Landslide Scars Research Articles

Application and comparison of shallow landslide susceptibility models in weathered granite soil under extreme rainfall events

Landslide susceptibility maps are important for risk management and land-use planning in mountainous countries such as Korea. Many of the slopes of the mountainous areas of Korea are composed of residual soils that originate from weathering of the granite bedrock. The purpose of this study was to compare the application of the SHALSTAB and SINMAP models to slope stability evaluation in Deokjeok-ri Creek. The SINMAP and SHALSTAB models are terrain stability models that are based on a steady-state hydrologic model coupled with an infinite-slope stability equation. A digital elevation model based on LIDAR data was used to calculate the slope and wetness indices. In situ and laboratory tests were performed to find the geotechnical parameters. A geographic information system-based landslide inventory map of 748 landslide locations was prepared using data from previous reports, aerial photographs, and extensive field work. This inventory of landslide scars was used to document sites of instability and to provide an evaluation of model performance by comparing the observed landslide locations with predictions from the models. Both SHALSTAB and SINMAP models identified the northern part of the catchment as the most unstable area. Receiver operator curve plots were used to show that the susceptibility maps produced using SHALSTAB had better prediction accuracy (82.4 %) than the SINMAP model (62.58 %) had. It is concluded that SHALSTAB was relatively successful in delineating slopes where shallow landslides have been observed previously. SHALSTAB is therefore more applicable to weathered granite soil.

Temporal and spatial variation of infilling processes in a landslide scar in a steep mountainous region, Japan

AbstractThe duration of the soil‐depth recovery needed for reoccurrence of shallow colluvial landslides at a given site in humid regions is much longer than the return period of rainfall needed to generate sufficient pore water pressure to initiate a landslide. Knowledge of the rate of change in soil depth in landslide scars is therefore necessary to evaluate return intervals of landslides. Spatial variation in sediment transport at the Kumanodaira landslide scar in central Japan was investigated by field observations. Spatial distribution of the rate of change in soil depth was estimated using sediment transport data and geographic information system (GIS) analysis. Observations revealed that the timing of sediment transport differed for shallow and deep soil layers. Near‐surface sediment transport (mostly dry ravel and some shallow soil creep at depths ≤0·05 m) measured in sediment traps was active in winter and early spring and was affected by freezing–thawing; soil creep of subsoil (i.e. >0·05 m), monitored by strain probes, was active in summer and autumn when precipitation was abundant. Near‐surface sediment flux was estimated by a power law function of slope gradient. Deeper soil creep was more affected by relative location to the landslide scar, which influences soil depth, than by slope gradient. Our study indicated that the rate of soil‐depth recovery is high just below the head scarp of the landslide. Abrupt changes in the longitudinal slope topography immediately above, within and just below the head scarp became smoother with time due to degradation proximate to the landslide head scarp and flanks, as well as aggradation just below the head scarp. Copyright © 2014 John Wiley & Sons, Ltd.

Fluvial incision history that controlled the distribution of landslides in the Central Range of Taiwan

Hillslope processes, which are affected by long-term river incision, give rise to the risk of landsliding in active orogens. We studied the river incision history and the subsequent response of rock slopes in the upstream Dahan River catchment, north Taiwan, by analyzing river long profiles, hillslopes, and landslide scars. The results were combined with chronological data from several landform surfaces to reconstruct the history of landscape evolution. At the study area, the landscape comprises three levels of knickpoints and corresponding slope breaks. These knickpoints propagated upstream along trunk and tributary rivers, undercutting and destabilizing nearby slopes, of which the oldest is a paleosurface dated to ca. 150kyr by cosmogenic nuclide dating. Consequently, three levels of V-shaped inner gorges (up to 600m deep) are incised into the paleosurface. The inner slopes of the three levels of gorges have mean inclinations of 35.6°, 37.7°, and 39.8°, and steepen from the higher to the lower inner gorges. These three series of knickpoints and corresponding slope breaks suggest the occurrence of three phases of river incision. Based on analyses of the steepness indices of the river long profiles, cosmogenic nuclide dating, and the regional tectonic and climatic history, the two earlier phases of incision are inferred to have been caused by prevailing tectonic uplifts during the middle to late Pleistocene, and the most recent phase by climate change in addition to uplift. The long-term history of river incision has controlled the distribution of deep-seated gravitational slope deformation and landslides. Many areas of deep-seated gravitational slope deformation and deep-seated rockslide-avalanches are aligned along the higher and middle slope breaks, and debris slide avalanches are concentrated along the middle and lower slope breaks.

