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Spatial analysis and controlling factors of landslides in East Icelandic fjords

Analyse spatiale et facteurs de contrôle des glissements de terrain dans les fjords de l'est de l'Islande
Emilie Portier, Denis Mercier et Armelle Decaulne

Résumés

Les fjords de l’est de l’Islande concentrent de nombreux glissements de terrain. Ces phénomènes visibles par observation à distance d’images satellites et d’un Modèle Numérique de Terrain (MNT) ont été inventoriés dans cette étude, puis mesurés et analysés. Au total, 290 glissements de terrain ont été recensés au sein d’une base de données sous forme de tableur et dans un Système d’Informations Géographiques (SIG). Pour chaque glissement de terrain ont été relevées sa localisation, sa morphométrie (longueur, largeur, superficie, épaisseur, volume estimé, etc.), ainsi que de potentielles variables de contrôle notamment géologiques (lithologie, pendage), ou explicatives (orientation, âge de la déglaciation du versant affecté par le glissement de terrain). Ces variables et leur répartition ont été étudiées par analyse spatiale et statistique. Cette étude met en avant une plus forte densité de glissements de terrain dans le nord de la zone étudiée, qui pourrait s’expliquer par les directions d’écoulement de la calotte glaciaire passée, qui auraient renforcé la pression exercée sur les versants entraînant le phénomène de décompression postglaciaire. Un autre constat est une surreprésentation de glissements orientés vers l’ouest et le sud, amenant l’hypothèse d’un contrôle climatique : les versants davantage ensoleillés auraient subi une déglaciation plus rapide favorisant l’instabilité des versants. Les données recueillies suggèrent également que la lithologie (basaltes tertiaires) est une variable de contrôle et que la période pendant laquelle la zone de glissement de terrain est déglacée est une variable explicative de l'initiation du glissement de terrain. Les glissements de terrain observés s’inscriraient ainsi dans une dynamique paraglaciaire d’instabilité des versants après leur déglaciation.

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Notes de la rédaction

Manuscrit reçu le 14 novembre 2022, reçu dans sa forme révisée le 4 juillet 2023, définitivement accepté le 2 octobre 2023

Texte intégral

This research is part of the
"PAraglacial Rock-slope failures dating in Iceland" programme
(2022-2026) funded by the French Polar Institute Paul-Emile Victor
(IPEV).

1. Introduction

1Landslides are a common sight in Iceland. However, scientific knowledge and studies of these phenomena remain limited in relation to the potential they represent. The numerous landslides in Iceland have been inventoried at the island scale, as well as at more local scales. Jónsson (1957, 1976) published the first Iceland-wide inventory, which was later reworked by Whalley et al. (1983). An inventory has already been carried out in in the Skagafjörður area in northern Iceland with 158 landslides identified (Mercier et al., 2013, 2017; Cossart et al. 2014; Feuillet et al., 2014), the Westfjords with 186 landslides recognised (Peras et al., 2016), and in the East (170 identified landslides) and North Fjords with 390 landslides known (Youinou, 2020). Nevertheless, these inventories are incomplete and the digital data now available will enable us to compile the most exhaustive database possible for the fjords of East Iceland in this article. The geographical interest of these inventories is to seek explanations for the spatial distribution and triggering of landslides. Still, landslides, when they no longer pose a risk to current populations and infrastructure, remain geomorphosites of interest, informing in particular about past and present climate changes (Morino et al, 2022).

2The term landslide is defined for this study as the movement of a mass of rock or debris along a slope, organized according to a rupture zone characterized by a scar, and an accumulation zone including the material displaced during the slide (Cruden and Varnes, 1996). The landslides inventoried in Iceland are essentially of two types: rotational and translational (Cruden, 1991; Cruden and Varnes, 1996; Martins-Campina, 2005; Lopez, 2006; Hungr et al., 2014).

3Several hypotheses have already been put forward. For instance, Jónsson in 1957 suggested that landslides are triggered during deglaciation periods following the disappearance of the ice cap, and are concentrated in areas where the bedrock is made up of Tertiary lava flows. In the northern Skagafjörður region, central north Iceland, Mercier et al. (2013, 2017), Decaulne et al. (2016), Coquin et al. (2016), and Cossart et al. (2017) carried out dating revealing that the majority of landslides occurred between 13,000 and 5,000 BP, which corresponds to the deglaciation and postglacial periods in Iceland.

4At another scale, the question of the relationship between deglaciation and rock-slope failures is still in progress in several deglaciated mountains areas, in the Alps (Blondeau et al., 2021; Fioraso, 2017; Crosta et al., 2013), in British Columbia (Holm et al., 2004), in British Mountains (Wilson, 2005; Wilson and Smith, 2006; Jarman and Harrison, 2019), in Ireland (Ballantyne et al., 2013; Wilson, 2017), in central Italy (Guzzetti et al., 2008), in New Zealand (Korup, 2005); in Norway (Braathen et al., 2004; Blikra et al., 2006; Böhme et al., 2011; Curry, 2020), in Pakistan (Hewitt, 2009); in Pyrenees (Jarman et al., 2014), in Romania (Bǎlteanu et al., 2010; Gunnell et al., 2022), in Scotland (Sellier and Lawson, 1998; Ballantyne et al., 1998, 2009, 2014, 2018, 2021; Jarman, 2006; Cave and Ballantyne, 2016).

5The creation of inventories to record landslides is a work widely considered for its various interests (knowledge of the phenomena and associated risks, study of locations, occurrences, triggering factors, etc.) (Guzetti et al., 2012; Van Den Eeckhaut et al, 2012). Thus, the ambition of this work is (i) to create a new landslide database at the scale of the East Icelandic fjords; (ii) to carry out morphometric measurements, geological characterization and dating estimations on each of these phenomena, in order to collect and compile a multitude of data on landslides at the scale of the East Icelandic fjords; (iii) the challenge is also to identify potential controlling factors based on a set of explanatory variables.

2. Study area

6The study area lies between 65°79'N - 64°24'N and 15°W - 13°49'W, from Vopnafjörður to the Vatnajökull ice cap (Fig. 1), and corresponds to the Eastern Icelandic fjords, an area of about 20,000 km². It is delimited by Tertiary basaltic rocks about 3.5 million years old. The geology of the study area corresponds to tholeiitic, porphyritic and olivine basalts. Cut into fjords, the plateaux of eastern Iceland rise to an average altitude of 1,000 m. The slopes are steep and uneven.

Fig. 1 - Location of the inventoried Eastern Fjords.
Fig. 1 – Localisation des fjords de l’Est inventoriés.

Fig. 1 - Location of the inventoried Eastern Fjords.Fig. 1 – Localisation des fjords de l’Est inventoriés.

