Failure Scars Research Articles

Erosional and Depositional Features along the Axis of a Canyon in the Northern South China Sea and Their Implications: Insights from High-Resolution AUV-Based Geophysical Data

Autonomous Underwater Vehicle (AUV)-based multibeam bathymetry, sub-bottom profiles, and side-scan sonar images were collected in 2009 and 2010 to map the geomorphic features along the axial zone of a canyon (referred to as C4) within the canyon system developed on the northern slope of the South China Sea. These data significantly improved the spatial resolution of acoustic data, leading to a better understanding of the sedimentary processes within the modern canyon system. The bathymetric data reveal that sections across the canyon axis exhibit either asymmetrical or symmetrical characteristics, which differ from the overall asymmetrical pattern of the entire canyon. This suggests that the overall asymmetrical pattern of the canyon is not primarily due to axial incision. Various morphological elements were identified along the canyon axis, including failure scars, undulating features, knickpoints, flat terraces, furrows, and mass transport deposits (MTDs). Landslides, predominantly located in the upper canyon, were formed after at least 5000 years BP. Between the beginning of the canyon and a water depth of approximately 1300 m, there are alternating flat terraces and knickpoints. The large knickpoints’ low slope gradients are likely formed by the presence of undulating features. The internal configurations of undulating features suggest that they are depositional structures rather than sediment deformation. The formation of small-scale furrows below a depth of 1200 m may be associated with occasional gravity flows down the canyon. It is suggested that the canyon was generally inactive during the Holocene but experienced sporadic processes of sediment erosion, transport, and re-deposition in the axial zone that were triggered by landslide events occasionally in the upper canyon.

Large-scale margin collapses along a partly drowned, isolated carbonate platform (Lansdowne Bank, SW Pacific Ocean)

The Lansdowne Bank is a partly drowned, isolated carbonate platform of around 4000 km2 located 300 km west of New Caledonia, in the SW Pacific Ocean, in water depths of 20 to 100 m. New multibeam bathymetric data, high resolution seismic reflection profiles and sediment gravity cores have been acquired on the bank top and adjacent slopes. This dataset reveals an almost continuous 4 km wide outer reef rim located in ca. 50 m water depth, surrounding a gently deepening inner platform, reaching up to 100 m water depth. The bank is bordered by very steep slopes showing numerous erosional morphologies such as canyons, channels and gullies. Along with these bypass features, spectacular bank margin collapses and slope failures are evidenced by up to 20 km-wide bank edge and intraslope failure scars, respectively, resulting in a typical “scalloped” geometry of the bank margin. These failure scars can lead to a complete collapse of the outer reef rim and impact subsequent reef development. Bank margin collapses are evidenced by hectometer to kilometer-scale blocks and debris shed on the slope, likely emplaced by rock fall/avalanching processes originating from the brittle failure of early cemented bank edge and upper slope sediments. In turn, failures triggered on the un-cemented mud-prone middle to lower slopes likely generate more cohesive, submarine debris flows that could be at the origin of erosive morphologies within the debris fields. Estimated individual failure volumes can reach up to 3 km3. Quaternary sea-level lowstands, that would have led to platform exposure, fracturing and karstification, and the development of an erosional sea cliff, as well as subsequent rising sea-level are believed to play a significant role in mass wasting event emplacement, yet “bottom up” submarine processes such as the upslope propagation of bypass morphologies by retrogressive headward erosion cannot be ruled out. In terms of geomorphic and stratigraphic constraints, the documented bank margin collapses affect a terrace located in 70 m water depth around the bank, which, depending on its age and origin, could provide a minimum age for collapse events. Finally, considering the shallow water depth of failure headscarps, the volumes of material involved in the slides as well as their vicinity to the nearby main island of New Caledonia, numerical simulations of the tsunamigenic potential of submarine slides have been performed. They showed that these slides would have been able to produce a meter-scale wave that would reach the northern coast of the island in less than an hour.