Control of sedimentation by active tectonics, glaciation and contourite-depositing currents in Endurance Basin, South Georgia

Endurance Basin is an elongate broadly WNW–ESE trending basin located on the northern margin of the Scotia Sea, adjacent to the southern margin of the South Georgia micro-continent. Bathymetric and TOPAS sub-bottom profile data acquired in 2010 by the British research ship RRS James Clark Ross map this basin and its sedimentology for the first time. Endurance Basin contains a number of sub-basins and a substantial glaciogenic fan. The northern margin of Endurance Basin is formed by a series of steep slopes and intervening troughs. These are interpreted as a left-stepping en echelon array of oblique, strike–slip faults whilst the sub-basins are separated by compressional dip–slip faults. It appears that South Georgia is moving NW with respect to the basin. We interpret five seismic facies from TOPAS data, which are associated with distinct sedimentologies. The most striking units in the basin fill are: substantial contourite drifts located in the NW of the basin and on its southern margin; and two distinct mass transport deposits that pond in the centre of the basin. Combined with the known regional oceanographic setting, the contourites provide evidence of broadly eastward flowing bottom currents, entering the basin from at least two locations. Although landslide scars are present on the steep northern basin margin, the imaged mass transport deposits are interpreted to have been sourced from the glaciogenic fan, located in the SE of the basin, and from a contourite unit located on the basin's southern margin. Sediments from these events are transported at least 40km. The contourite drift sequence is at least 100m thick in the west of the basin and may contain a palaeoenvironmental archive of Antarctic Circumpolar Current (ACC) flow and the climate of South Georgia extending to the Pliocene. Such an archive would allow the reconstruction of ACC flow through the Pleistocene glaciations and provide a means of linking ocean circulation and climate records in the sub-Antarctic Polar Front region.

Evolution of a large landslide in the High Himalaya of central Nepal during the last half-century

Episodic and catastrophic landslides are considered to be one of the main sources of sediment in the steep, mountainous landscapes of the Himalayas. However, the evolution of a single landslide through time and its contribution to erosional processes remain poorly constrained. In this study, we focus on a single, large (0.5km2) landslide in a small catchment on the southern flank of the Annapurnas in Nepal (the Khudi valley) in order to quantify its importance in the overall erosion of this steep Himalayan catchment.The evolution of the Saituti landslide has been continuously monitored by remote sensing for the past 46years. During that period, the Saituti landslide displayed sustained activity, such that the area of the landslide scar increased by a factor of 4. This retrogressive failure, a consequence of several sporadic flank and crown collapses, has not been continuous. Rather, acceleration phases have alternated with more quiescent periods. Simultaneously, the upper edge moved upward by 900m. Based on field evidence from recent activity (such as scarps and open tension cracks above the landslide) and on an analysis of slope angles, at least the next 500m is expected to fail.Volume losses within the landslide were estimated from differences between digital elevation models (DEMs) and from changes in landslide area, using a calibrated power law relationship between landslide area and volume. Corresponding landslide-induced erosion rates at the scale of the whole Khudi catchment were found to be 2.6±0.9mm/y for the past half-century. Those rates are similar to denudation rates obtained from sediment load measurements between 1999 and 2004. Those results, along with the lack of other major landslides in the valley for the last 46years, suggest that the Saituti landslide plays a dominant role in the modern erosion of the High Himalayan Khudi catchment for the last years and possibly for the past few decades.We propose that continuous and sustained activity of a few major landslides over the past few decades might represent a significant contribution to the erosion of the High Himalayan range. This long-lasting, landslide-induced erosion should be taken into account when interpreting suspended load measurements, results from provenance analysis, or cosmogenic nuclides in river sand. Such processes should also be included in landscape evolution models when annual to secular problems are explored.