This area of the Eastern Fjords has been largely shaped by successive glaciations and deglaciations. The chronology of Iceland's deglaciation used in this study is based on the studies of Geirsdóttir et al. (2007) and Pétursson et al., (2015): the area entirely glaciated during the Last Glacial Maximum (LGM), is characterized by a period of gradual deglaciation until around 14,000 BP. Then deglaciation occurs inland, and some valleys are progressively deglaciated, following a period of glacial readvance during the Recent Dryas (11,700 BP). Then, during the Preboreal (10,000 BP), only the highlands were glaciated. This was followed by a gradual retreat of the ice-covered area, until today.
Cette zone des fjords orientaux a été largement façonnée par des glaciations et déglaciations successives. La chronologie de la déglaciation de l'Islande utilisée dans cette étude est basée sur les travaux de Geirsdóttir et al., (2007) et Pétursson et al., (2015) : la région entièrement englacée pendant le dernier maximum glaciaire (LGM), est caractérisée par une période de déglaciation progressive jusqu'à environ 14 000 BP. Ensuite, la déglaciation se produit à l'intérieur des terres, et certaines vallées sont progressivement désenglacées à la suite d'une reprise glaciaire pendant le Dryas récent (11 700 BP). Ensuite, pendant le Préboréal (10 000 BP), seuls les hauts plateaux ont été englacés. Cette période a été suivie d'un retrait progressif de la zone couverte de glace, jusqu'à aujourd'hui.

3. Data and methods

3.1. Landslide inventory

3.1.1. Data

7Two sources of images were mainly used for this work. The aerial photographs available via the geographic national organism Loftmyndir ehf (https://map.is/​base/​) (Fig. 2A), and the SENTINEL 2A images aquired from the CNES database (pep's). GEBCO bathymetric data were used for the reconstruction of sea levels according to the deglaciation chronology. On the other hand, the 10x10 m Digital Elevation Model (DEM) from the cooperation of the National Land Survey of Iceland, the Icelandic Meteorological Office and the Polar Geospatial Center was used (Fig. 2B).

Fig. 2 - Summary of data used for rock-slope failure deposits inventory.
Fig. 2 - Synthèse des données utilisées pour l’inventaire des glissements de terrain.

Fig. 2 - Summary of data used for rock-slope failure deposits inventory.Fig. 2 - Synthèse des données utilisées pour l’inventaire des glissements de terrain.

A: Available data used for the creation of the database; B: Resolution of the DEM used (Landmælingar Íslands) and processing carried out.
A : Données disponibles utilisées pour la réalisation de la base de données ; B : Précision du MNT utilisé (Landmælingar Íslands) et traitement réalisés.

3.1.2. Inventory method

8Landslides were delineated by geomorphological interpretation based on the Varnes (1978) classification. By aggregating them, each observed phenomenon assumed to correspond to a landslide is first located in a GIS as a vector layer. The observed shapes were then delineated (Fig. 3A). The SENTINEL 2A images were converted into NDVI bands in ESA's SNAP software in order to color them according to the value of the vegetation index (Fig. 3B). This manipulation enables to highlight the contours of the landslide deposits, and is an aid to decision making on landslide boundaries. Finally, previously determined criteria and variables were entered for each phenomenon: morphometry (length, width, surface area, altitudes, etc.), geology (lithology, dip), estimates of displaced volumes, chronology, orientation, etc.

3.1.3. Analysis of the spatial distribution of the inventory: Kernel Density Estimation

9Once the landslides have been inventoried, in order to study the distribution of landslides in Eastern Iceland and to highlight possible clusters or on the contrary a natural distribution of landslides, the closest events of the point cluster were grouped to characterize a density weight. In statistics, this corresponds to the kernel estimation method, also called Kernel Density Estimation (KDE). The calculations are performed in QGIS from the landslide inventory of the study area. Given the large number of individuals studied, estimating the density is a good method to summarize the distribution and pattern of landslides at the regional scale. This method enables, for physical phenomena such as landslides, to show concentration points which can be revealing and feed hypotheses.

3.2. Morphometric measurements and analysis

3.2.1. Morphometric measurement methods

10The measurements to be taken are indicated manually in the GIS, then the expected values are obtained automatically (Fig. 3C). Thus, for length measurements, two points are placed manually, one at the level of the scar and one at the distal elevation of the deposit for the total length of the landslide; one at the apex elevation of the deposit and one at the distal elevation of the deposit for the length of the deposit. The same applies to width and area measurements. The attribute table of the layers created are then automatically filled in for the desired measurements. Then, additional variables are calculated: such as different average (eq. 1) or tear-off area (eq. 2).

11The elevations are recorded in three steps: a point at the highest location of the landslide scar, a point at the highest location of the deposit and a point at the lowest location of the deposit. The orientation of the landslides with respect to the north is calculated by measuring the angle value of the vector layers created for the landslide lengths. The total altitude difference (eq. 3), the slope difference of the tear-out scar (eq. 4), the slope difference of the deposit (eq. 5), and finally a slope gradient (eq. 6) which enables knowing the inclination of the slope with respect to the horizontal are calculated using the following formulas:

12The collection of these data enables to estimate the distance travelled by the landslide in order to highlight the average runout distance and the exceptional events (Fig. 3D). This distance is calculated by studying the horizontal and vertical ratio of the phenomenon (eq. 7). This involves dividing the difference in height travelled by the horizontal length travelled, taking into account the tangent in the 32° angle from the summit altitude of the tear-out scar (Hsü, 1975, 1978). This information on the distance travelled by the displaced material has been translated into an index that highlights the relationship between horizontal and vertical distances travelled.

Fig. 3 - Landslide delineation and morphometric parameters extraction.
Fig. 3 - Méthode de localisation et d’extraction des paramètres morphométriques des glissements de terrain.

Fig. 3 - Landslide delineation and morphometric parameters extraction.Fig. 3 - Méthode de localisation et d’extraction des paramètres morphométriques des glissements de terrain.

A: Method for locating and delineating a landslide; B: Location of a landslide based on NDVI processing of SENTINEL 2 images; C: Location and method of measurements taken for landslides; D: Topographic section of the Haugur landslide, in the Smjörfjöll massif. 1. Landslide delineation; 2. Deposit limits observed; 3. Landslide area (m²); 4. Deposit area (m²); 5. Landslide length (in meters); 6. Deposit length (in meters); 7. Average landslide scar width (in meters); 8. Average width at the top of the deposit (in meters); 9. Average width at the base of the deposit (in meters); 10. Altitude at the top landslide scar (in meters); 11. Altitude at the top of the deposit (in meters); 12. Altitude at the base of the deposit (in meters).
A : Méthode de localisation et délimitation d’un glissement de terrain ; B : Localisation d'un glissement de terrain d'après un traitement NDVI d'images SENTINEL 2 ; C : Localisation et méthode des mesures réalisées pour les glissements de terrain ; D : Coupe topographique du glissement de terrain Haugur, dans le massif du Smjörfjöll. 1. Délimitation du glissement de terrain ; 2. Limites observées du dépôt ; 3. Superficie du glissement de terrain (m²) ; 4. Superficie du dépôt (m²) ; 5. Longueur du glissement (en m) ; 6. Longueur du dépôt (en m) ; 7. Largeur moyenne de la cicatrice d’arrachement (en m) ; 8. Largeur moyenne au sommet du dépôt (en m) ; 9. Largeur moyenne à la base du dépôt (en m) ; 10. Altitude de la cicatrice d’arrachement (en m) ; 11. Altitude du sommet du dépôt (en m) ; 12. Altitude de la base du dépôt (en m).