Endoscopy-guided diode laser-assisted transcaruncular StopLoss Jones tube implantation for canalicular obstructions in primary surgery

PurposeTo introduce and evaluate a minimally-invasive endoscopy-guided transcaruncular laser-assisted StopLoss Jones tube (SLJT) implantation technique for severe canalicular obstructions in primary surgeries.MethodsWe retrospectively identified 12 adult patients (12 eyes) with severe epiphora secondary to long-segment canalicular obstructions. All the 12 eyes underwent an endoscopy-guided transcaruncular SLJT implantation with an 810-nm diode laser’s assistance as the primary surgical approach. Surgical and functional success rates, intraoperative and postoperative complications, as well as the need for secondary surgery, are evaluated.ResultsPrimary surgical success was achieved in 11 of the 12 cases (92%); one patient (8%) required secondary surgery to replace an SLJT with a shorter one. Ultimately, all cases showed well-placed functioning tubes. Three of the 12 cases (25%) presented conjunctival scarring, conjunctival granulation tissue, with or without tube-associated irritation of the ocular surface. We observed no sink-in, extrusion, nor crack of the tube. Complete functional success was achieved in 83%, and moderate functional success in 17% of all patients. The functionally unsuccessful outcome was not present in this study.ConclusionEndoscopy-guided transcaruncular diode laser-assisted SLJT implantation seems to be a promising minimally invasive approach for primary treatment of severe canalicular dacryostenosis. This novel technique shows high functional success rates. It seems to avoid the risk of tube malposition and extrusion, septal and turbinate injury, nasal adhesion, drainage failure, ethmoiditis, postoperative bleeding, and cutaneous scars.

Latest Quaternary mass-transport processes of fan-shaped body in the western margin of the Ulleung Basin, East Sea (Japan Sea)

In the western margin of the Ulleung Basin, a detailed analysis of cores with geophysical data from a fan-shaped body, just downslope of a submarine gully associated upslope with failure scars, reveals various modes of mass-transport processes. The arcuate failure scars occurs in water depths exceeding 600 m. The fan-shaped body, less than ca. 10 km long in radius, displays strong backscatter intensity in sonar images, and corresponds to the uppermost transparent mass in Chirp sub-bottom profiles. Sediment cores penetrating to the uppermost transparent mass consist mostly of various facies of mass-transport deposits (MTDs), causing the strong back-intensity in the sonar images. The interval of MTD facies comprises the upper and lower units without hemi-pelagic muds between them, implying that the fan-shaped body was probably deposited during a single event separated in at least two stages without a significant time break. The lower unit shows brittle to plastic deformation of soft muds (slides/slumps), whereas the upper units exhibits fully fragmented soft mud clasts (low viscous debris flows). Both the upper and lower units involve same original lithology (i.e., soft hemi-pelagic mud) prior to failures, suggesting that the lithology could not significantly affect depositional processes. The fully fragmented soft mud clasts of the upper unit are probably indicative of more shearing than the brittle to plastic deformation of soft muds in the lower unit. Considering the small dimension of the failure scars/gully and the same original lithology, the more shearing of the upper unit was most likely caused by longer transport distance than that of the lower unit. The rare turbidites with absence of channellevee systems in the fan-shaped body and the failure scars confined in the upper to middle slopes suggest that the submarine gully probably formed by slope failures, not by erosion of turbidity currents.

Qualifying mass failures on loess gully sidewalls using laboratory experimentation

The morphology of the failure scar has been a long-debated issue concerning the Loess Plateau of China, and the lack of normative data has hampered vital research in this area. In this study, a series of laboratory experiments were conducted to observe failure geometries and volumes under rainfall simulations. The following six failure-scar types occurred: The sequence of the types described here may be same to that of the following text, namely Tf, Cd, Cu, Ia, Ps, and Co. The number of failure masses in the experiment for scar types Tf, Cd, Cu, Ia, Ps, and Co amounted to 45, 26, 23, 3, 2, and 1% of the total, and the corresponding volumes were 58, 16, 20, 2, 3, and 1% of the total, respectively. This implies that Tf, Cd, and Cu were the three major types of failure scars during the process of gravity erosion on a steep loess slope, and the scar Tf was the most significant. The sensitivity coefficient – the degree of variation in the target value caused by a change in a crucial factor when other conditions remain unchanged – of the slope gradient and rainfall intensity for the total volume of the Tf scar were 4.5 and 3.7, respectively. In other words, a relatively dangerous failure scar (Tf) might appear if the slope became steeper, or if the rainfall became more intensive. In addition, the sensitivity coefficients of the rainfall duration for the total volume of the Cu scar was significant, being 12.4. This shows that a long-duration storm could easily induce a large-volume failure with a Cu scar. The experimental results obtained here provide a morphogenic insight into the gravity-erosion control on a loess gully sidewall.