Physically based shallow translational landslide susceptibility analysis in Tibo catchment, NW of Portugal

The Northwest of Portugal is a steep slope mountain area where high amounts of rainfall (3,000 mm/year) occur especially on winter, promoting high natural propensity to slope failure, mainly shallow landslides. This article aims to assess shallow landslide susceptibility, in Tibo catchment, Serra da Peneda, North of Portugal, using two physically based models—SHAllow Landslide STABility (SHALSTAB) and Safety Factor (SF)—applying a set of mechanical and hydrological parameters, assessed in situ and laboratory testing of soil samples collected in the field, calibrated by back analysis of landslides inventoried in the study area, as well as accurate topographic information derived from a high-resolution digital elevation model (DEM). The validation of results was made using shallow landslide scars, directly inventoried in the field. Both susceptibility model results were validated by scar concentration (SC) and landslide potential (LP). SHALSTAB model was also validated by minimum log q/T. SHALSTAB predicts 50 % of the area to be on unstable classes (log q/T < −2.5), 77 % of the SC on unstable classes and a LP index of 7 and 4.7 % for the two most unstable classes. By minimum log q/T, SHALSTAB predicts 91 % of the scars to occur on unstable classes. Safety factor predicts 47.99 % of the area as unstable, 79.9 % of the SC for unstable classes, and a LP index on unstable classes of 4.63 and 2.77 % on partially unstable class. For the most unstable classes of both models, the greatest values of LP were between 3.5 and 7 %. The simple physically based models used in this study (SHALSTAB and SF) proved to be effective as shallow landslide susceptibility predictors, being in consequence useful tools for municipal planning on landslide hazards, but their application requires, beyond detailed topographical information, good estimates of the mechanical and hydrological soil properties.

High-resolution near-surface velocity model building using full-waveform inversion—a case study from southwest Sweden

Full-waveform inversion (FWI) is an iterative optimization technique that provides high-resolution models of subsurface properties. Frequency-domain, acoustic FWI was applied to seismic data acquired over a known quick-clay landslide scar in southwest Sweden. We inverted data from three 2-D seismic profiles, 261-572 m long, two of them shot with small charges of dynamite and one with a sledgehammer. To our best knowledge this is the first published application of FWI to sledgehammer data. Both sources provided data suitable for waveform inversion, the sledgehammer data containing even wider frequency spectrum. Inversion was performed for frequency groups between 27.5 and 43.1 Hz for the explosive data and 27.5-51.0 Hz for the sledgehammer. The lowest inverted frequency was limited by the resonance frequency of the standard 28-Hz geophones used in the survey. High-velocity granitic bedrock in the area is undulated and very shallow (15-100 m below the surface), and exhibits a large P-wave velocity contrast to the overlying normally consolidated sediments. In order to mitigate the non-linearity of the inverse problem we designed a multiscale layer-stripping inversion strategy. Obtained P-wave velocity models allowed to delineate the top of the bedrock and revealed distinct layers within the overlying sediments of clays and coarse-grained materials. Models were verified in an extensive set of validating procedures and used for pre-stack depth migration, which confirmed their robustness.

Comparative analysis of SHALSTAB and SINMAP for landslide susceptibility mapping in the Cunha River basin, southern Brazil

The Shallow Landsliding Stability Model (SHALSTAB) and Stability Index Mapping (SINMAP) models have been applied to various landslide management and research studies. Both models combine a hydrological model with an infinite slope stability model for predicting landslide occurrence. The objectives of the present study were to apply these two models to the Cunha River basin, Santa Catarina State, southern Brazil, where many landslides occurred in November 2008, and perform a comparative analysis of their results. Soil samples were collected to determine the input parameters. The models were calibrated with a landslide scar inventory, and rainfall data were obtained from three rain gauges. A comparison of their results obtained from the models was undertaken with the success and error index. Based on the maps of stability and instability areas for the study basin, the models performed well. Since the initial equations of both models are not particularly different, their results are similar. Locations with steep slopes, as well as areas with concave relief that tend to have larger contribution areas and moisture, have lower stability indexes. SHALSTAB classified only ~13 % of the total area of the Cunha River basin as unstable, while SINMAP classified ~30 % as unstable. The analysis of maps based on the results of the two models shows that if SHALSTAB is correctly calibrated, based on hydrological parameters, its results could be more accurate than SINMAP in the prediction of landslide areas. Although SINMAP showed better calibration of the landslide scars, its classification over the basin results in an overestimation of stability areas. The conclusion is that SHALSTAB is more suitable than SINMAP for the prediction of landslides in the Cunha River basin, Brazil.