3.2.2. Estimation of displaced volumes

13The planar morphometric measurements were used to estimate volumes: estimate of the volumes displaced and deposited by the landslides was carried out (eq. 8 and 9). The equation proposed by Cossart et al. (2017), enabling the quantification of sediment transfers at the scale of a watershed for landslides is as follows:

14where L = length of the deposit (m), W = mean width of the deposit (m), T = mean thickness observed along the mountain slope (m), Tmax = maximal estimated thickness at the toe of the deposit (m), Tmin = apparent thickness at the toe of the deposit (m).

15The thickness data of the landslide deposits used were estimated from measurement carried out from the DEM, and compared to the research of Decaulne et al. (2016), according to which the landslide studied in Vatn, Iceland, has a thickness at its end of about 15 to 20 meters. By comparison with this estimate, and measurements taken from the DEM, the thickness information retained in the database has been generalized as follows: estimated average thickness of the deposit of 15 m, estimated maximum thickness of the deposit of 30 m and apparent thickness of the deposit of 10 m. To complete these estimates, calculations of the swelling factor were carried out, reported by Cruden and Varnes (1996), based in particular on the work of Church (1981). This swelling factor of the material moved consists in estimating the increase in the volume of the material during and after its displacement. There are different ways of estimating the swelling factor, for example by taking into account a fixed slip percentage of 67% (Church, 1981), which corresponds to an average taken from data on the displacement of solid rock, or 33% (Nicoletti and Sorriso-Valvo, 1991). These percentages give an overview of the phenomenon of swelling but their reliability is relative, if only because of the difference between these two values proposed. Cruden and Varnes (1996) thus propose a formula for estimating material swelling in each case (eq. 10):

16where Dr = Depth of rupture surface, Wr = Width of rupture surface, Lr = Lenght of rupture surface, Dd = depth of displaced mass, Wd = Width of displaced mass, Ld = Lenght of displaced mass

17After applying the formulas, in particular taking into account the swelling factor of the material moved, a value was estimated for each landslide, considering an average between the minimum volume of the landslide deposit according to Cossart et al. (2017) and the maximum volume removed based on the deposition volume swelling factor.

3.3. Identification of the landslides controlling factors

3.3.1. Relating landslides to deglaciation information of affected slopes

18A chronology of the occurrence of landslides has been established. They are based on the common assumption that the landslides studied in the inventory occurred after the Last Glacial Maximum (LGM), and that the deposits have not been reworked by a new glaciation (Benn and Evans, 2010). The potential timing of occurrence of landslides was therefore estimated using the same method as for the inventory of Peras et al. (2016): a potential maximum age of a landslide can be estimated by examining the deglaciation history of the region. This deglaciation history of the East Iceland fjords is proposed by Geirsdóttir (2007).

19The delineations of glacial maxima according to the periods studied are thus highlighted by intersecting landslide locations and ice sheet evolution: each identified landslide is classified into a timeframe. Indeed, by taking into account the extent of the ice cap, it is possible to estimate when a slope was deglaciated, and consequently the earlier boundary of the timeframe on which a landslide may have occurred over the post-glacial maxima period.

3.3.2. Geological characterization of landslides

20To obtain the lithology information, the geological maps made available online by the Iceland GeoSurvey institute (ISOR, https://arcgisserver.isor.is/​) were imported by georeferencing screenshots into the GIS including the landslide locations, in order to determine the nature of the rocks in which they occur.

21In order to determine the dip of the landslides, the maps made by Náttúrufræðistofnun Íslands and available online (https://jardfraedikort.ni.is/​), were consulted to identify the dip orientation in the tear-out scar of each landslide. Three modalities are presented: landslide non-compliant, compliant or perpendicular to the dip.

3.3.3. Statistical analysis and chi² test

22The database characterizes each landslide, which constitutes a geographical unit, by indicators and characteristics. Descriptive analyses and statistics of the inventory enable to summarize the different series of morphological information.

23The statistical analysis of data enables, in particular, the study of the dependence between two variables using a chi² test. In order to carry out this test, the method followed is that developed by Feuillet et al. (2014) for the study of the predisposing factors of landslides in Northern Iceland. Within a GIS, a mesh is made at the scale of one, here the test was made for the Smjörfjöll massif in the north of the study area. Each mesh is fixed at 250 m by 250 m, and alluvial areas and lakes are excluded from the mesh: in total, 8,837 cells of the mesh have been exploited. For each grid cell, information is given on whether it is a stable or unstable area, i.e., whether it is affected by a landslide or not, as well as information on its lithology, and then an estimate of its deglaciation age. Then, in an Excel spreadsheet, the chi² test method is applied, in the form of contingency tables: observed population / expected population. The method of selecting the grids can affect the results, which is why this conclusion must be qualified. This mesh selects more stable pixels than unstable ones, the selection proportions are unequal. On the other hand, it must be taken into account that part of the massif was reglaciated in the Preboreal period, and that the landslides in the heart of the massif have been identified as posterior to the landslides in the periphery of the massif. Finally, this test is performed on pixel units, and not on landslide units, which modifies the reading of the results, since data on landslide area are taken into account.

4. Results

4.1. Landslide inventory

24We propose here a database of 290 landslides (Fig. 4), spread heterogeneously across the survey area. The database is a GIS spatial database containing a total of 33 variables and criteria for each landslide.

Fig. 4 - Landslide inventory of the East Fjords of Iceland.
Fig. 4 - Inventaire des glissements de terrain des fjords de l'Est de l'Islande.

Fig. 4 - Landslide inventory of the East Fjords of Iceland.Fig. 4 - Inventaire des glissements de terrain des fjords de l'Est de l'Islande.

1. Landslide; 2. Current ice cap.
1. Glissement de terrain ; 2. Calotte glaciaire actuelle.

25The landslide density map shows areas of high landslide concentration and areas of voids (Fig. 5). Two main clusters are observed in the northernmost massifs: the Smjörfjöll massif has up to 15 landslides for a 600 km², and the second focus has up to 17 landslides for the same area. On the contrary, the South of the East Fjords has about ten landslides for a hundred km². Thus, the distribution is heterogeneous, leading to over-representation of landslides in some areas.

Fig. 5 - Density of listed landslides.
Fig. 5 - Densité des glissements de terrain répertoriés.

Fig. 5 - Density of listed landslides.Fig. 5 - Densité des glissements de terrain répertoriés.