Recent morphology and sedimentary processes along the western slope of Great Bahama Bank (Bahamas)

AbstractCarbonate slopes and associated resedimented deposits have recently gained renewed interest because they represent volumetrically significant parts of carbonate platforms. Carbonate slopes are highly variable compositionally, architecturally and spatially due to a spectrum of sediment sources, resedimentation processes and controlling factors. Here, new high resolution acoustic data (including EM302 multi‐beam echo‐sounder and very high resolution seismic) and piston cores document highly diverse and complex morphological features along the north‐western slope of Great Bahama Bank. The recent morphology of the slope is the result of the interplay between depositional and erosive processes that vary through time and along strike. The different sedimentary processes are recorded as a Pleistocene lowstand surface, characterized by many erosional features and a Holocene sedimentary wedge along the upper to middle slope that partially covers the underlying Pleistocene surface. Sedimentary processes during the Holocene are dominated by density cascading flows, which export muddy aragonitic sediment from the platform top towards the slope. Sedimentation rates, however, vary along strike due to platform top morphology combined with the variable strength of the basinal current. Reefs and islands in the Bimini area block off‐bank sediment export, and shoals and tidal deltas from Cat Cay to the south reduce the density cascading processes. Numerous small and large slope failure scars show the instability of the steep slopes of Great Bahama Bank. Bottom currents dominate the lower slope and the basin. Striations and moats are the morphological expressions of current directions, while areas of non‐deposition document strong current and concomitant removal of off‐bank transported sediment along parts of the slope, while the Santaren Drift and the drift on the north‐western edge of Great Bahama Bank act as the depositional locus for the fine‐grained sediments transported in the current.

Carbonate slope morphology revealing a giant submarine canyon (Little Bahama Bank, Bahamas)

New high-quality multibeam data detail the morphology of the giant 135-km-long Great Abaco Canyon (GAC) located between Little Bahama Bank (LBB, Bahamas) and Blake Plateau. Knickpoints, chutes, and plunge pools mark the canyon main axis, which is parallel to the LBB margin. The canyon head covers a large area but does not represent the main source of the modern sediments. The material supplied through the LBB canyon systems originates below this head, which only shows erosive lineaments related to the pathway of currents running along the seafloor and restricted failure scars. Most of the sediment supply originates from the canyon sides. The northern canyon flank incises the Blake Plateau, which comprises contourites on top of a drowned Cretaceous carbonate platform. These deposits are susceptible to translational slides and form dissymmetric debris accumulations along the northern edge of the canyon. A large tributary drains the Blake Plateau. Two large tributaries connecting the southern flank of the GAC directly to the LBB upper slope form additional sources of sediments. Subbottom profiles suggest the presence of a sedimentary levee on the tributary canyon and of sediment gravity flow deposits. The GAC has been a permanent structure since the drowning of the Cretaceous platform, and its size and morphology are comparable to those of canyons in siliciclastic environments. The orientation of the GAC parallel to large-scale regional tectonic structures suggests a structural control. The size of the observed structures, especially plunge pools at the base of gigantic chutes, is unusual on Earth. The presence of deposits downflow of the pools suggests that the GAC results from or at least is maintained by persistent and sustained submarine gravity flows rather than by retrogressive erosion.

