation zone of the Greenland Ice Sheet is closely associated with water from surface melt introduced to the bed in summer, yet melting of basal ice also generates subglacial water year-round. Assessments of basal melt rely on modelling with results strongly dependent upon assumptions with poor observational constraint. Here we use surface and borehole measurements to investigate the generation and fate of basal meltwater in the ablation zone of Isunnguata Sermia basin, Western Greenland. The observational data are used to constrain estimates of the heat and water balances, providing insights into subglacial hydrology during the winter months when surface melt is minimal or non-existent. Despite relatively slow ice flow speeds during winter, the basal meltwater generation from sliding friction remains many fold greater than that due to geothermal heat flux. A steady acceleration of ice flow over the winter period at our borehole sites can cause the rate of basal water generation to increase by up to 20 %. Borehole measurements show high but steady basal water pressure, rather than monotonically increasing pressure. Ice and groundwater sinks for water do not likely have sufficient capacity to accommodate the meltwater generated in winter. Analysis of basal cavity dynamics suggests that cavity opening associated with flow acceleration likely accommodates only a portion of the basal meltwater, implying a residual is routed to the terminus through a poorly connected drainage system. A forcing from cavity expansion at high pressure may explain observations of winter acceleration in Western Greenland.

Increasing surface melting on the Greenland ice sheet requires better constraints on seasonally evolving basal water pressure and sliding speed. Here we assess the potential of using inverse methods on a dense time series of surface speeds to recover the seasonal evolution of the basal conditions in a well-documented region in southwest Greenland. Using data compiled from multiple satellite missions, we document seasonally evolving surface velocities with a temporal resolution of two weeks. We then apply the inverse control method using Elmer/Ice to infer the basal sliding and friction corresponding to each of the 24 surface-velocity data sets. Near the margin where the uncertainty in the velocity and bed topography are small, we obtain clear seasonal variations that can be mostly interpreted in terms of a effective-pressure based hard-bed friction law. We find for valley bottoms or "troughs" in the bed topography, the changes in basal conditions directly respond to local water pressure variations, while the link is more complex for subglacial "ridges" which are often non-locally forced. At the catchment scale, in-phase variations of the water pressure, surface velocities, surface-runoff variations are found.Our results show that time-series inversions of observed surface velocities can be used to understand the evolution of basal conditions over different timescales and could therefore serve as an intermediate validation for subglacial hydrology models to achieve better coupling with ice-flow models.

Since the Last Glacial Maximum ∼20,000 years ago, the Antarctic Ice Sheet has undergone extensive changes, resulting in a much smaller present-day configuration. Improving our understanding of basic physical processes that played important roles during that retreat is critical to providing more robust model projections of future retreat and sea-level rise. Here, a limited-area nested ice sheet model was applied to the last deglacial retreat of the West Antarctic Ice Sheet in the Amundsen Sea Embayment (ASE), at 5 km resolution. The ice sheet response to climate and sea-level forcing was examined at two sites along the flowlines of Pine Island Glacier and Pope Glacier, close to the Hudson Mountains and Mount Murphy respectively, and the simulated responses compared with ice sheet thinning histories derived from glacial-geological data. The sensitivity of results to selected model parameters was also assessed. The model simulations predict a broadly similar response to ocean forcing in both the central and eastern ASE, with an initial rapid phase of thinning followed by a slower phase to the modern configuration. Although there is a mismatch of up to 5,000 years between the timing of simulated and observed thinning, the modeling suggests that the upstream geological records of ice surface elevation change reflect a response to retreat near the grounding line. The model-data mismatch could potentially be improved by accounting for regional variations in mantle viscosity, sea-surface heights and basal sliding properties across the continental shelf.

We develop Bayesian statistical models that are designed for the inference of ice softness and basal sliding parameters, important glaciological quantities. These models are applied to Langjökull, the second largest temperate ice cap in Iceland at about 900 squared kilometers in area. The models make use of a relationship between physical parameters and ice velocity as stipulated by a shallow ice approximation that is generally applicable to Langjökull. The posterior distribution for ice softness concentrates around 18.2 × 10−25s−1Pa−3; moreover, spatially varying basal sliding parameters are inferred allowing for the decomposition of velocity into a deformation component and a sliding component, with spatial variation consistent with previous studies. Bayesian computation is conducted with a Gibbs sampling approach. The paper serves as an example of statistical inference for ice softness and basal sliding parameters at temperate, shallow glaciers using surface velocity data.