4.2. Distributions and statistical analyses of morphometric data

26The database enables analyses to highlight possible trends or links between variables. The statistical treatment of the descriptive variables shows that the longest landslides reach 3,570 m, three times the average size calculated at 965 m, and that more than half of the landslides are less than one kilometer long (Fig. 6A-1). The shortest landslide is 171 m. Half of the landslide deposits are less than 625 m in length, with values widely dispersed. (Fig. 6A-2). The distribution of values is relatively similar to that of total lengths. About 80% of the different landslide widths are less than 710 m (Fig. 6A-3). However, the average widths are somewhat higher for deposits, implying that the material displaced by the landslide extends in width once it has been torn away (Fig. 6A-4). The landslides average a total area of 0.583 km², and the largest one covers 6.149 km² (Fig. 6A-5). The distribution of landslide deposit areas is similar to that of total areas (Fig. 6A-6). The estimated volumes displaced by landslides are for 86% of them less than 1,300,000 m3, and the largest would have mobilized 7,500,000 m3, almost six times more material than average. The distribution of landslides according to their surface area shows some foci of very large landslides, particularly in the north of the eastern fjords, especially in the Smjörfjöll massif (Fig. 6B). The travel distances were mapped for each slide to highlight their distribution (Fig. 6C).

27Finally, the elevation data collected highlight the fact that landslides where the apex of the deposits is between 300 and 500 meters of altitude are the majority, and the elevations of the landslide deposit bases are mostly very low, even reaching sea. Thus, more than half of the landslides have a difference in height of between 50 and 200 m.

Fig. 6 - Statistical and spatial distribution of different variables for the 290 landslides.
Fig. 6 - Distribution statistique et spatiale de différentes variables pour les 290 glissements de terrain.

Fig. 6 - Statistical and spatial distribution of different variables for the 290 landslides.Fig. 6 - Distribution statistique et spatiale de différentes variables pour les 290 glissements de terrain.

A: Univariate statistics of the variables: length of the landslide (1), length of the deposit (2), average width of the tearing scar (3), average width of the deposit according to their area (4), area (5), area of the deposit (6), displaced volume (7); B: Distribution of landslides according to their deposit area : 1. Deposit area (m²); C: Distribution of landslides according to the travel distances of the material moved: 1. Travel distance index (km).
A : Statistiques univariées des variables : longueur du glissement (1), longueur du dépôt (2), largeur moyenne de la cicatrice d'arrachement (3), largeur moyenne du dépôt d'après leur superficie (4), superficie (5), superficie du dépôt (6), volume déplacé (7) ; B : Représentation des glissements de terrain en fonction de leur superficie de dépôt : 1. Deposit area (m²) ; C : Représentation des glissements de terrain en fonction des distances de parcours du matériel déplacé : 1. Distance de parcours (en km).

4.3. Control factors

4.3.1. Orientation

28The distribution of landslides according to their orientation is heterogeneous (Fig. 7), so it is an interesting factor to study in order to look for factors playing a role in the triggering of landslides. A trend is emerging, with a predominance of landslides oriented between west and south: they represent 65% of the landslides. On the other hand, the orientation from northwest to southeast is under-represented (35%).

Fig. 7 - Landslide orientation.
Fig. 7 - Orientation des glissements de terrain.

Fig. 7 - Landslide orientation.Fig. 7 - Orientation des glissements de terrain.

4.3.2. Geology

29The geological characterization of each inventoried landslide gives the following distributions for the lithology of the landslide and the dip orientation of the affected slope. First, regarding lithology (Fig. 8A): 47% of Middle Miocene basalt, 28% of Late Miocene basalt, 24% of Pleiostocene silicic extrusives, and 1% of silicic intrusions. Next, regarding the dip (Fig. 8B): 39% of the landslides are compliant with the dip of the affected slope, while 23% of the landslides are counter-dip and 38% perpendicular.

30Carrying out a chi² test of dependence between the mesh stability and mesh lithology variables gives the following result: the two variables of lithology and slope stability are not independent, i.e., that is, they are associated (Fig. 8C). We can therefore deduce that the lithology plays a role in the stability of the slope.

Fig. 8 - Geological characterization of landslides based on lithology and dip information (modified from the geological maps from Jóhannesson and Sæmundsson, 1998).
Fig. 8 - Caractérisation géologique des glissements de terrain selon des informations de lithologie et de pendage (modifié de la carte géologique de Jóhannesson et Sæmundsson, 1989).

Fig. 8 - Geological characterization of landslides based on lithology and dip information (modified from the geological maps from Jóhannesson and Sæmundsson, 1998).Fig. 8 - Caractérisation géologique des glissements de terrain selon des informations de lithologie et de pendage (modifié de la carte géologique de Jóhannesson et Sæmundsson, 1989).

A: Estimation of the lithology of the landslides inventoried from the geological data of Iceland; B: Estimation of landslide dip at the pullout scar; C: Results of the chi² test for the lithology and slope stability variables. 1. Basaltic and Intermediate extrusive rocks and sediments. Middle Miocene, older than 11 Myr; 2. Silicic intrusions, rhyolite and granophyre; 3. Silicic extrusives, Tertiary and Pleistocene, older than 11,500 years; 4. Basaltic and Intermediate extrusive rocks and sediments. Late Miocene, 5.3-11 Myr; 5. Domain of central volcano; 6. Late Pleistocene and Lower Pleistocene bedrock, 0.8-3.3 Myr; 7. Upper Pleistocene bedrock, younger than 0.8 m.y.; 8. Upper Miocene and Lower Pliocene bedrock, 3.3-8.5 Myr; 9. Upper and Middle Miocene bedrock, 10-15 Myr; 10. Upper Miocene bedrock, 8.5-10 Myr; 11. Current ice surface; 12. Non-compliant dip landslide; 13. Compliant dip landslide; 14. Perpendicular dip landslide; 15. Dip direction.
A : Estimation de la lithologie des glissements de terrain inventoriés d'après les données géologiques de l'Islande ; B : Estimation du pendage des glissements de terrain à la cicatrice d'arrachement ; C : Résultats du test chi² pour les variables lithologie – stabilité du versant. 1. Roches et sédiments extrusifs basaltiques et intermédiaires. Miocène moyen, plus de 11 Ma ; 2. Intrusions siliciques, rhyolite et granophyre ; 3. Extrusives siliciques, Tertiaire et Pléistocène, plus de 11 500 ans ; 4. Roches et sédiments extrusifs basaltiques et intermédiaires. Miocène tardif, 5,3-11 Ma ; 5. Domaine du volcan central ; 6. Substrat rocheux du Pléistocène supérieur et du Pléistocène inférieur, 0,8-3,3 Ma ; 7. Socle rocheux du Pléistocène supérieur, inférieur à 0,8 Ma ; 8. Socle rocheux du Miocène supérieur et du Pliocène inférieur, 3,3-8,5 Ma ; 9. Socle rocheux du Miocène supérieur et moyen, 10-15 Ma ; 10. Socle rocheux du Miocène supérieur, 8,5-10 Ma ; 11. Calotte glaciaire actuelle ; 12. Glissement de terrain au pendage non conforme ; 13. Glissement de terrain au pendage conforme ; 14. Glissement de terrain au pendage perpendiculaire ; 15. Orientation du pendage.