RELIEF DEVELOPMENT OF THE BABIA GÓRA MASSIF, WESTERN CARPATHIAN MOUNTAINS

Abstract The paper discusses structural considerations relating to landform development on Mt. Babia Góra (1,725 m a.s.l.), the highest massif in the flysch section of the Western Carpathian Mountains. The Babia Góra massif consists of folded Palaeogene-age sediments, including resistant Magura sandstone and less resistant sub-Magura layers, with numerous tectonic faults. The area has inverse-type geomorphology. The monoclinal ridge of the massif itself consists of the resistant Magura sandstone dipping southwards. Since the Miocene, the development of the massif’s relief has involved a number of processes, including: tectonic uplifting, removal of a thick layer of rocks, exposure of sub-Magura layers over a large area, the staged development of valleys dissecting the pediments surrounding the ever higher ridge, and slope retreat due to deep landsliding. Axes of linear terrain forms, escarpments of landslide scars and of headwater areas follow two main intersecting fracture lines present within the massif. Large quantities of colluvial material are transported away from the massif along these lines. The development of the land relief has led to the elongation of slopes as local elevation differences increased. Landsliding has developed in an uphill direction, which means that the youngest relief is observed on the highest sections of steep slopes. The development of the massif’s northern slope, which has formed a high and precipitous cuesta, began to accelerate only after a nearly complete exposure of the sub-Magura layers at its foot. Following this exposure the profile of the massif’s N-S cross-section has begun to become asymmetrical. With time, the degree of general remodelling of the massif has tended to decrease. As a result, the geomorphological contrast between the northern and southern sides of the massif has become well established. The description of the probable development of the Babia Góra relief is based on the author’s fieldwork, an analysis of geological maps and aerial photos, and on literature.

A case study on an open hillside landslide impacting on a flexible rockfall barrier at Jordan Valley, Hong Kong

A case study on an open hillside landslide impacting on a flexible rockfall barrier at Jordan Valley, Hong Kong is presented in this paper. The landslide occurred sometime in June 2008. This is so far the only case history of landslide debris having been intercepted by a flexible rockfall barrier in Hong Kong. The landslide scar is 10 m wide and 7 m long, and the landslide volume is about 110 m3. The landslide debris was largely retained by the barrier but two of the barrier posts were severely damaged and failed. Debris mobility analysis and structural analysis of this case history have been undertaken with a view to obtaining a better understanding of the possible landslide dynamics and behaviour of flexible barrier upon debris impact. The analyses appear to have reproduced some of the salient field observations. The probable key contributory factors to the failure are highlighted and discussed. Through the study, the possible range of equivalent pseudo-static impact pressure exerted on the flexible barrier by the landslide debris is assessed. The site observations and results of the analyses provide insights pertaining to the importance of robustness in the design and detailing of flexible debris-resisting barriers.

Large-scale catastrophic flank collapses in a steep volcanic ridge: The Pico–Faial Ridge, Azores Triple Junction

Large-scale flank collapses are common in the geological evolution of volcanic ocean islands in the Atlantic. To date, catastrophic lateral collapses in the Azores Islands have been difficult to identify, leading to suggestions that a lack of events may relate to the relatively small size of the islands. Here we show evidence for two major collapses on the northern flank of Pico Island (Pico–Faial volcanic ridge, central Azores), suggesting that this island had a collapse incidence similar to that of other Atlantic volcanic islands.The study is based on the analysis of: (1) offshore and onshore high-resolution digital elevation models; (2) field data focused on the N flank; and (3) new K–Ar ages on selected lava flow samples.Pico sub-aerial northern flank is marked by two conspicuous arcuate shaped depressions concave towards the sea, here interpreted as landslide scars. A main debris field is observed offshore the largest depression. This deposit has 20km of maximum length, covers ca. 150km2, is composed of meter to hectometer blocks, and has an exposed volume here estimated between 4 and 10km3, though the actual volume probably exceeds 10km3. Debris flow towards the ESE was apparently determined by the slope of the narrow WNW–ESE S. Jorge channel.Young lava flows cascade over the interpreted scars, thus concealing the older volcanic sequence(s) affected by the landslide(s). New K–Ar ages measured on these lava flows provide a minimum age of ca. 70ka for the large-scale collapse(s) in Pico's northern flank.

