Submarine Mass Movements and Their Consequences

Current liquefaction analyses of slopes are generally based on undrained soil response. Recent experimental studies have shown that small net flow of water into an element will result in additional pore pressure generation and further reduce its strength. Many flow slides have occurred in submarine slopes, most of which were induced by monotonic loadings. Tidal variations can cause unequal pore pressure generation with depth in unsaturated seabed soils. An effective stress approach is presented to model flow liquefaction of sand under a range of drainage conditions to evaluate the triggering of liquefaction during low tides for an unsaturated underwater slope.

To simulate earthquake-induced, retrogressive submarine slope failures, element removal capabilities of a finite element program are used to model a soil mass that fails and then flows away, causing upper parts of slope to fail retrogressively due to loss of support. It is explained how an initial failure leads to subsequent failures of a flat or gently sloping seafloor. Effects of a silt layer and gently sloping seafloor on the extension of retrogression in a sand deposit are studied. The extension of failure increases significantly for a gently sloping seafloor with the presence of a silt layer.

A lack of data representing natural phenomena, that give rise to transformation of the continental slope appears to be the most serious challenge in obtaining the full pattern of the process. The latest advanced instruments have insured more intensive, though not yet enough systematic observations of the submarine mass movement. Therefore, it seems reasonable to seek for indirect means of evaluation of sediment dynamics on the shelf and continental slopes by specially derived equations. An article deals with the study of statistical and dinamical conditions and some criterion of stability of the sedimentary forms on the submarine slope in deltaic areas.

COSTA is addressing the questions of why seafloor slope failures occur where they do, and with what frequency they occur. The original program has been complemented by COSTA Canada. One of the tasks involves the study of the initiation of slope instability through numerical and centrifuge modelling.This paper reviews previous centrifuge studies related to submarine slope failure and presents the preparations for this task. The initiation of submarine slope instability has been attributed to triggers such as earthquakes, erosion, oversteepening, wave loading, and sedimentation. Centrifuge modelling has been used to simulate most of these loading conditions in similar boundary value problems.

Loose submarine sand deposits, such as those found in the Fraser River Delta, Canada, are often susceptible to flow or cyclic liquefaction. These deltaic soils often contain both free and dissolved gas, most commonly methane, which will alter the soil strength and hence its potential for liquefaction. This paper evaluates the liquefaction resistance of loose gassy sands using laboratory and constitutive modeling results and presents a preliminary method for evaluating the liquefaction potential of gassy sands.

This paper presents the results of a laboratory testing program on the influence of stratification on cyclic strength of soil samples. Reference undrained cyclic triaxial tests were conducted on fine Ottawa sand samples and a much finer silica silt sample. Both samples were prepared by pluviation under water. Undrained cyclic triaxial tests conducted on stratified sand-silt samples revealed that layering induced a much lower cyclic resistance than that developed in either of the materials. Differential pore pressure generation in each soil unit suggest that water migration occurred from the sand layer to the silt layer and caused this strength reduction. The experimental data have significant implication for field conditions, especially for submarine slopes.

With the growing development of offshore natural resources, use of sea-floor transport and communication routes, considerations for the environment and the effects of global climate changes, and will for protecting populations and their infrastructures, the need for assessing risk associated with submarine mass movements is increasing. The present paper proposes an approach for the assessment of risk associated with submarine mass movements based on geotechnical characterisation.

This paper presents a finite element analysis of a landslide that occurred in a newly dredged submarine slope for port development. The landslide was a multiple retrogressive landslide. The marine slope comprised recent marine and river mouth delta deposits. The landslide occupied a plan area 200 m long and 150 m wide. A volume of about 240,000 m3 soil slipped into a newly dredged open space in the sea. The finite element analysis indicates that the backfill pre-loading significantly increased the failure zone in the dredged slope and the soil lateral displacements toward to the open space in the sea.

A mathematical model is derived to describe the generation and propagation of water waves by a submarine landslide in two horizontal dimensions (2HD). The model consists of a depth-integrated continuity equation and a vector momentum equation, in which the ground movement is the forcing function. These equations include high nonlinear, but weak frequency dispersion effects. The model is capable of describing wave propagation from relatively deep water to shallow water. These Boussinesq-type equations are coupled with a moving boundary algorithm, allowing for the prediction of wave runup along the shoreline. The numerical model is able to accurately simulate the tsunamis generated by subaerial slides, as is shown by comparison with an analytical solution. A variety of 2HD slide setups will be run, including both submerged and subaerial slides. The maximum vertical thickness of the slide, as well as the length to width ratio are varied for a number of different slopes, and the results will be discussed.