4.3.3. Landslides and slope deglaciation

31The majority of landslides are located in areas that were free of ice in the early stages of deglaciation after the Last Glacial Maximum (Fig. 9A-B). A chi² test carried out on the Smjörfjöll massif only, this time for the variables “stability of the slope” and “relative period of deglaciation”, is also conclusive: the supposed period of deglaciation of the slope and its stability are two related variables (Fig. 9C). Deglaciation therefore has an impact on the stability of the slopes and therefore on the triggering of landslides. It should be taken into account that this test is based on pixels and not on landslides: thus, perhaps the oldest landslides are also the most extensive (thus counting for more pixels), which could explain a higher proportion of unstable pixels in the Younger Dryas compared to the Preboreal.

Fig. 9 - Relative ages of landslides according to deglaciation.
Fig. 9 - Âges relatifs des glissements de terrain par rapport à la déglaciation.

Fig. 9 - Relative ages of landslides according to deglaciation.Fig. 9 - Âges relatifs des glissements de terrain par rapport à la déglaciation.

A: Reconstruction of the chronology of occurrence of landslides since the Last Glacial Maximum considering the evolution of the ice cap and sea level; B: Maximum potential age of landslides inventoried according to the deglaciation chronology; C: Result of the chi² test for the age - stability values for Smjörfjöll massif (45 landslides).
A : Reconstitution de la chronologie d'apparition des glissements de terrain depuis le Dernier Maximum Glaciaire en prenant en compte l'évolution de la calotte glaciaire et le niveau de la mer ; B : Âge maximal potentiel des glissements de terrain inventoriés selon la chronologie de la déglaciation ; C : Résultats du test chi² pour les variables stabilité du versant et début de sa déglaciation (45 glissements de terrain).

5. Discussion

5.1. Landslides inventoried and control factors studied: predisposing factors?

32The dip characteristics of each landslide in the database enables the analysis of its relationship with landslide initiation. The parameter of whether or not the landslide conforms to the dip does not seem to be a conclusive element to explain the triggering of a landslide. Indeed, the results is similar to that obtained by Peras et al. (2016) for the inventory carried out in the Icelandic Westfjords: only 33% of the landslides recorded were compliant with the dip of the slope, for 25% contrary to the dip.

33Almost all landslides are located in Tertiary basalts, which corresponds well with previous observations by Whalley et al. (1983) based on the research of Jónsson (1957 and 1976), by Mercier et al. (2013) and Feuillet et al. (2014), at different scales. The hypothesis of the relationship between the lithology of the slope and the probability of landslide occurrence is widespread: in Romania (Bălteanu et al., 2010), in Sogn and Fjordane County in Norway where rock slope instability is located in relatively weak rocks units (Böhme et al., 2011), in the Upper Tiber River basin in central Italy (Guzzetti et al., 2007) where landslides appear more in dip-slopes and in soft and weak rocks crop out. In addition to this lithological predisposition hypothesis, there is also a lithological contact hypothesis: the contact between two different substrates could make the slopes more prone to landslide initiation (Peras et al., 2016). This hypothesis was put forward by Peras et al. (2016), based on a regional review of the Westfjords. It was found that many landslides were favored either by lithological contacts between basic and acidic bedrock due to volcanic activity, or by weak sedimentary horizons. However, Jarman (2019), while studying British mountains, states that geology is only a secondary control. The study proposes to discuss the common belief that 'geological structure has exercised a fundamental control' (Ballantyne, 1997). The hypothesis brought is that large landslide clusters appear more on over zones of deformed and fractured bedrock rather than defined basal surfaces (Braathen et al., 2004; Jarman and Wilson, 2015). Thus, geology influences the spatial distribution of landslides, but its role is not primary.

34Due to the close proximity of the ocean, the slopes are steep and uneven: on the scale of the Eastern Fjords, the slopes have an average gradient of 31%, for altitudes ranging from sea level to 1,200 m a.s.l. The topography can create a vacuum on the slopes, which can lead to landslides (Zhang et al., 2012; Kafle et al., 2020). The horizontal distance and the vertical drop ratio highlights landslides with above-average travel distances (index of 3 on average, with a range of 0.8 to 7.7): this information could be used to support some working hypotheses by cross-referencing this with data on lithology, dip, altitude, etc (Wong and Ho, 1996; Peruzzetto et al., 2020; Gao et al., 2021).

5.2. Morphometric data

35The database that has been compiled offers many opportunities for further research and analysis. Morphometric indices should be developed: indices of compactness, elongation, roughness, etc. It will then be possible to carry out typological classifications of landslides according to their different characteristics, or to study further the correlations between the different variables (Nicoletti et al, 1994; Quantin et al, 2004, Shafieiganjeh et al, 2022)

36The spatialized database of landslides in the fjords of eastern Iceland thus created offers considerable potential.

5.3. Control factors

5.3.1. Timing of landslides occurrence

37One of the starting hypotheses of this work is that the landslides studied are directly linked to deglaciation, and that they are therefore so-called paraglacial landslides, i.e., non-glacial processes that are nevertheless conditioned by glaciation (Church and Ryder, 1972; Ballantyne, 2002; Mercier, 2008; McColl, 2012). In Iceland, Jónsson put forward the hypothesis of the instability of rocky slopes linked to deglaciation in 1957.

38This hypothesis has been further developed in several research studies (Mercier et al., 2013 and 2017; Cossart et al., 2013, 2014 and 2017; Feuillet et al., 2014; Coquin et al., 2015, 2016 and 2019; Decaulne et al., 2016), particularly through dating tests (radiocarbon, tephrochronology, etc.). Some landslides have already been dated in Iceland, in the northern region (Mercier et al., 2013; Mercier et al. 2017; Decaulne et al. 2016; Coquin et al., 2016), revealing that they occurred between 13,000 and 5,000 BP.

39This hypothesis of landslide development at the beginning of the deglaciation has been the subject of numerous studies. In northwest Ireland, nine postglacial landslides have been dated using cosmogenic 10BE, and occurred within ~5000 years following ice-sheet deglaciation at ~17.4 ka (Ballantyne et al., 2013). In Scotland, most landslides occurred soon after deglaciation (Jarman, 2006: Ballantyne, 2019; 2021) and in North-West England, studies of landslides concluded that they all post-dated the Last Glacial Maximum (LGM; c.21±3 cal. ka BP) (Wilson, 2005; Wilson and Smith, 2006). The state of the art by Pánek (2016, 2019) mentions that 90% of landslides in the British Islands occurred within a few millennia after deglaciation (Ballantyne et al., 2014). In the Alps, postglacial decompression weakens the walls and favors the appearance of paraglacial damage to the slopes (Grämiger et al., 2017).