Forensic analysis of rockfall scars

We characterise and analyse the detachment (scar) surfaces of rockfalls to understand the mechanisms that underpin their failure. Rockfall scars are variously weathered and comprised of both discontinuity release surfaces and surfaces indicative of fracturing through zones of previously intact rock, known as rock bridges. The presence of rock bridges and pre-existing discontinuities is challenging to quantify due to the difficulty in determining discontinuity persistence below the surface of a rock slope. Rock bridges form an important control in holding blocks onto rockslopes, with their frequency, extent and location commonly modelled from the surface exposure of daylighting discontinuities. We explore an alternative approach to assessing their role, by characterising failure scars. We analyse a database of multiple rockfall scar surfaces detailing the areal extent, shape, and location of broken rock bridges and weathered surfaces. Terrestrial laser scanning and gigapixel imagery were combined to record the detailed texture and surface morphology. From this, scar surfaces were mapped via automated classification based on RGB pixel values.Our analysis of the resulting data from scars on the North Yorkshire coast (UK) indicates a wide variation in both weathering and rock bridge properties, controlled by lithology and associated rock mass structure. Importantly, the proportion of rock bridges in a rockfall failure surface does not increase with failure size. Rather larger failures display fracturing through multiple rock bridges, and in contrast smaller failures fracture occurs only through a single critical rock bridge. This holds implications for how failure mechanisms change with rockfall size and shape. Additionally, the location of rock bridges with respect to the geometry of an incipient rockfall is shown to determine failure mode. Weathering can occur both along discontinuity surfaces and previously broken rock bridges, indicating the sequential stages of progressively detaching rockfall. Our findings have wider implications for hazard assessment where rock slope stability is dependent on the nature of rock bridges, how this is accounted for in slope stability modelling, and the implications of rock bridges on long-term rock slope evolution.

Sedimentary characteristics and processes of submarine mass-transport deposits in the Ulleung Basin and their relations to seismic and sediment physical properties

Three successive Pliocene MTD units (M2, M3, and M4 in ascending order) in the Ulleung Basin are characterized by continuous strong-amplitude negative basal reflections with transparent to chaotic internal seismic facies. Mapping of these MTD units in a dense 2-D grid of seismic reflection data reveals that each MTD unit generally thickens upslope to a maximum of 113m and wedges out downslope against a structurally uplifted pre-existing surface. Although the source failure scars cannot be traced on the slope, the minimum runout distances of the MTD units from the base of the upper slope near the 1000m isobath are estimated to be >120km.Each MTD unit is characterized in the logging-while-drilling (LWD) data by a gradual increase in bulk density, P-wave velocity, and resistivity values at the basal part and an abrupt decrease at the lower boundary. Nine sedimentary facies are identified in the MTD units, which indicate a variety of mass-transport processes such as sliding with brittle to plastic deformations and high-to low-viscosity debris flows. Vertical distribution of sedimentary facies shows a repeated generalized pattern in all MTD units. The pattern is interpreted to represent initial deep failures involving sand-prone consolidated sediments near the shelf break and successive shallow failures involving soft heterogeneous muddy sediments that were deposited in a deeper downslope area. The presence of sand-prone mass-transport deposits accounts for the basal densification and strong-amplitude negative basal reflections. Absence of such sand-prone basal parts in MTDs at other locales implies unpredictability of lithologic characteristics in MTDs with similar seismic reflection signatures.

Catastrophic Rock-Slope Failures in NW Scotland: Quantitative Analysis and Implications

ABSTRACT126 postglacial catastrophic rock-slope failures (PRSFs) and 66 debris-free failure scarps (DFFSs) were identified using Google Earth™ imagery on Lewisian Gneiss (LG), Torridon Sandstone (TS), Cambrian Quartzite (CQ) and Moine Schist (MS) terrains in NW Scotland. Normalisation of these data for area and gradient shows that RSF density on slopes ≥25° decreases in the sequence MS > LG ≈ CQ > TS. Most are apparently discordant with structure, suggesting that failure along slope-(sub)parallel joints was more common than failure along bedding or foliation planes. DFFSs represent failure scars from which runout debris has been removed by later glaciation. Eighteen were recorded outside the limits of the Loch Lomond Readvance, implying that they survived the last ice-sheet glaciation, and support the view that many steep upper rock slopes have been dominated by gravity-driven rock detachment rather than glacial erosion. We argue that many RSFs were probably triggered by Lateglacial earthquakes due to fault (re)activation by crustal rebound, as 74% of recorded PRSFs and DFFSs are located at or near slope crests. 44% of recorded PRSFs and DFFSs occur in cirques, suggesting that cirque enlargement during successive glacial-interglacial cycles involved deepening by glacial erosion, headwall retreat by rock-slope failure and removal of RSF debris during subsequent glaciation.