Effect of Revegetation on Soil Development and Water Infiltration Rates in Shallow Landslide Scars of Slopes Covered by Volcanic Ash and Pumice Fall

Effect of Revegetation on Soil Development and Water Infiltration Rates in Shallow Landslide Scars of Slopes Covered by Volcanic Ash and Pumice Fall

Condor seamount (Azores, NE Atlantic): A morpho-tectonic interpretation

High-resolution datasets collected by multibeam and acoustic backscatter surveys were used to produce fine-scale seafloor nature and morpho-tectonic interpretations of the Condor seamount. Condor constitutes an elongated volcanic ridge that extends for 39km and rises more than 1800m from the surrounding seafloor. Constructive morphologies include (i) linear eruptive centres, (ii) volcanic cones with or without summit depressions, (iii) lava flows and (iv) hummocky sectors. Eruptive type is interpreted to vary with depth. On the deeper seamount extremities, the predominance of highly acoustically backscattering volcanic cones and hummocky terrain is interpreted to result from effusive eruptions not yet covered by sediment deposits. In contrast, the smoother relief of the central seamount flanks is interpreted as draping and infilling of the underlying effusive relief by (i) primary volcaniclastic deposits produced by explosive eruptions on the shallowest parts of the ridge, together with (ii) secondary volcanigenic sediments resulting from truncation of the seamount top by swell erosion and (iii) sediments resulting from biogenic production.A set of WNW–ESE to NW–SE trending volcano-tectonic structures are shown to control most of the fissural volcanism that formed the ridge. A network of NNW–SSE trending faults is identified on the sea-floor around Condor but they show little relation with the distribution of volcanic edifices or with post-emplacement dismantling of the seamount. These fault sets are related to the transtensional regime acting on the Azorean segment of the Eurasia–Nubia plate boundary.Erosional features include (i) palaeo wave-cut platforms on the seamount summit, (ii) landslide scars produced by lateral collapses of the NE and SW-facing flanks, (iii) gullies and turbidity current channels and (iv) mass-wasting deposits. Iceberg drag and bump marks are also identified on the seamount upper flanks, representing the first reference to such features in the Azores and an additional low latitude record.Given the lack of major erosional and tectonic dismantling, Condor is suggested to be a relatively young seamount. A revised factoring of eustatic, erosional and isostatic processes does not exclude that the summit may have been eroded as late as the Last Glacial Maximum.

Continuous catchment‐scale monitoring of geomorphic processes with a 2‐D seismological array

AbstractDistributed activity of geomorphic processes with different spatiotemporal scales is hard to monitor in detail with conventional methods but might be detected with seismometers. From July to September 2010, we deployed 14 seismometers to evaluate the ability of a two‐dimensional array with small interstation distances (11 km) to continuously monitor geomorphic processes in a mountain catchment (370 km2) in Taiwan. Spectral analysis of seismic records highlights different sources with high‐frequency content (&gt;1 Hz), consistent with hillslope and river processes. Using a common detection algorithm and a location technique based on the timing of seismic amplitude, we have located 314 near‐surface events, most of which (69%) occurred during daily convective storms. Event activity was positively correlated with the precipitation intensity, but this relation was not uniform in the catchment. High‐resolution satellite images and air photos did not show geomorphic change during the study, which did not have any episodes of extreme precipitation. A majority of events (61%) were collocated with preexisting geomorphic features (landslide scars, gullies) within the uncertainty on location (9% of interstation distance). The combination of event location and timing suggests a geomorphic source of recorded signals and most events had the seismic characteristics of rockfall, debris avalanches, or slides. Reactivation of prior erosion sites by such processes is difficult to detect with imagery, but can possibly be resolved by seismic monitoring. When proven, this approach will allow a spatially comprehensive survey of geomorphic activity at the catchment scale, with temporal detail sufficient to evaluate the exact (meteorological) conditions under which process events occur.