Rock slides quite commonly transform into flow-like landslides along their runout paths. At three initial rock slides in northern British Columbia, which occurred between 2002 and 2005, around 50–70% of the entire runout distance is composed of debris avalanche and debris flow deposits, which is comparable to other composite landslides around the world. Saturated ground conditions at the time of rock sliding make entrainment or undrained loading as agents of flow transformation from initially dry rock slides to partially saturated debris avalanches, respectively, fully saturated debris flows likely. Hummocks occur as clusters within the rock slide parts, whereas the flow-like depositions have more subdued morphologies. We show that late- or post-emplacement motion of individual hummocks is possible and can even divert in direction from the dominant landslide trajectory by responding to the underlying topographic gradient. Analyses of the well-preserved deposits suggest hummock formation above a basal sliding plane (low-angle normal fault) and along subordinate shear zones (high-angle normal faults) within a largely translational mass movement, thereby supporting the hypothesis of hummock formation proposed by Paguican et al. (2014). Pebbles on top of rock slide-debris avalanche boulders and on top of snapped-off trees record much higher dynamic debris flow heights. These and other features are not recorded on ancient landslides due to rapid erosion.

Ice loss from the Antarctic ice sheet (AIS) is expected to becomethe major contributor to sea level in the next centuries. Projections of theAIS response to climate change based on numerical ice-sheet models remainchallenging due to the complexity of physical processes involved in ice-sheetdynamics, including instability mechanisms that can destabilise marine basinswith retrograde slopes. Moreover, uncertainties in ice-sheet models limit theability to provide accurate sea-level rise projections. Here, we applyprobabilistic methods to a hybrid ice-sheet model to investigate theinfluence of several sources of uncertainty, namely sources of uncertainty inatmospheric forcing, basal sliding, grounding-line flux parameterisation,calving, sub-shelf melting, ice-shelf rheology and bedrock relaxation, on thecontinental response of the Antarctic ice sheet to climate change over thenext millennium. We provide probabilistic projections of sea-level rise andgrounding-line retreat, and we carry out stochastic sensitivity analysis todetermine the most influential sources of uncertainty. We find that allinvestigated sources of uncertainty, except bedrock relaxation time,contribute to the uncertainty in the projections. We show that thesensitivity of the projections to uncertainties increases and thecontribution of the uncertainty in sub-shelf melting to the uncertainty inthe projections becomes more and more dominant as atmospheric and oceanictemperatures rise, with a contribution to the uncertainty in sea-level riseprojections that goes from 5 % to 25 % in RCP 2.6 to more than 90 % inRCP 8.5. We show that the significance of the AIS contribution to sea levelis controlled by the marine ice-sheet instability (MISI) in marine basins,with the biggest contribution stemming from the more vulnerable WestAntarctic ice sheet. We find that, irrespective of parametric uncertainty,the strongly mitigated RCP 2.6 scenario prevents the collapse of the WestAntarctic ice sheet, that in both the RCP 4.5 and RCP 6.0 scenarios theoccurrence of MISI in marine basins is more sensitive to parametricuncertainty, and that, almost irrespective of parametric uncertainty, RCP 8.5triggers the collapse of the West Antarctic ice sheet., info:eu-repo/semantics/published

We describe and evaluate version 2.1 of the Community Ice Sheet Model (CISM). CISM is a parallel, 3-D thermomechanical model, written mainly in Fortran, that solves equations for the momentum balance and the thickness and temperature evolution of ice sheets. CISM's velocity solver incorporates a hierarchy of Stokes flow approximations, including shallow-shelf, depth-integrated higher order, and 3-D higher order. CISM also includes a suite of test cases, links to third-party solver libraries, and parameterizations of physical processes such as basal sliding, iceberg calving, and sub-ice-shelf melting. The model has been verified for standard test problems, including the Ice Sheet Model Intercomparison Project for Higher-Order Models (ISMIP-HOM) experiments, and has participated in the initMIP-Greenland initialization experiment. In multimillennial simulations with modern climate forcing on a 4 km grid, CISM reaches a steady state that is broadly consistent with observed flow patterns of the Greenland ice sheet. CISM has been integrated into version 2.0 of the Community Earth System Model, where it is being used for Greenland simulations under past, present, and future climates. The code is open-source with extensive documentation and remains under active development.