A channel evolution model for subtropical macrochannel systems

A channel evolution model (CEM) represents stages of channel development in response to specific types of disturbance. In recent years, classic incised/disturbed CEMs have provided process-based understanding of channel adjustment and formed the cornerstone for river restoration and rehabilitation. While broadly applicable to alluvial systems in temperate and semi-arid regions, these models cannot be assumed to be universally applicable. Lockyer Creek in South East Queensland, Australia, has notable macrochannel morphology and is subject to high hydrological variability typical of many subtropical climates. The aim of this paper is to present a case study of channel adjustment and evolution in lower Lockyer Creek, to determine if existing CEMs adequately describe processes of channel adjustment and the associated trajectories of change typical of river systems in subtropical settings. Lockyer Creek has recently been subjected to a spate of flooding resulting in significant channel erosion. This offers an ideal opportunity to investigate the nature and rate of channel adjustment processes and place them in context of longer-term geomorphic adjustments in these systems. Specifically we address two questions. Firstly, do the classic incised/disturbed CEMs adequately represent the observed macrochannel adjustment? Secondly, if current CEMs are inadequate, what is the channel evolution model for these systems, of which lower Lockyer Creek is an example? Results show that these are non-incising systems where wet-flow bank mass failures (WBMFs) are the dominant process of channel adjustment. They occur within the channel bank top boundary resulting in no change to overall bank-top width. Furthermore, subsequent floods deposit sediment in the failure scars and failure headwalls generally do not retreat beyond channel bank-top. Channel adjustment has not followed the evolutionary stages for incised/disturbed channels and a new four stage macrochannel CEM is outlined for these subtropical systems. The proposed CEM illustrates a cyclical pattern of erosion by channel bank WBMF followed by re-aggradation, through deposition and oblique processes, contributing to bank rebuilding. This CEM provides sufficient information to determine the stage of macrochannel adjustment, enabling decisions to be made over whether intervention is required or will be successful.

Mass movements in small canyons in the northeast of Baiyun deepwater area, north of the South China Sea

The process of mass movements and their consequent turbidity currents in large submarine canyons has been widely reported, however, little attention was paid to that in small canyons. In this paper, we document mass movements in small submarine canyons in the northeast of Baiyun deepwater area, north of the South China Sea (SCS), and their strong effects on the evolution of the canyons based on geophysical data. Submarine canyons in the study area arrange closely below the shelf break zone which was at the depth of–500 m. Within submarine canyons, seabed surface was covered with amounts of failure scars resulted from past small-sized landslides. A complex process of mass transportation in the canyons is indicated by three directions of mass movements. Recent mass movement deposits in the canyons exhibit translucent reflections or parallel reflections which represent the brittle deformation and the plastic deformation, respectively. The area of most landslides in the canyons is less than 3 km2. The trigger mechanisms for mass movements in the study area are gravitational overloading, slope angle and weak properties of soil. Geophysical data indicate that the genesis of submarine canyons is the erosion of mass movements and consequent turbidity currents. The significant effects of mass movements on canyon are incision and sediment transportation at the erosion phases and fillings supply at the fill phases. This research will be helpful for the geological risk assessments and understanding the sediment transportation in the northern margin of the SCS.

Mapping Submarine Landslide: A Case Study in Northern Continental Slope of the South China Sea

Geographical Information Systems is a useful tool for marine geohazard mapping. This paper describes an integrated and systematic map-based approach for identification and characterization of submarine landslide with multidisciplinary data such as multibeam bathymetric data, multichannel seismic data, sidescan sonar and so on. Taking Liwan 3- 1 gas field in northern continental slope of Southern China Sea as a case study, a regional database is established to provide convenient services to manage search and extract the target data for geohazard mapping and identification. Bathymetric map and seismo-geomorphologic map are calculated and produced with GIS to establish an elementary understanding of geomorphologic features. With this integrated approach, dozens of landslides are identified in the study area and they are featured as small and multiple superimposed failure scars. Fundamental information of landslides can also be provided for further detail interpretation, quantification and assessment of their mechanism and potential risks.