40Thus, with the convergence of these data, the timing of Iceland's deglaciation (Geirsdóttir, 2004; Geirsdóttir et al, 2009) and the hypothesis of the relationship between slope instability and deglaciation converging, attempting from Geirsdóttir's (2004, 2009) data to propose temporal bounds for the landslides identified in the database seems consistent.

41The chi² test performed on the Smjörfjöll massif to study the relationship between slope stability and deglaciation chronology is similar to that of Peras et al. (2016), who obtained similar results: the stability data and the time of occurrence of landslides are not contradictory but related.

42The graphical presentation of the results obtained for the estimation of the time of occurrence of landslides shows a decreasing curve in time: the number of landslides follows a trend whereby the oldest deglaciated regions are those with the most landslides. Finally, by comparing the position of the landslides with the slope map, it appears logically that no landslide occurs on the central plateau with very low slopes (less than 4°), although the period of deglaciation would allow their eventual occurrence because the prerequisites are no longer met.

5.3.2. Hypothesis of a paraglacial origin

43The landslides appear to be paraglacial in origin, related to the post-glacial decompression phenomenon. An interesting approach is the comparison of the landslide density map with the flow map of the Icelandic ice sheet (Fig. 10). Indeed, according to the map proposed by Bourgeois et al. (2000), the northernmost sectors, which correspond to landslide clusters, are largely bypassed by the main ice currents, resulting in a curvature of the currents, and thus a modification of the pressure exerted on the slopes, and of the speed and strength of the flow. On the contrary, the sectors with the least number of landslides correspond to regular ice sheet currents, with shorter distances and straighter lines.

Fig. 10 - Ice cap main flow in Iceland (after Bourgeois et al., 2000 modified).
Fig. 10 - L’écoulement de la calotte glaciaire en Islande (modifiée d’après Bourgeois et al., 2000).

Fig. 10 - Ice cap main flow in Iceland (after Bourgeois et al., 2000 modified).Fig. 10 - L’écoulement de la calotte glaciaire en Islande (modifiée d’après Bourgeois et al., 2000).

1. Ice sheet flow.
1. Ecoulement de la calotte glaciaire.

5.3.3. Climate control hypothesis

44Furthermore, an explanatory hypothesis for the results of the landslide orientation analysis could be that of a climatic control in the occurrence of landslides. The over-representation of landslides oriented to the south and west could be explained by the degree and duration of sunshine on the slopes so oriented. This hypothesis is based on the logic of rapid deglaciation of these highly sunny slopes at the time of deglaciation. Thus, the rapidity of this melting on these slopes, which are particularly exposed to sunlight, would weaken the walls and therefore create instabilities.

6. Conclusion

45Based entirely on aerial photographs, DEMs and satellite images, 290 landslides have been identified in the fjords of eastern Iceland. The realization and analysis of this inventory enable to propose the following hypotheses:

46The density of landslides in the fjords can be explained on the one hand by the lithology, and on the other hand by the chronology of deglaciation, leading to the phenomenon of postglacial decompression of the slopes.

47The higher density of landslides in the north of the study area could be linked to the currents of the past ice cap, which would have reinforced the pressure exerted on the slopes of the massifs concerned.

48The spatial logic of landslides distribution related to climatic control seems likely due to their orientation.

49The topography of the fjords and the steep slopes probably create a vacuum phenomenon that favors the occurrence of landslides.

50The hypothesis of a higher probability of landslide occurrence when its tear-out scar is compliant with the dip cannot be confirmed.

51The resulting database provides a wealth of information that can be used to study the morphometry of landslides, for example. Further analysis and statistical testing will enable it to be put to even greater use.

52Finally, all evidence suggests that paraglacial conditions are the main cause of landslide initiation in the East Icelandic fjords. Further field work and dating would be necessary to support these hypotheses and to obtain more information on what conditions the studied slopes to a high susceptibility of instability.

*Auteur correspondant : Tel : +33 (0)1 49 78 11 18 emilie.portier@sorbonne-universite.fr (Emilie Portier)

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Annexe

Version française abrégée

Les nombreux glissements de terrain en Islande ont été partiellement inventoriés à différentes échelles. Jónsson (1957, 1976) a constitué le premier inventaire à l’échelle de l’Islande, retravaillé ensuite par Whalley et al. (1983). Un inventaire a d’ores et déjà été réalisé dans les Westfjords (Peras et al., 2016), dans le Skagafjörður (Mercier et al., 2013 ; Cossart et al. 2014) et dans les fjords de l’est et le nord (Youinou, 2020). Ce dernier inventaire était incomplet pour la zone étudiée, puisqu’il ne recensait que 170 glissements là où cette étude en identifie 290. L’intérêt géographique de ces inventaires est de chercher des explications à la répartition spatiale et au déclenchement des glissements de terrain. Ce travail a pour ambition de créer une nouvelle base de données qui soit la plus exhaustive possible à l’échelle des fjords de l’est de l’Islande ; de réaliser des mesures morphométriques, des caractérisations géologiques et estimations de datation sur chacun de ces phénomènes, afin de recueillir et compiler une multitude de données sur les glissements de terrain à l’échelle des fjords de l’est.

La zone étudiée et prospectée correspond à l’ensemble des fjords de l’est de l’Islande, soit une étendue d’environ 20 000 km². Elle ignore les frontières administratives, mais prend en compte la géologie : elle est délimitée par les roches basaltiques tertiaires d’environ 3,5 millions d’années. La géologie de la zone étudiée correspond à des basaltes tholeitiques, porphyritiques et à olivine. L’aire étudiée se situe entre 65°79’N – 64°24’N and 15°W – 13°49’W, du Vopnafjörður à la calotte glaciaire du Vatnajökull (Fig. 1). Cette région des fjords de l’est a été largement façonnée par la présence de glaciers et les successions de glaciations et déglaciations.

Deux sources de données ont principalement été utilisées pour ce travail : les photographies aériennes map.is (Fig. 2A) et les images SENTINEL 2A téléchargées depuis la base de données du CNES (pep’s) ; et d’autre part, un Modèle Numérique de Terrain (MNT) d’une précision de 10x10m (Fig. 2B).

Chaque phénomène observé supposé correspondre à un glissement de terrain est dans un premier temps localisé dans un SIG. Les formes observées sont ensuite délimitées (Fig. 3A), notamment avec l’aide des images SENTINEL 2A converties en bandes NDVI (Normalized Difference Vegetation Index) (Fig. 3B). Les mesures réalisées sont automatisées et localisées dans le SIG, afin d’étudier et comprendre la morphométrie et l’organisation du glissement (Fig. 3C, 3D).