Contemporary ice stream flow is directly linked to conditions at the ice/bed interface, yet this environment is logistically difficult to access. Instead, we investigate subglacial processes important for ice stream flow by studying tills on the deglaciated Antarctic continental shelf. We test currently-accepted hypotheses surrounding subglacial processes and till properties with a Ross Sea dataset. Till shear strengths indicate a continuum of simultaneous processes acting at the bed, rather than discrete ‘deformation’ and ‘lodgement’ end-members. We identify a threshold water content representing saturated pore spaces, leading to basal sliding and meltwater channelization. Based on observations of till properties relative to glacial landforms, we challenge the assumption that low shear strength is linked to intense deformation. Spatial variability in landform morphology reflects variability in deforming processes at the sub-ice stream scale and suggests a maximum deforming bed thickness of 2 m at the grounding line. Regional till properties generally correlate with seafloor geology and deglacial history; the western Ross Sea is characterized by higher and more variable shear strengths and water contents, while lower-shear strength till was preserved in the Eastern Basin. These observations inform till interpretation and provide context for deforming beds beneath the modern ice sheet and on glaciated continental shelves.

Among the most important challenges faced by ice flow models is how to represent basal and rheological conditions, which are challenging to obtain from direct observations. A common practice is to use numerical inversions to calculate estimates for the unknown properties, but there are many possible methods and not one standardised approach. As such, every ice flow model has a unique initialisation procedure. Here we compare the outputs of inversions from three different ice flow models, each employing a variant of adjoint-based optimisation to calculate basal sliding coefficients and flow rate factors using the same observed surface velocities and ice thickness distribution. The region we focus on is the Amundsen Sea Embayment in West Antarctica, the subject of much investigation due to rapid changes in the area over recent decades. We find that our inversions produce similar distributions of basal sliding across all models, despite using different techniques, implying that the methods used are highly robust and represent the physical equations without much influence by individual model behaviours. Transferring the products of inversions between models results in time-dependent simulations displaying variability on the order of or lower than existing model intercomparisons. Focusing on contributions to sea level, the highest variability we find in simulations run in the same model with different inversion products is 32 %, over a 40-year period, a difference of 3.67 mm. There is potential for this to be improved with further standardisation of modelling processes, and the lowest variability within a single model is 13 %, or 1.82 mm over 40 years. While the successful transfer of inversion outputs from one model to another requires some extra effort and technical knowledge of the particular models involved, it is certainly possible and could indeed be useful for future intercomparison projects.

Ice streams are the arteries through which a large fraction of the ice lost from Antarctica is discharged. With the introduction of "higher order" mechanics, the representation of ice streams in ice sheet models appears to have become more robust, eliminating previously ubiquitous grid effects. The detailed processes that control ice stream formation --- and the minimal ingredients that a model requires to represent them faithfully --- remain incompletely explored. Here we focus on "pure" ice streams, not confined to topographic troughs. We study two mechanisms that can cause their formation through feedbacks between enhanced dissipation and faster sliding, and study the minimal model capable of reproducing both mechanisms. In the first mechanism, increased dissipation raises basal temperature before the melting point is reached, and subtemperate sliding is in turn facilitated by these higher temperatures, leading to yet more dissipation. This mechanism has received very limited attention in the literature, and is not fully incorporated in at least some commonly used ice sheet model. The second, better-studied mechanism involves basal effective pressure rather than temperature as the degree of freedom that creates a positive feedback: increased dissipation produces additional meltwater. Draining that excess water requires a lower effective pressure in typical "distributed" draiange ssytems. Reduced effective pressure in turn leads to faster sliding, and yet more dissipation. The two mechanisms are distinct and one can operate in the absence of the other, but both can cause the formation of ice streams whose trunks have very similar features. Using a novel, hybrid `shallow/"full Stokes" flow' model derived from first principles, we show how accelerated flow due to either feedback leads to advection of cold ice to the bed, and demonstrate that this is the key negative feedback that controls ice steam formation due to its role in cooling the bed. Downward advection occurs both along the axis of the incipient ice stream, and in the transverse plane. There, a significant secondary flow towards the ice stream centre develops, which is of equal importance to along-flow advection in controlling heat transport. Our model is unique in its ability to fully resolve that secondary flow while still using the "shallowness" of the flow to simplify computations of ice stream physics. The formation of ice streams can be understood as "spatial" instabilities in which small-scale structure is amplified in the downflow direction, for which we derive an analytical criterion. Our model self-consistently predicts the formation of a sharply-defined ice stream margin and very cold-bedded ice ridges over a relatively short downstream distance from the onset of patterning for both mechanisms. The model also shows how basal dissipation in the margin leads to appreciable stream widening in the downstream direction, while englacial dissipation in combination with advection can lead to a pronounced peak in basal water supply some distance inside the margins. We demonstrate additionally that the emergent patterns can be unstable in time, and identify the properties required of a model that can handle such temporal instabilities.