The analysis of rainfall-induced slope failures at Iva Valley area of Enugu State, Nigeria

This paper analyses the dominant mechanisms of slope failures and identifies potential obstacles to landslide-hazard reduction at the Iva Valley area, Enugu, Nigeria. The landscape is replete with landslide scars and gullies of varied sizes and the slope deposits comprise unconsolidated, friable sands inter-bedded with thin units of montmorillonitic claystone. Forty-three landslide events were identified in the study area with most being shallow, short run-out movements with slip-surface depth <2 m. The study found the landslides mainly occur in the beginning of rainy season characterized by short duration, high intensity rainfall. An integrated approach comprising field mapping, laboratory tests and numerical analyses reveals that the barren nature of the slopes prior to the outset of rainy season, high rainfall intensity, erosion, overgrazing, soil characteristics and the site’s unique lithologic sequence are the main causes of instability. Shearing tests under several conditions showed that the soils strongly strain-soften until low steady-state strength is achieved. A computer code, based on this strength reduction technique, used input parameters obtained from the field and laboratory studies to simulate a landslide with similar structure, travel distance and distribution area. It is noted that urbanization has gradually increased the vulnerability of the society’s poor to landslide hazards as they now erect unplanned residence (tents and blocks) on the slopes. This work is part of a regional study aimed at finding ways of protecting the vulnerable by generating data that could be used to build future landslide susceptibility map.

Application of first-arrival tomography to characterize a quick clay landslide site in Southwest Sweden

First-arrival traveltime tomography was applied to high-resolution seismic data acquired over a known quick-clay landslide scar near the Gota River in southwest Sweden in order to reveal the geometry and physical properties of clay-related normally consolidated sediments. Investigated area proved to be a challenging environment for tomographic imaging because of large P-wave velocity variations, ranging from 500 to 6000 m/s, and relatively steeply-dipping bedrock. Despite these challenges, P-wave velocity models were obtained down to ca. 150 m for two key 2D seismic profiles (each about 500-m long) intersecting over the landslide scar. The models portrait the sandwich-like structure of marine clays and coarse-grained consolidated sediments, but the estimated resolution (20 m) is too small to distinguish thin layers within this structure. Modelled velocity structures match well the results of reflection seismic processing and resistivity tomography available along the same profiles.

High-resolution reflection seismic investigations of quick-clay and associated formations at a landslide scar in southwest Sweden

We present high-resolution reflection seismic data from four lines (total 1.9 km) that cross a quick-clay landslide scar located close to the shore of the Göta River in southwest Sweden, and compare the results with geotechnical data from boreholes. The seismic data allow the imaging of bedrock topography and normally to weakly consolidated sediments to a subsurface depth of about 100 m. Different types of seismic sources, including sledgehammer, accelerated weight-drop and dynamite were utilized and compared with each other. Analysis of their power spectra suggests that weight-drop and dynamite have higher frequency content and energy than the sledgehammer, which makes these two sources suitable also for waveform tomography and surface-wave data analysis. The shallowest non-bedrock reflector is observed at about 10–20 m below the surface, it overlays the bedrock, and is interpreted to originate from the contact between clay formations above and a coarse-grained layer below. The coarse-grained layer appears to be spatially linked to the presence of quick-clays. It is a regional scale formation, laterally heterogeneous, which deepens to the west of the study area and correlates well with the available geotechnical data. Continuity of the coarse-grained layer becomes obscured by the landslide scar. There may be a link between the coarse-grained layer and landslides in the study area, although this possibility requires further hydrogeological and geotechnical investigations. Reflectors from the top of the bedrock suggest a depression zone with its deepest point below the landslide scar and a bowl-shaped structure in the northern portion of one of the seismic lines.

Submarine structure of Vulcano volcano (Aeolian Islands) revealed by high-resolution bathymetry and seismo-acoustic data