Ainsi, le résultat de ce travail est une base de données inventoriant une population de 290 glissements de terrains (Fig. 4). La base de données se présente sous forme de tableur Excel comprenant une trentaine de variables et critères pour chaque individu. Des analyses et statistiques descriptives de l’inventaire permettent de résumer les différentes séries d’information morphologiques (Fig. 6A-C).

L’analyse de cette base de données permet de proposer quelques hypothèses. La distribution des glissements de terrain selon leur orientation est hétérogène (Fig. 7), une tendance se dessine, avec une prédominance de glissements de terrain orienté entre l’ouest et le sud : ils représentent 65 % des glissements. Une hypothèse explicative pourrait être celle d’un contrôle climatique dans l’apparition des glissements de terrain.

Si l’on compare la carte de densité des glissements de terrain recensés (Fig. 5) à la carte d’écoulement de la calotte glaciaire islandaise (Fig. 10), deux foyers principaux de glissements de terrain ont été mis en avant au nord de la zone étudiée : ils correspondent également aux deux massifs contournés par les écoulements majeurs de la calotte glaciaire d’après la carte de Bourgeois et al. (2000). Cela s’expliquerait par le phénomène de décompression post-glaciaire des versants.

Concernant les résultats liés à la géologie des glissements : 39 % des glissements sont conformes au pendage du versant affecté, pour 23 % de glissements contraires au pendage (Fig. 8A). Ce résultat est similaire à celui obtenu par Peras et al. (2016) pour l’inventaire réalisé dans les Westfjords. D’autre part, la quasi-totalité des glissements de terrain sont localisés dans des basaltes tertiaires (Fig. 8B), ce qui correspond bien aux observations réalisées préalablement par Whalley et al. (1983) d’après les recherches de Jónsson (1957, 1976) et par Mercier et al. (2013) et Feuillet et al. (2014) en Islande.

Enfin, l’historique de la déglaciation des fjords de l’est de l’Islande proposé par Geirsdóttir (2007) a été réutilisé dans un SIG afin de donner pour chaque glissement une estimation de la date de déglaciation du versant qu’il affecte et donc potentiellement celle de son apparition (Fig. 9A). L’occurrence des glissements de terrain est décroissante dans le temps, c'est-à-dire que la majorité des glissements apparaissent dans les secteurs qui ont été libérés dans les premiers temps de la déglaciation après le Dernier Maximum Glaciaire, et leur occurrence ne fait ensuite que réduire dans le temps (Fig. 9B).

A l’échelle du massif du Smjörfjöll des tests chi² ont été réalisés afin d’étudier la dépendance entre deux variables, selon la méthode de Feuillet et al. (2014) : un premier pour les variables stabilité du versant et lithologie et un second pour stabilité et âge relatif de la déglaciation. Les deux tests sont concluants (Fig. 8C et 9C) : la lithologie jouerait donc un rôle dans la stabilité du versant, tout comme la date supposée de sa déglaciation.

Pour conclure, à partir d’un travail entièrement réalisé par photo-interprétation, 290 glissements de terrain ont été recensés dans les fjords de l’est de l’Islande. Si la densité des glissements de terrain dans cette région pourrait en partie s’expliquer par la lithologie, les pentes et leur orientation, et la chronologie de la déglaciation, le pendage ne semble pas quant à lui influencer leur déclenchement.