Microseismicity, induced by the sliding of a glacier over its bed, can be used to characterize frictional properties of the ice‐bed interface, which are a key parameter controlling ice stream flow. We use naturally occurring seismicity to monitor spatiotemporally varying bed properties at Rutford Ice Stream, West Antarctica. We locate 230000 micro‐earthquakes with local magnitudes from –2.0 to –0.3 using 90 days of recordings from a 35‐station seismic network located ∼40 km upstream of the grounding line. Events exclusively occur near the ice‐bed interface and indicate predominantly flow‐parallel stick‐slip. They mostly lie within a region of interpreted stiff till and along the likely stiffer part of mega‐scale glacial landforms. Within these regions, micro‐earthquakes occur in spatially (

Complex processes occur at the base of glaciers and ice sheets that influence their flow and hydrological behaviour and determine the landforms left behind by glaciation. This chapter considers mathematical models of basal sliding, subglacial water drainage and till deformation, and introduces various subglacial landforms due to the interaction of ice, water and sediment. The treatment extends to models of large-scale phenomena including glacier outburst floods and glacier surges.

Models describing natural phenomena can depend on parameters that cannot be directly measured, hence the necessity to develop inverse techniques to determine them. Our goal is to utilize such techniques to enable better initialization of ice sheet models for Antarctica. This will help such models to produce better forecasts as part of climate studies. The parameters of interest are the basal sliding coefficient, which characterizes the contact of the ice sheet with the bed underneath, and the diffusion coefficient which dictates the dynamics within the mass-continuity partial differential equation describing the movement of ice sheets. A Lyapunov based approach is proposed to control the convergence of the 1D and 2D inhomogeneous transport models toward a feasible equilibrium matching the measurements of surface topography of the Antarctic ice sheet. Our work offers a new 1D update law for the basal sliding coefficient inversion. We also use adaptive distributed parameter inversion to retrieve basal sliding from diffusion in 1D and 2D models. These two methods are tested on study cases and real data. Our results show that the methods proposed are successful in inverting for sliding and diffusion while replicating the available data.; Les modèles décrivant certains phénomènes naturels peuvent dépendre de paramètres non mesurables, d'où la nécessité de les estimer par méthodes inverses. Notre objectif est d'utiliser de telles techniques pour permettre une meilleure initialisation des modèles de simulations des calottes glaciaires en Antarctique. Cela permettra l'obtention de meilleures prévisions dans le cadre des études climatiques. Nous nous intéressons au paramètre de glissement basale qui caractérise le contact de la calotte glaciaire avec le socle rocheux. De même qu'au paramètre de diffusion qui dicte la dynamique au sein de l'équation différentielle partielle de continuité de masse décrivant son mouvement. Une approche basée sur la théorie de Lyapunov est proposée pour contrôler la convergence des modèles de transport inhomogènes 1D et 2D, vers un équilibre correspondant aux mesures de la topographie de surface de la calotte glaciaire de l'Antarctique. Notre travail propose une nouvelle loi pour l'inversion en 1D du coefficient de glissement basal. Nous utilisons également l'inversion adaptative de paramètres distribués pour récupérer le glissement basal depuis le paramètre de diffusion dans des modèles 1D et 2D. Ces deux méthodes sont testées sur des cas d'études et des données réelles. Nos résultats montrent que les méthodes proposées réussissent à inverser les paramètres de glissement et de diffusion tout en reproduisant les données disponibles.