A series of developments that led to both new understandings of mass gravity flows in the marine environment and in the techniques to quantitatively analyze them. Methods have been developed that exploit the newly emerging technical capabilities to perform very precise and detailed field investigations in deep continental slope settings that support numerical models which simulate the flows and their deposits. This combination of technologies provides a means to create engineering design criteria for mass gravity flows. In this paper, the methods that have been developed for these analyses are described and illustrated with examples.

The propagation of solidified zones in the course of subaqueous liquefied sediment flow is theoretically discussed. The coupled systems of Navier-Stokes equations and consolidation equation are numerically solved under moving boundary conditions, with consideration of the concurrent evolutions of the flow surface as well as of the internally formed interface between the fluid and solidified zones. The analyses for collapse of a body of liquefied soil into ambient fluid under gravity show that the liquefied sediment flow manifests itself as a decelerating gravity flow due to the dynamic interaction between the flowing liquefied soil and the progressively solidified zones in the sediment.

We present a theoretical study of the thermodynamic chemical equilibrium of gas hydrate in soil by taking into account the influence of temperature, pressure and pore water chemistry. The second part of the paper shows an application of the model through a back-analysis of the giant Storegga Slide on the Norwegian margin. Two of the most important changes during and since the last deglaciation (hydrostatic pressure due to the change of the sea level and the increase of the sea water temperature) were considered in the calculation.

Large-scale laboratory experiments were conducted measuring the waves generated by two wedge-shaped bodies sliding down a 2:1 slope. The waves near the body propagating offshore and the run-up generated on the slope were measured. It was determined that the maximum “draw-down” wave over the body was essentially generated before the body had moved more than one-half its length downslope. In addition to shape, the important parameters that controlled the run-up were the initial submergence and the initial acceleration of the body.

The nature of the tsunami sources (displacement, duration, area and volume) is reviewed for selected past earthquakes, slumps and slides, from the point of view of tsunami generation. It is concluded that slides and slumps differ significantly form earthquakes in: (1) the velocity of spreading, (2) the balance of the uplifted material, and (3) the vertical displacements per unit horizontal area.

Abundant mass-transport deposits characterize the seafloor of the northwestern continental slope and rise of Gulf of Mexico. Most of the deposits reveal a cohesive nature, consisting mainly of a silty-clay matrix with dominant to rare mud-clasts. Five types of cohesive gravity-flow deposits have been distinguished, based on their structural characteristics, nature of the mud matrix, and mud-clast percentage. Each type of the cohesive gravity-flow deposits implies different rheological behavior for the flows from which they originate, depending on their viscosity, mud-clast abundance, and external factors (e.g. hydroplaning), thus revealing that these deposits have resulted from a wide range of different types of flows.

We illustrate and compare several natural examples of recent submarine mass movements along the NW Mediterranean margin and discuss the respective role of the main parameters controlling slope failures, including slope angle and high pore-fluid pressure. We then present results of pilot experiments testing how an increase in pore fluid pressure can trigger spontaneous downslope gliding of sediments along the margin’s slope.

The paper presents a model describing the mechanical behaviour of gassy soil under drained and undrained conditions. Special attention was devoted to the mechanical behaviour of gassy soil during undrained unloading. Simulation results show that for gassy soil a decrease of the total stress under undrained conditions induces a decrease of the shear soil resistance.

Characteristics of water waves caused by subaerially generated debris flows vary with distance from the debris-flow entry point. Three hydrodynamically distinct regions (splash zone, near field, and far field) may be identified. Experiments demonstrate that characteristics of the near-field water wave-the only coherent wave to emerge from the splash zone-depend primarily on submerged volume and submerged travel time of the debris flow and on water depth where debris-flow motion stops. Near-field wave characteristics commonly may be used as a proxy source for computational tsunami propagation. An example application explores hazards associated with potential debris flows entering a reservoir.

Probabilistic analyses of underwater landslides can yield probability distributions of landslide tsunami amplitudes. About 35% of all earthquakes may generate landslide tsunamis with a maximum tsunami amplitude that surpasses vertical coseismic displacement. A finite probability exists for underwater landslides to generate tsunamis with amplitudes in excess of 10 meters, as in the 1998 Papua New Guinea event. Indicators of prospective tsunamigenic landslides, such as sediment shear strength, improve our ability to predict future events and to assess their impact on coastal populations and infrastructure. Probabilistic analyses may play an important role in tsunami risk assessment from landslide tsunamis.