Canyon morphology on a modern carbonate slope of the Bahamas: Evidence of regional tectonic tilting

New high-quality multibeam data presented here depict the northern slope of the Little Bahama Bank (Bahamas). The survey reveals the details of large- and small-scale morphologies that look like siliciclastic systems at a smaller scale, including large-scale slope failure scars and canyon morphologies, previously interpreted as gullies and creep lobes. The slope exhibits mature turbidite systems built by mass-fl ow events and turbidity currents. The sediment transport processes are probably more complex than expected. Slope failures show sinuous head scarps with various sizes, and most of the scars are fi lled with recent sediment. Canyons have amphitheater-shaped heads resulting from coalescing slump scars, and are fl oored by terraces that are interpreted as slump deposits. Canyons rapidly open on a short channel and a depositional fan-shaped lobe. The entire system extends for ~40 km. The development of these small turbidite systems, similar to siliciclastic systems, is due to the lack of cementation related to alongshore current energy forcing the transport of fi ne particles and fl ow differentiation. Detailed analyses of bathymetric data show that the canyon and failurescar morphology and geometry vary following a west-east trend along the bank slope. The changing parameters are canyon length and width, depth of incision, and canyon and channel sinuosity. Accordingly, failure scars are larger and deeper eastward. These observations are consistent with a westward tectonic tilt of the bank during the Cenozoic.

Hydrological Environment in Subsurface Steep Slope - Groundwater Flow Passageway on Slope Behind Kiyomizudera -

The hydrological environment must be understood before water flow can be adequately controlled to prevent slope failure without impacting unduly on the hydrological mountain slope environment. We conducted field studies to determine current sites and measurement of ground temperature 1 meter deep to clarify groundwater flow passageways on the slope behind the cultural heritage temple Kiyomizudera in Kyoto. Results showed anomalous temperature 1 meter deep bands on the slope and several springs that are extensions of these bands. Several of these bands coincide with terrain deformations such as gullies and slope failure scars indicating the probability of relationships between groundwater flow and topological deformation.

Recent sedimentary processes along the Makran trench (Makran active margin, off Pakistan)

A geophysical and geological survey (CHAMAK) has been carried out on the Makran accretionary wedge off Pakistan in order to understand the structure of the margin and the recent sedimentary processes in this self-maintaining prism disconnected from the modern Indus inputs (Qayyum et al., 1997; Gaedicke et al., 2002a; Schluter et al., 2002). Morphostructural analysis, based on the interpretation of bathymetric data and backscatter imagery, as well as a 3.5 kHz echo-character mapping, allow us to distinguish three structural domains, from north to south, where sedimentary processes differ: (1) the accretionary wedge to the north, (2) the trench and (3) the northern Murray Ridge at the seaward edge of the trench. The accretionary wedge is cut by canyons responsible for an important erosion of the prism especially in the eastern part of the wedge. Within the trench, sediments transported by the canyons generate sediment waves and are transported westward, parallel to the E–W axis of the trench. The eastern part of the abyssal plain is eroded by strong turbidity currents whereas important sediment deposition occurs in the western part of the abyssal plain, as a consequence of a decrease in the current energy. Nearly no mass transport deposits are recognized in the study area except near the ridges forming the accretionary wedge. Small-scale slope failure scars are described. The prevalence of turbiditic processes and the existence of a morphological barrier formed by the Murray Ridge allow the confinement of turbidites within the trench. Migrating sediment waves seem to be common sedimentary structures in this setting. These features might be produced by important velocity decrease of turbidity currents when reaching the trench.

Size distribution of submarine landslides along the U.S. Atlantic margin

Assessment of the probability for destructive landslide-generated tsunamis depends on the knowledge of the number, size, and frequency of large submarine landslides. This paper investigates the size distribution of submarine landslides along the U.S. Atlantic continental slope and rise using the size of the landslide source regions (landslide failure scars). Landslide scars along the margin identified in a detailed bathymetric Digital Elevation Model (DEM) have areas that range between 0.89 km 2 and 2410 km 2 and volumes between 0.002 km 3 and 179 km 3. The area to volume relationship of these failure scars is almost linear (inverse power-law exponent close to 1), suggesting a fairly uniform failure thickness of a few 10s of meters in each event, with only rare, deep excavating landslides. The cumulative volume distribution of the failure scars is very well described by a log–normal distribution rather than by an inverse power-law, the most commonly used distribution for both subaerial and submarine landslides. A log–normal distribution centered on a volume of 0.86 km 3 may indicate that landslides preferentially mobilize a moderate amount of material (on the order of 1 km 3), rather than large landslides or very small ones. Alternatively, the log–normal distribution may reflect an inverse power law distribution modified by a size-dependent probability of observing landslide scars in the bathymetry data. If the latter is the case, an inverse power-law distribution with an exponent of 1.3 ± 0.3, modified by a size-dependent conditional probability of identifying more failure scars with increasing landslide size, fits the observed size distribution. This exponent value is similar to the predicted exponent of 1.2 ± 0.3 for subaerial landslides in unconsolidated material. Both the log–normal and modified inverse power-law distributions of the observed failure scar volumes suggest that large landslides, which have the greatest potential to generate damaging tsunamis, occur infrequently along the margin.