A new definition of the morpho-structural submarine setting of the Vulcano volcano is presented based on the integration of multibeam swath bathymetry, high-resolution seismic profiles and sea-floor sampling. A number of unknown volcanic features have been mapped on the submarine flanks of Vulcano Island as volcanic outcrops and isolated cones. For most of them a relatively old age is suggested (especially to the west of the island), while evidence of relatively recent volcanic activity characterizes the north-eastern submarine sector, offshore Vulcanello. The morphology and distribution of these features, either aligned along NNW–SSE trends or radially elongated, suggest the prevalence of regional structural control or local stresses related to the volcano structure, respectively, in the different sectors of the volcanic edifice. Shallow-water (< 120 mbsl) insular shelves, related to wave action during Late Quaternary sea-level fluctuations, were recognized in correspondence of the oldest sectors of the edifice, while they are lacking in the younger sectors. Beyond the shelf edge, a strong increase of slope gradients characterizes the submarine flanks, promoting the formation of erosive-depositional and mass-wasting features at different scale, such as landslide scars, gullies, headless channelized features, crescent-shaped bedforms and volcaniclastic fans. In particular, two large volcaniclastic fans were identified on the north-eastern and south-western flanks, where marine retrogressive processes are very active and sediment is funnelled from the coasts to the foot of the volcano. These fans are created by the gradual stacking of gravity flows fed by repeated small- and medium-scale mass-wasting events, instead of being related to large-scale flank instability events. The lack of large-scale lateral collapse on the submarine flanks of Vulcano volcano has been related to the morpho-structural and volcano-tectonic setting of the volcanic edifice, characterized by the occurrence of summit caldera collapses limiting its total height and by the absence of well-defined rift zones. ► The submarine portions of Vulcano account for ≈ 85% of its areal extent. ► Multibeam and seismic data show new details of the submarine flanks. ► Interacting constructional and erosive-depositional processes are documented. ► Insular shelf development gives constraints on the evolution of the edifice. ► No large-scale lateral collapse deposits have been recognized on submarine flanks.

High-resolution 3D reflection seismic investigation over a quick-clay landslide scar in southwest Sweden

Quick-clay landslides often occur in the northern hemisphere in areas that were covered by Pleistocene glaciations. They are particularly common along the shorelines of the Göta River in southwestern Sweden. Characterization of potential landslide areas and identification of features that indicate high risk are necessary to better understand the triggering mechanisms of these events. Therefore, an intensive characterization project was initiated at the Fråstad landslide in Sweden. Part of the characterization program included the acquisition of 3D reflection seismic data to image structures in the normally consolidated sediments, as well as the bedrock topography below the landslide scar. Two seismic horizons within the glacial and postglacial sediments were observed. The shallowest seismic horizon (here, referred to as S1) corresponds to a coarse-grained layer that was previously detected by eight geotechnical boreholes located within the 3D survey area. Discontinuities in S1, mapped by the 3D reflection seismic data, occur across a zone that correlates with the landslide scar boundary, suggesting that this zone may have played a role in triggering and/or in limiting the extension of the landslide. If S1 is truncated by or mixed with clays in this zone, then the outflow of water from the permeable S1 into the clays above may have increased the amount of quick clays above this zone. The increased outflow of water may also have caused a higher pore-water pressure south of the zone, which in turn could have acted as a trigger for the landslide. We evaluated the potential of using the 3D reflection seismic method as a complement to drilling and other geophysical methods when performing landslide site investigations. We also demonstrated the importance of further investigating the relationship between 3D subsurface geometries and landslide development.

Physical and geotechnical characterization of unconsolidated sediments associated with the 2005 Mbonjo landslide, Limbe, Cameroon

Laboratory analyses of unconsolidated sediments from the ruptured zone of the 2005 Mbonjo Landslide scar were carried out to characterize and evaluate their role as contributory factors in enhancing the slide. Colour, particle size (PS) and particle size distribution (PSD), Atterberg limits [liquid limit (LL), plastic limit (PL) and plasticity index (PI)] and basic geotechnical properties [bulk density (ρ), dry bulk density (ρd), dry unit weight (γd), moisture content (Mc), specific gravity (Gs), void ratio (e) and porosity (n)] were determined. Colour ranged from reddish brown to yellowish brown. Texturally, samples were classified as loam, silty clay loam and silty clay. Liquid limit ranged from [56 to 74 weight percentage (wt%)], PL (37 to 50 wt%) and PI (15 to 36 wt%). Values for geotechnical properties were ρ (2 to 2.25 g/cm3), ρd (1.75 to 1.95 g/cm3), Mc (14 to 15 wt%), Gs (2.67 to 2.69 g/cm3), e (0.37 to 0.52) and n (0.27 to 0.34). From results of colour, PS and PSD, Atterberg limits and geotechnical analyses, the samples were reflective of clay-rich sediments with high plasticity and sliding potential. Based on the interpretative data, the studied sediments were qualified as landslide prone with clay interbeds serving as the slide surface. Key words: Cameroon volcanic line, clay interbeds, geotechnical properties, ruptured zone, slide surface.