This paper introduces three advanced laboratory testing systems for measuring the stress-strain-strength behavior of soils. The three systems are: (a)a new Double Cell Triaxial System (DCTS) for continuous measurement of the volume change of a gaseous (unsaturated or saturated) soil specimen in triaxial testing (this system is developed by the author).(b)a Hollow Cylinder Apparatus (HCA) for measuring the behavior of a hollow soil specimen under conditions of pure shearing, plain strain, rotation of the middle principle stress, etc., and(c)a Truly Triaxial System (TTS) for measuring the behavior of a brick-shape soil specimen under independent control of the three principle stresses. Calibration results on the DCTS are presented and discussed.

In this paper a numerical model for predicting waves generated by nearshore submarine mass-movements is described. The model is based on the Reynolds Averaged Navier Stokes (RANS) equations with the k - ε turbulence model. The volume of fluid (VOF) method is employed to track the free surface. The submarine mass movement is prescribed. Numerical results obtained from the present model are validated with laboratory experiments and analytical solutions. Very good agreements are observed.

The Newmark method for predicting seismically induced displacement of slopes is enhanced by introducing of the yield (threshold) acceleration on the real soil strength, accounting for the effects of pore water pressure. Seismically induced pore water pressure build-up and its dissipation after the earthquake are calculated as a function of soil properties and ground motion characteristics, based on well-recognized practical methods. The proposed model provides more realistic predictions of slope displacements. It is validated based on centrifuge experimental results.

The Afen Slide lies in deep waters within the Faroe Shetland Channel (FSC) and was first recognised on TOBI data in 1996. Subsequently, it was observed on a 3D seismic survey that had been acquired in 1995. This paper presents the latest image of the slide generated from 3D seismic and the methodology used in attenuation of geophysical artefacts. It is demonstrated that 3D seismic has the potential to produce highly detailed images of seafloor features in deep-water areas comparable with swath or TOBI.

The Holocene Storegga Slide is the last of a series of slides occurring in the same area during the last 500ky. The objectives of the present paper are to present the current understanding of the trigger mechanisms and development of the Storegga Slide, and to show the link between the sliding and Pleistocene climatic fluctuations in the area. Instability is created by the rapid loading of fine-grained hemipelagic deposits and oozes by rapid glacial deposition during peak glaciations. Postglacial earthquake activity was the most likely trigger. Although slide development is complicated and involves a number of slide mechanisms and processes, the overall development is retrogressive, starting at the mid- to lower slope. Sliding stops when the headwall reaches the flat lying, overconsolidated glacial deposits of the shelf.

A huge slide (volume of 2400 km3 and run-out 450 km) was released in the Storegga area off the western coast of Norway during early Holocene, followed by numerous smaller debris flows. We perform numerical simulations of the giant slide using a Bingham model for the clay material. Agreement with present deposit distribution and run-out is found by assuming that the shear resistance between the debris flow and the seabed decreases during the flow, and we suggest sediment remolding or hydroplaning as possible explanations. Debris velocities are predicted and possible applications to the associated tsunami event are investigated.

Statistical analysis of the lobes of the Storegga slide reveals a power-law dependence of the runout distance on the release volume. For small to moderate volumes, visco-plastic models with a (remoulded) yield strength of about 10 kPa reproduce this dependence quite well, in contrast to granular-friction models. However, either progressive wetting of the bottom shear layer or hydroplaning has to be invoked to explain the extreme runout distance and the sediment distribution of the largest slide phase. Preliminary es timates of the turbidite volume put severe constraints on the formation rate and density of the turbidity currents accompanying the slide.

Downhole shear-strength profiles, obtained from cone-penetrometer and lab tests were tied to sedimentary facies from adjacent continuous cores. The geotechnical response of mass-transport deposits was investigated. In the Campos Basin, sediments have evacuated from the upper continental slope and have accumulated as folded deposits on the middle slope. Sediment removal is recognised by an abrupt step-like change in shear-strength at the level of the unconformity. The folded deposits are characterised by a belly-shaped increase in shear-strength coinciding with a zone of intense lamination within the deposit, induced by internal shearing and fluid loss (strain hardening). In contrast, highly disintegrated muddy debris-flow deposits are indistinguishable, in terms of shear-strength, from normal hemipelagic slope sediments.