Deep-seated landslide is one of most catastrophic and disastrous geohazards. Probing the spatial extent and basal sliding interface of the deep-seated landslide is not only particularly critical for understanding landslide size (i.e., volume and collapsed area), but also crucial for landslide hazard assessment. The conventional investigations such as the borehole drilling and seismic profiles are usually challenging for investigating landslide body comprehensively in space due to the expensive cost and the limitations of geophysical exploration. Recent studies of ambient seismic noise monitoring have provided an additional tool to monitor the subsurface medium in a non-invasive and relatively inexpensive way, which advances the investigating landslide geological structure. Here, we applied the ambient seismic noise monitoring technique to deep-seated landslide at Fanfan, Ilan area in northeastern Taiwan. The multiple geophysical, geotechnical and geodetic approaches including active multi-channel analysis of surface wave (MASW), real-time kinematic (RTK) measurement, campaign GPS, borehole time-domain reflectometer (TDR) and groundwater level (GWL) gauge are adopted during our monitoring period. A series of relation analysis found that the variations of frequency-dependent seismic velocity changes (dv/v), TDR sliding behavior, time series of groundwater level associated to two heavy rainfall episodes concurrently. With the available shear-wave velocity model (VS) derived from MASW, the depth range sensitive to different frequency band for surface wave can be certainly determined. Clear 3-5 Hz dv/v measurement at seismic station of V01 collocated with GWL gauge can be found with the largest reduction of ~ 1%, coinciding with 1 m GWL increasing. Models with different thickness layer (H), basal depth (d), Vs perturbation (dVs) were exercised, and a good fit between predicted spectral dv/v and the frequency-dependent dv/v measurements at seismic station V02 with H = 0.5 m, d = 21 m and dVs = 0.5. TDR measurement showed the obvious sliding signals is consistent with the shear zones identified by borehole log with the depth ranging from 48 to 50 m. These results demonstrate that multidisciplinary perspectives are needed to increase a better understanding of landslide structure. Consequently, a model linking variations of dv/v and TDR measurements is proposed to better understand sliding characteristics, which could potentially toward failure prediction of deep-seated landslide.

This paper presents macro and microsedimentological data recording subglacial deformation of a sediment succession at Przeszkoda in north-central Poland under the Vistula Ice Stream at the southern fringe of the Scandinavian Ice Sheet during the Weichselian glaciation. The sediment succession consists of (i) undeformed outwash sand, (ii) deformed outwash sand, (iii) bedded till, and (iv) massive till. These deposits were examined in a closely sampled vertical profile for kinematic structures, macrofabrics, grain shape characteristics, and micromorphology (detrital components, microstructures, clast microfabrics). The analyzed parameters suggest that the deposits were generated by a progressive homogenization and mixing of local material with far-travelled detritus successively released from the active ice sole. Grain crushing and abrasion characteristics lack any obvious trends in the vertical profile, indicating that the deformation was focused in a thin (cm-scale) layer which progressively migrated upwards parallel with sediment accretion. The closely spaced thin sections were collected in the deformed outwash sand, bedded till and massive till units. These revealed formation and evolution of stress-induced foliation in the glacier bed whereby three microfabric generations including S1 (earliest), S2 and S3 (latest) foliation were recognized. The geometry of these microfabrics is consistent with their formation in response to the development of subhorizontal Y-type shears, and up-ice dipping P-type and down-ice dipping R-type Riedel shears within the subglacial shear zone. The geometry and cross-cutting relationships between the microfabrics throughout the succession reflects changing deformation styles within a thin deforming bed that migrated time-transgressively up the profile during till accretion. Strain magnitude accumulated in the whole sediment succession was low, in the range of 101 to 102, suggesting that enhanced basal sliding was an important component contributing to fast flow of the ice stream.

Water flowing below glaciers exerts a major control on glacier basal sliding. However, our knowledge of the physics of subglacial hydrology and its link with sliding is limited because of lacking observations. Here we use a 2-year-long dataset made of on-ice-measured seismic and in situ-measured glacier basal sliding speed on Glacier d'Argentière (French Alps) to investigate the physics of subglacial channels and its potential link with glacier basal sliding. Using dedicated theory and concomitant measurements of water discharge, we quantify temporal changes in channels' hydraulic radius and hydraulic pressure gradient. At seasonal timescales we find that hydraulic radius and hydraulic pressure gradient respectively exhibit a 2- and 6-fold increase from spring to summer, followed by comparable decrease towards autumn. At low discharge during the early and late melt season channels respond to changes in discharge mainly through changes in hydraulic radius, a regime that is consistent with predictions of channels' behaviour at equilibrium. In contrast, at high discharge and high short-term water-supply variability (summertime), channels undergo strong changes in hydraulic pressure gradient, a behaviour that is consistent with channels behaving out of equilibrium. This out-of-equilibrium regime is further supported by observations at the diurnal scale, which prove that channels pressurize in the morning and depressurize in the afternoon. During summer we also observe high and sustained basal sliding speed, which supports that the widespread inefficient drainage system (cavities) is likely pressurized concomitantly with the channel system. We propose that pressurized channels help sustain high pressure in cavities (and therefore high glacier sliding speed) through an efficient hydraulic connection between the two systems. The present findings provide an essential basis for testing the physics represented in subglacial hydrology and glacier sliding models.