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Table des illustrations

Titre Fig. 1 - Location of the inventoried Eastern Fjords.Fig. 1 – Localisation des fjords de l’Est inventoriés.
Légende This area of the Eastern Fjords has been largely shaped by successive glaciations and deglaciations. The chronology of Iceland's deglaciation used in this study is based on the studies of Geirsdóttir et al. (2007) and Pétursson et al., (2015): the area entirely glaciated during the Last Glacial Maximum (LGM), is characterized by a period of gradual deglaciation until around 14,000 BP. Then deglaciation occurs inland, and some valleys are progressively deglaciated, following a period of glacial readvance during the Recent Dryas (11,700 BP). Then, during the Preboreal (10,000 BP), only the highlands were glaciated. This was followed by a gradual retreat of the ice-covered area, until today.Cette zone des fjords orientaux a été largement façonnée par des glaciations et déglaciations successives. La chronologie de la déglaciation de l'Islande utilisée dans cette étude est basée sur les travaux de Geirsdóttir et al., (2007) et Pétursson et al., (2015) : la région entièrement englacée pendant le dernier maximum glaciaire (LGM), est caractérisée par une période de déglaciation progressive jusqu'à environ 14 000 BP. Ensuite, la déglaciation se produit à l'intérieur des terres, et certaines vallées sont progressivement désenglacées à la suite d'une reprise glaciaire pendant le Dryas récent (11 700 BP). Ensuite, pendant le Préboréal (10 000 BP), seuls les hauts plateaux ont été englacés. Cette période a été suivie d'un retrait progressif de la zone couverte de glace, jusqu'à aujourd'hui.
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Titre Fig. 2 - Summary of data used for rock-slope failure deposits inventory.Fig. 2 - Synthèse des données utilisées pour l’inventaire des glissements de terrain.
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Légende A: Available data used for the creation of the database; B: Resolution of the DEM used (Landmælingar Íslands) and processing carried out.A : Données disponibles utilisées pour la réalisation de la base de données ; B : Précision du MNT utilisé (Landmælingar Íslands) et traitement réalisés.
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Titre Fig. 3 - Landslide delineation and morphometric parameters extraction.Fig. 3 - Méthode de localisation et d’extraction des paramètres morphométriques des glissements de terrain.
Légende A: Method for locating and delineating a landslide; B: Location of a landslide based on NDVI processing of SENTINEL 2 images; C: Location and method of measurements taken for landslides; D: Topographic section of the Haugur landslide, in the Smjörfjöll massif. 1. Landslide delineation; 2. Deposit limits observed; 3. Landslide area (m²); 4. Deposit area (m²); 5. Landslide length (in meters); 6. Deposit length (in meters); 7. Average landslide scar width (in meters); 8. Average width at the top of the deposit (in meters); 9. Average width at the base of the deposit (in meters); 10. Altitude at the top landslide scar (in meters); 11. Altitude at the top of the deposit (in meters); 12. Altitude at the base of the deposit (in meters).A : Méthode de localisation et délimitation d’un glissement de terrain ; B : Localisation d'un glissement de terrain d'après un traitement NDVI d'images SENTINEL 2 ; C : Localisation et méthode des mesures réalisées pour les glissements de terrain ; D : Coupe topographique du glissement de terrain Haugur, dans le massif du Smjörfjöll. 1. Délimitation du glissement de terrain ; 2. Limites observées du dépôt ; 3. Superficie du glissement de terrain (m²) ; 4. Superficie du dépôt (m²) ; 5. Longueur du glissement (en m) ; 6. Longueur du dépôt (en m) ; 7. Largeur moyenne de la cicatrice d’arrachement (en m) ; 8. Largeur moyenne au sommet du dépôt (en m) ; 9. Largeur moyenne à la base du dépôt (en m) ; 10. Altitude de la cicatrice d’arrachement (en m) ; 11. Altitude du sommet du dépôt (en m) ; 12. Altitude de la base du dépôt (en m).
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Titre Fig. 4 - Landslide inventory of the East Fjords of Iceland.Fig. 4 - Inventaire des glissements de terrain des fjords de l'Est de l'Islande.
Légende 1. Landslide; 2. Current ice cap.1. Glissement de terrain ; 2. Calotte glaciaire actuelle.
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Titre Fig. 5 - Density of listed landslides.Fig. 5 - Densité des glissements de terrain répertoriés.
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Titre Fig. 6 - Statistical and spatial distribution of different variables for the 290 landslides.Fig. 6 - Distribution statistique et spatiale de différentes variables pour les 290 glissements de terrain.
Légende A: Univariate statistics of the variables: length of the landslide (1), length of the deposit (2), average width of the tearing scar (3), average width of the deposit according to their area (4), area (5), area of the deposit (6), displaced volume (7); B: Distribution of landslides according to their deposit area : 1. Deposit area (m²); C: Distribution of landslides according to the travel distances of the material moved: 1. Travel distance index (km).A : Statistiques univariées des variables : longueur du glissement (1), longueur du dépôt (2), largeur moyenne de la cicatrice d'arrachement (3), largeur moyenne du dépôt d'après leur superficie (4), superficie (5), superficie du dépôt (6), volume déplacé (7) ; B : Représentation des glissements de terrain en fonction de leur superficie de dépôt : 1. Deposit area (m²) ; C : Représentation des glissements de terrain en fonction des distances de parcours du matériel déplacé : 1. Distance de parcours (en km).
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Titre Fig. 7 - Landslide orientation.Fig. 7 - Orientation des glissements de terrain.
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Titre Fig. 8 - Geological characterization of landslides based on lithology and dip information (modified from the geological maps from Jóhannesson and Sæmundsson, 1998).Fig. 8 - Caractérisation géologique des glissements de terrain selon des informations de lithologie et de pendage (modifié de la carte géologique de Jóhannesson et Sæmundsson, 1989).
Légende A: Estimation of the lithology of the landslides inventoried from the geological data of Iceland; B: Estimation of landslide dip at the pullout scar; C: Results of the chi² test for the lithology and slope stability variables. 1. Basaltic and Intermediate extrusive rocks and sediments. Middle Miocene, older than 11 Myr; 2. Silicic intrusions, rhyolite and granophyre; 3. Silicic extrusives, Tertiary and Pleistocene, older than 11,500 years; 4. Basaltic and Intermediate extrusive rocks and sediments. Late Miocene, 5.3-11 Myr; 5. Domain of central volcano; 6. Late Pleistocene and Lower Pleistocene bedrock, 0.8-3.3 Myr; 7. Upper Pleistocene bedrock, younger than 0.8 m.y.; 8. Upper Miocene and Lower Pliocene bedrock, 3.3-8.5 Myr; 9. Upper and Middle Miocene bedrock, 10-15 Myr; 10. Upper Miocene bedrock, 8.5-10 Myr; 11. Current ice surface; 12. Non-compliant dip landslide; 13. Compliant dip landslide; 14. Perpendicular dip landslide; 15. Dip direction.A : Estimation de la lithologie des glissements de terrain inventoriés d'après les données géologiques de l'Islande ; B : Estimation du pendage des glissements de terrain à la cicatrice d'arrachement ; C : Résultats du test chi² pour les variables lithologie – stabilité du versant. 1. Roches et sédiments extrusifs basaltiques et intermédiaires. Miocène moyen, plus de 11 Ma ; 2. Intrusions siliciques, rhyolite et granophyre ; 3. Extrusives siliciques, Tertiaire et Pléistocène, plus de 11 500 ans ; 4. Roches et sédiments extrusifs basaltiques et intermédiaires. Miocène tardif, 5,3-11 Ma ; 5. Domaine du volcan central ; 6. Substrat rocheux du Pléistocène supérieur et du Pléistocène inférieur, 0,8-3,3 Ma ; 7. Socle rocheux du Pléistocène supérieur, inférieur à 0,8 Ma ; 8. Socle rocheux du Miocène supérieur et du Pliocène inférieur, 3,3-8,5 Ma ; 9. Socle rocheux du Miocène supérieur et moyen, 10-15 Ma ; 10. Socle rocheux du Miocène supérieur, 8,5-10 Ma ; 11. Calotte glaciaire actuelle ; 12. Glissement de terrain au pendage non conforme ; 13. Glissement de terrain au pendage conforme ; 14. Glissement de terrain au pendage perpendiculaire ; 15. Orientation du pendage.
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Titre Fig. 9 - Relative ages of landslides according to deglaciation.Fig. 9 - Âges relatifs des glissements de terrain par rapport à la déglaciation.
Légende A: Reconstruction of the chronology of occurrence of landslides since the Last Glacial Maximum considering the evolution of the ice cap and sea level; B: Maximum potential age of landslides inventoried according to the deglaciation chronology; C: Result of the chi² test for the age - stability values for Smjörfjöll massif (45 landslides).A : Reconstitution de la chronologie d'apparition des glissements de terrain depuis le Dernier Maximum Glaciaire en prenant en compte l'évolution de la calotte glaciaire et le niveau de la mer ; B : Âge maximal potentiel des glissements de terrain inventoriés selon la chronologie de la déglaciation ; C : Résultats du test chi² pour les variables stabilité du versant et début de sa déglaciation (45 glissements de terrain).
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Titre Fig. 10 - Ice cap main flow in Iceland (after Bourgeois et al., 2000 modified).Fig. 10 - L’écoulement de la calotte glaciaire en Islande (modifiée d’après Bourgeois et al., 2000).
Légende 1. Ice sheet flow.1. Ecoulement de la calotte glaciaire.
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Référence électronique

Emilie Portier, Denis Mercier et Armelle Decaulne, « Spatial analysis and controlling factors of landslides in East Icelandic fjords »Géomorphologie : relief, processus, environnement [En ligne], 1 | 2023, mis en ligne le 09 octobre 2023, consulté le 11 décembre 2023. URL : http://journals.openedition.org/geomorphologie/17634 ; DOI : https://doi.org/10.4000/geomorphologie.17634

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Auteurs

Emilie Portier

Sorbonne Université, Laboratoire de Géographie Physique (LGP) UMR 8591 CNRS, 3 rue Henri Dunant, 94320 Thiais, France.

Denis Mercier

Sorbonne Université, Laboratoire de Géographie Physique (LGP) UMR 8591 CNRS, 3 rue Henri Dunant, 94320 Thiais, France.

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Armelle Decaulne

CNRS, LETG-Nantes UMR 6554, Université de Nantes, Chemin de la Censive du Tertre, 44300 Nantes, France.

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Le texte et les autres éléments (illustrations, fichiers annexes importés), sont « Tous droits réservés », sauf mention contraire.

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