The Traenadjupet Slide (14,100 km2) remobilised an up to 180 m thick package comprising late Weichselian glacigenic sediments and an underlying late Saalian — late Weichselian contourite drift. Rapid burial of the contourites and the presence of gas, is inferred to have caused development of excess pore pressure of the contourites which probably were the “weak layer” that initially failed. During triaxial compressional tests the contourite sediments show contractive behaviour and shear band development. Shear band development due to porewater pressure increase and liquefaction of contractive sediments is therefore regarded a possible mechanism for initial failure and sediment mobilisation of the Trænadjupet Slide.

Large-scale submarine slides occurred during the Holocene on the continental slope offshore mid-Norway, north and south of the Vøring Plateau. The Trænadjupet slide event that affected an area of 14100 km2 is located north of the Vøring Plateau. It occurred about 4,300 years B.P., 4000 years after the giant Storegga slide that affected an area of about 112,500 km2. A slope stability evaluation was performed in order to explain why the sliding took place on a very gentle slope (1 degree). This was done first with the deterministic approach using the Limit Equilibrium and the Finite Element methods, for static, pseudo-static and dynamic cases. Then the probabilistic approach was applied using the limit equilibrium method with the 1st and 2nd order reliability methods (FORM and SORM) and the Monte Carlo simulation to include the parameter uncertainties (soils parameters, seismic loading). The Finite Element modelling indicates that the slide triggering impacted preferably the upper 40 meters of the sediment column. The trigger could have been caused by one large earthquake of magnitude larger than M _s 5.8 (retrogressive failures) but cyclic loading due to several earthquakes could also explain the slide, affecting the shearing resistance in the NYK contourite drift unit (weak layer) by excess pore pressure generation.

As part of the STRATAFROM project, the Hudson Apron area was selected for a detailed slope stability analysis. Results indicate that high pore pressure is necessary to trigger a failure. Under normal conditions, an excess pore pressure of more than 90% would be required for failure. On the other end, the actual strength profile would indicate a remaining marginal stability. Aggravating factors were the high sedimentation rate cyclicity and esulting layering inducing high excess pore pressures, and potentially gas pressures and earthquakes.

Submarine slides varying from 0.002km3 to more than 360km3 have been identified in the Faroe — Shetland Channel using a wide range of surveys. Although not a major constraint to seabed use slides need to be considered as a potential geohazard. Dynamic loading of contouritic horizons is considered the triggering mechanism of these thin layer failures.

Published accounts and unpublished multibeam bathymetry and seismic-reflection profiles have been used to assemble a G.I.S. database documenting the geographic extent of surface and buried Quaternary submarine mass movements on the eastern Canadian margin. These range from small failures in fiords to enormous mass-transport deposits on the continental rise. The patterns of distribution are interpreted in terms of failure processes and large scale physiographic, geologic and glaciologic variability.

High-resolution seismic reflection profiles, multibeam bathymetry, piston cores, and biostratigraphy from petroleum wells are used to date submarine mass movements on the continental slope off southeastern Canada. Several different styles of mass movement are recognised in a variety of geological settings. The chronology allows evaluation of potential forcing processes, including earthquakes triggered by glacial loading on the continental shelf and dissociation of gas hydrates related to sea level or bottom-water-temperature changes.

Numerous sediment failures have contributed to the morphology of the flanks of the intraslope basins of the northwestern continental slope of Gulf of Mexico. The main cause of slope instabilities is the oversteepening of the flanks by halokinetic processes. High-resolution seismic information (3.5 kHz profiles and side-scan sonar imageries), long sediment cores, and high-resolution bathymetric maps, acquired from the Bryant Canyon area, northwest Gulf of Mexico, are presented in this paper. They provide an excellent example, and better insight into the morphology, generation, and evolution of slope instability processes in a complex deep-water environment.

The Afen slide submarine slope failure has comprehensive 2D and 3D seismic coverage as well as geotechnical data from sediment cores. This allows a detailed assessment of the phases, modes and characteristics of failure. These findings can be applied to the recognition and understanding of other submarine slope failures.

The Cumbre Vieja volcano, on La Palma in the western Canary Islands, is an unstable area that may develop into a future landslide, generating a tsunami that could cause damage far from the source. However, volcaniclastic turbidites that are directly correlated with the two most recent Canary Islands landslides, show stacked sub-units within a single turbidite bed. This may indicate multiple stages of landslide failure. Similar findings have previously been reported from volcaniclastic turbidites linked to Hawaiian landslides. Consequently the potential tsunami hazard from such failures may be lower than previously predicted.