The Viedma glacier is the largest glacier in Argentina and one of the most dynamic in the Southern Patagonian Icefield. It is a strategic water source. This study analyzes the interactions of glaciological variables that help better explain the behavior of the Viedma glacier. This paper presents the study of surface flow speed and ice surface temperatures during the 2015–2016 warm season by using remote sensing data and includes the statistical correlation between those variables. In addition, a reconstruction of the front fluctuation between 1979 and 2016 through CORONA and LANDSAT images is included. Finally, the displacement modes dominating the Viedma glacier are suggested. Our results showed that the mean surface flow speed during the study period was of around 1.2 ± 0.3 md−1 and reached maximum values of 5.5 ± 0.3 md−1 and 3 ± 0.3 md−1 at middle basin and terminus respectively. Thermal data revealed minimum values of −2 °C in the uppermost section of the study area, whilst the glacier's terminus showed supraglacial melt (values above pressure melting point). Front fluctuation analysis revealed the largest front retraction of the last 40 years between 2010 and 2016: around 281 my−1. Statistical correlation analysis and glaciological data suggested that the middle and lower basins of the Viedma glacier are widely dominated by basal sliding. Fil: Lo Vecchio Repetto, Andrés. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Lenzano, María Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Durand, Jorge Marcelo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Lannutti, Esteban Damián. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Bruce, Roberto Haroldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Lenzano, Luis Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina

Water flowing below glaciers exerts a major control on glacier basal sliding speeds. However, our knowledge on the physics of subglacial hydrology and its link with sliding is limited by lacking observations. Here we use a two-year long dataset made of on-ice measured seismic and in-situ measured glacier basal sliding speed records on the Glacier d’Argentière (French Alps) to investigate the physics of subglacial channels and its potential link with glacier basal sliding. Using dedicated theory and concomitant measurements of water discharge, we quantify temporal changes in channels hydraulic radius and hydraulic pressure gradient. At seasonal timescales we observe, for the first time, that hydraulic radius and hydraulic pressure gradient present a four-fold increase from spring to summer, followed by a comparable decrease towards autumn. At low discharge during the early and late melt season channels respond to changes in discharge mainly through changes in hydraulic radius, a regime that is consistent with predictions of channels behaving at equilibrium. In contrast, at high discharge and high short-term water-supply variability (summertime), channels undergo strong changes in hydraulic pressure gradient, a behavior that is consistent with channels being out-of-equilibrium. This out-of-equilibrium regime is further supported by observations at the diurnal scale, which demonstrate that channels pressurize in the morning and depressurize in the afternoon. During summer we also observe high and sustained basal sliding speeds, supporting that the widespread inefficient drainage system (cavities) is likely pressurized concomitantly with the channel-system. We propose that pressurized channels help sustain high pressure in cavities (and therefore high glacier sliding speeds) through an efficient hydraulic connection between the two systems. Using the two regimes herein observed in channels seasonal-dynamics as constraints for subglacial hydrology/ice dynamics models may allow to strengthen our knowledge on the physics of subglacial processes.

Ice stream evolution is a major uncertainty in projections of the future of the Greenland and Antarctic Ice sheets. Accurate simulation of ice stream evolution requires an understanding of a number of “ingredients” that control the location and behaviour of ice stream flow. Here, we test the influence of geothermal heat flux, grid resolution, and bed hydrology on simulated ice streaming. The palaeo-record provides snapshots of ice stream evolution, with a particularly well constrained ice sheet being the British-Irish Ice Sheet (BIIS). We implement a new basal sliding scheme coupled with thermo-mechanics into the BISICLES ice sheet model, to simulate the evolution of the BIIS ice streams. We find that the simulated location and spacing of ice streams matches well with the empirical reconstructions of ice stream flow in terms of position and direction when simple bed hydrology is included. We show that the new basal sliding scheme allows the accurate simulation for the majority of BIIS ice streams. The extensive empirical record of the BIIS has allowed the testing of model inputs, and has helped demonstrate the skill of the ice sheet model in simulating the evolution of the location, spacing, and migration of ice streams through millennia. Simulated ice streams also prompt new empirical mapping of features indicative of streaming in the North Channel region. Ice sheet model development has allowed accurate simulation of the palaeo record, and allows for improved modelling of future ice stream behaviour.