Mass movement‐induced fold‐and‐thrust belt structures in unconsolidated sediments in Lake Lucerne (Switzerland)

AbstractHigh‐resolution seismic imaging and piston coring in Lake Lucerne, Switzerland, have revealed surprising deformation structures in flat‐lying, unconsolidated sediment at the foot of subaqueous slopes. These deformation structures appear beneath wedges of massflow deposits and resemble fold‐and‐thrust belts with basal décollement surfaces. The deformation is interpreted as the result of gravity spreading induced by loading of the slope‐adjacent lake floor during massflow deposition. This study investigated four earthquake‐triggered lateral mass‐movement deposits in Lake Lucerne affecting four sections of the lake floor with areas ranging from 0·25 to 6·5 km2 in area. Up to 6 m thick sediment packages draping the subaqueous slopes slid along the acoustic basement. The resulting failure scars typically lie in water depths of >30 m on slopes characterized by downward steepening and inclinations of >10°. From the base‐of‐slope to several hundred metres out onto the flat plains, the wedges of massflow deposits overlie deeply (10–20 m) deformed basin‐plain sediment characterized by soft sediment fold‐and‐thrust belts with arcuate strikes and pronounced frontal thrusts. The intensity of deformation decreases towards the more external parts of the massflow wedges. Beyond the frontal thrust, the overridden lake floor remains mostly undisturbed. Geometrical relationships between massflow deposits and the deformed basin‐plain sediment indicate that deformation occurred mainly during massflow deposition. Gravity spreading induced by the successive collapse of the growing slope‐adjacent massflow wedge is proposed as the driving mechanism for the deformation. The geometry of fjord‐type lakes with sharp lower slope breaks favours the deposition of thick, basin‐marginal massflow wedges, that effectively load and deform the underlying sediment. In the centre of the basins, the two largest massflow deposits described are directly overlain by thick contained (mega‐)turbidites, interpreted as combined products of the suspension clouds set up by subaqueous mass movements and related tsunami and seiche waves.

Near-surface geology and sediment-failure geohazards of the central Scotian Slope

The central Scotian Slope demonstrates a complex seafloor morphology superimposed on a regional gradient of 24 across the margin. The west-central Scotian Slope is characterized by a relatively smooth seafloor, but with numerous 1080-m (33260-ft)-high escarpments representing slide failure scars. In the east, the seafloor is highly dissected by canyons. Throughout the region are scars and deposits of sediment mass failures, including retrogressive headwalls, rotational slumps, slides, creep, debris-flow deposits, and turbidites.The complexity of failure styles and triggering mechanisms identified underscores the need for comprehensive site assessments for situating seabed facilities. Critical factors that need to be taken into account include local terrain analysis and shallow subbottom stratigraphy. Slope-stability analysis has shown the surface sediment to be statically stable, except on steep escarpments and canyon walls. There is evidence, however, of sediment failures that approximately correlate to glacial advances (2512, 75, and 130 ka), providing some clue as to potential triggering mechanisms. Sparse, passive-margin, tectonic earthquakes, however, are the likely cause for large-scale, regionally correlated failures and failure escarpments.

Submarine slides in the eastern continental margin, Korea

Abstract Closely spaced seismic‐reflection profiles across the eastern continental margin of Korea reveal that large‐scale slope failure scars occur ubiquitously on steep slopes (gradient, >5°) in water depths greater than 400 m. The slope failure scars are well defined with irregular scour planes and sometimes coalesce to form a large escarpment (maximum area, 275 km2 ). The failed material was deposited farther downslope as slide and mass‐flow deposits. These submarine slope failures are probably due to both seismicity associated with faulting and the steep gradient of the slope.