The largest uncertainty in the ice sheet models used to predict future sea level rise originates from our limited understanding of processes at the ice/bed interface. Near glacier termini, where basal sliding controls ice flow, most predictive ice sheet models use a parameterization of sliding that has been theoretically derived for glacier flow over a hard bed. We find that this sliding relation does not apply to the 140 Greenland glaciers that we analyzed. There is no relationship between basal sliding and frictional stress at the glacier bed, contrary to theoretical predictions. There is a strong relationship between sliding speed and net pressure at the glacier bed. This latter finding is in agreement with earlier observations of mountain glaciers that have been largely overlooked by the glaciological community.

In times of warming in polar regions, the prediction of ice sheet discharge is of utmost importance to society, because of its impact on sea level rise. In simulations the flow rate of ice is usually implemented as proportional to the differential stress to the power of the exponent n=3. This exponent influences the softness of the modeled ice, as higher values would produce faster flow under equal stress. We show that the stress exponent, which best fits the observed state of the Greenland Ice Sheet, equals n=4. Our results, which are not dependent on a possible basal sliding component of flow, indicate that most of the interior northern ice sheet is currently frozen to bedrock, except for the large ice streams and marginal ice. Ice in the polar ice sheets flows towards the oceans under its own weight. Knowing how fast the ice flows is of crucial importance to predict future sea level rise. The flow has two components: (1) internal shearing flow of ice and (2) basal motion, which is sliding along the base of ice sheets, especially when the ice melts at this base. To determine the first component we need to know how "soft" the ice is. By considering the flow velocities at the surface of the northern Greenland Ice Sheet and calculating the stresses that cause the flow, we determined that the ice is effectively softer than is usually assumed. Previous studies indicated that the base of the ice is thawed in large parts (up to about 50%) of the Greenland Ice Sheet. Our study shows that that is probably overestimated, because these studies assumed ice to be harder than it actually is. Our new assessment reduces the area with basal motion and thus melting to about 6-13% in the Greenland study area.

The Whillans Ice Plain (WIP) is unique among Antarctic ice streams because it moves by stick-slip. The conditions allowing stick-slip and its importance in controlling ice dynamics remain uncertain. Local basal seismicity previously observed during unstable slip is a clue to the mechanism of ice stream stick-slip and a window into current basal conditions, but the spatial extent and importance of this basal seismicity are unknown. We analyze data from a 2010–2011 ice-plain-wide seismic and GPS network to show that basal micro-seismicity correlates with large-scale patterns in ice stream slip behavior: Basal seismicity is common where the ice moves the least between unstable slip events, with small discrete basal micro-earthquakes happening within 10s of km of the central stick-slip nucleation area and emergent basal tremor occurring downstream of this area. Basal seismicity is largely absent in surrounding areas, where inter-slip creep rates are high. The large seismically active area suggests that a frictional sliding law that can accommodate stick-slip may be appropriate for ice stream beds on regional scales. Variability in seismic behavior over inter-station distances of 1–10 km indicates heterogeneity in local bed conditions and frictional complexity. WIP unstable slips may nucleate when stick-slip basal earthquake patches fail over a large area. We present a conceptual model in which basal seismicity results from slip-weakening frictional failure of over-consolidated till as it is eroded and mobilized into deforming till.

Long-term kinematic research of slowmoving debris slide is rare despite of the widespread global distribution of this kind. This paper presents a study of the kinematics and mechanism of the Jinpingzi debris slide located on the Jinsha river bank in southwest China. This debris slide is known to have a volume of 27×106 m3 in active state for at least one century. Field survey and geotechnical investigation were carried out to define the structure of the landslide. The physical and mechanical properties of the landslide materials were obtained by in-situ and laboratory tests. Additionally, surface and subsurface displacements, as well as groundwater level fluctuations, were monitored since 2005. Movement features, especially the response of the landslide movement to rainfall, were analysed. Relationships between resisting forces and driving forces were analysed by using the limit equilibrium method assuming rigid-plastic frictional slip. The results confirmed a viscous component in the long-term continuous movement resulting in the quasioverconsolidated state of the slip zone with higher strength parameters than some other types of slowmoving landslides. Both surface and subsurface displacements showed an advancing pattern by the straight outwardly inclined (rather than gently or reversely inclined) slip zone, which resulted in low resistance to the entire sliding mass. The average surface displacement rate from 2005 to 2016 was estimated to be 0.19~0.87 mm/d. Basal sliding on the silty clay seam accounted for most of the deformation with different degrees of internal deformation in different parts. Rainfall was the predominant factor affecting the kinematics of Jinpingzi landslide while the role of groundwater level, though positive, was not significant. The response of the groundwater level to rainfall infiltration was not apparent. Unlike some shallow slow-moving earth flows or mudslides, whose behaviors are directly related to the phreatic groundwater level, the mechanism for Jinpingzi landslide kinematics is more likely related to the changing weight of the sliding mass and the downslope seepage pressure in the shallow soil mass resulting from rainfall events.

Fast ice flow and substantial surface runoff form distinct and extensive supraglacial rivers and lakes on the northeast Greenland Ice Sheet (GrIS). These supraglacial rivers and lakes play an important role in controlling surface meltwater routing and storage. However, the hydromorphology of these complex supraglacial features remains poorly quantified. In this study, we mapped the multi-temporal supraglacial rivers and lakes on the northeast GrIS (area ~ 3 × 104 km2) using sixty-five 10 m Sentinel-2 multispectral satellite images acquired during the 2017 summer, and quantified their primary hydromorphology, including meltwater area fraction, meltwater volume, supraglacial river length, river sinuosity, river depth, supraglacial lake area, lake shape, and lake depth. The quantified hydromorphology was compared with variable ice flow regimes and regional climate model (RCM) runoff. The results indicate that widespread supraglacial rivers and lakes maximally cover 5.3% of the northeast GrIS, which is more than double the previous estimates and only slightly lower than that for the strongly melting southwest GrIS (~7.0%). As the surface runoff increases and the snowline retreats, supraglacial rivers and lakes migrate from the ice sheet margin (~400 m) to high elevations (~1400 m). The satellite-mapped surface meltwater area fraction for the entire study area is positively correlated with the RCM surface runoff before the timing of peak melt (August 1), but decreases relatively slowly (by 32–45%) compared to the rapidly decreasing runoff (by 84–96%) in August. Large elevational variations were found for the peak magnitude and timing of the meltwater extent and volume. High (800–1400 m) elevation bands achieve peak values over two weeks later than low (400–800 m) elevation bands, and the 800–1000 m elevation band yields maximum peak values due to the wide distribution of supraglacial rivers. Furthermore, the supraglacial drainage patterns vary with different ice flow regimes. There are long rivers and few lakes in areas where ice deformation controls the ice flow velocity while there are short rivers and large, narrow lakes in areas where basal sliding is a major component of the ice flow. Meanwhile, long, sinuous rivers and small lakes form on the floating ice. In general, this study conducts a preliminary investigation of supraglacial rivers and lakes on the poorly-studied northeast GrIS and reveals that surface melt and ice flow control the hydromorphology of supraglacial rivers and lakes.

Hydraulic fracture (or hydrofracture) can promote the propagation of meltwater-filled surface crevasses in glaciers and, in some cases, lead to full-depth penetration that can enhance basal sliding and iceberg calving. Here, we propose a novel poro-damage phase field model for hydrofracturing of glacier crevasses, wherein the crevasse is represented by a nonlocal damage zone and the effect of hydrostatic pressure due to surface meltwater is incorporated based on Biot’s poroelasticity theory. We find that the elastic strain energy decomposition scheme of Lo et al. (2019) with an appropriate fracture energy threshold can adequately represent the asymmetric tensile–compressive fracture behavior of glacier ice subjected to self-gravity loading. We assessed the performance of the model against analytical linear elastic fracture mechanics solutions by comparing their predictions of maximum crevasse penetration depth. The model simulates both surface crevasse propagation in the interior region of the glacier, as well as cliff failure in the terminus region. The excellent performance of the proposed model for air/water-filled surface crevasses in idealized land- and marine-terminating grounded glaciers illustrates its applicability to studying the dynamic response of glaciers to atmospheric warming.