This paper describes numerous submarine landslides in French Polynesia. This inventory shows an evolution of the landslide type with oceanic islands age. Submarine active volcanoes are subject to superficial landslides of fragmental material whereas young islands exhibit marks of mass wasting corresponding to giant lateral collapses producing debris avalanche during the period of volcanic activity. Later, erosional processes generate fine grained debris avalanches and carving the stellate morphology observed on atolls and guyots. In addition, the shape of Tupai atoll and Rurutu Island have been deeply modified by giant slumps that deeply modify their shape.

Many mass movement features have been mapped in the Monterey Bay region of central California. Most of these features have the appearance of being displaced by fluid flow. Therefore, fluids may have played a major role in facilitating mass movement along this tectonically active continental margin and will do so in the future. We selected three major areas of mass movement located within submarine canyons and modeled their potential to generate tsunamis. Run-up extent is dependent upon slump geometry, depth and size, and we believe that two could have produced tsunamis. The third area exhibits multiple retrogressive failures and future tsunami occurrences appear feasible.

Submarine landslides are becoming recognized as a potential source of damaging local tsunamis. However, there are presently few documented case studies of landslide events that have caused historic tsunamis or likely caused prehistoric tsunamis. We present three case studies of submarine landslide environments off the west coast of the United States, including Alaska. Each environment has been imaged using multibeam technology allowing excellent resolution of the morphology of the seafloor. Based on this imagery and the historic record, we document the character of these environments and the resulting tsunamis. In the case of one of the failures, we present a model of the motion of the landslide and the size of tsunami that this motion would have produced.

Characteristic features of landslide tsunamis are examined from a set of documented cases. The studied events affected a coastal zone of no more than 10-15 km at both sides of the slump area. The only two tsunamis that were observed over a considerably greater distance are the Izmit 1999 and particularly PNG 1998 tsunamis, both of them being possibly associated with co-seismic fault displacement component. Slump volume seems to control both maximum wave height and maximum length of affected coastline. A rapid quasi-exponential attenuation of wave heights with distance from the source is observed because of strong wave dispersion.

Sediments from Effingham Inlet, an anoxic fjord on the west coast of Vancouver Island, were studied for their possible relation to paleoseismic activity. Diatomaceous sediments carry a complex history of episodic turbidites and deformation structures set against a background of varved sedimentation. The turbidity currents were initiated as fjord sidewall slope failures many of which were earthquake-induced. Accurate dating of 24 turbidites over approximately 3,300 years of the inner basin record was obtained. Known earthquakes recorded by turbidites include the AD 1946 Vancouver Island earthquake and AD 1700 plate-boundary earthquake. Comparison of this event history and a similar record from Saanich Inlet (130 km away) yielded 8 contemporaneous slope failures over the past 1,500 years.

After considerable controversy over the origin of the July 1998 PNG tsunami, there is now a large body of evidence that supports a sediment slump offshore of the devastated area. Between 1999 and 2000, four surveys were carried out offshore of the affected area, acquiring bathymetry, sediment cores, 3.5kHz seismic, multi-channel seismic and seabed imagery. In 2001, the same area was surveyed using single channel seismic that has been used to interpret the northern margin of PNG and the internal architecture of the slump. The susceptibility to slumping of the area offshore of northern PNG can be more definitively assessed.

Mass movement is an important process controlling the Quaternary sedimentary structure of the Ebro continental slope. Seismic indicates that about 37% of the slope surface is affected by mass movement features, which are variable in distribution, type, size and morphology. Physical and geotechnical properties define two areas: upper slope and the lower slope. The geotechnical modelling only explains certain aspects of mass-movement features, but it is insufficient to explain their variability. In order to have a good knowledge of geotechnical and sedimentary characteristics of the area and a good approach to a future mapping of instability hazard it is required study individually each depositional environment and failure event.

Morpho-bathymetric and stratigraphic data reveal small-scale mud reliefs in the toe region of the late-Holocene mud wedge on the Adriatic shelf. The reliefs are elongate features with acoustically-transparent cores. They are present in two geologic settings: seaward of shore-parallel undulations within a thick mud wedge and seaward of basement highs where the mud wedge is thin. In both settings, the reliefs define clusters sub-perpendicular to the regional contours, possibly indicating an origin related to escape of fluids from an impregnated unit at the base of the late-Holocene wedge. Shore-parallel bottom-hugging currents appear to modify the reliefs following their episodic growth.

A morpho-stratigraphic study carried out on the Western Pontine Archipelago continental slope (Eastern Tyrrhenian Margin) revealed a suite of instability/erosional features producing the complete cannibalization of a whole span of continental slope, from the shelf break down to the abyssal plain (located at 3600 m w.d.); only 2% of the whole area (2.000 sq. km) is not affected by these processes. One of the main controlling factors seems to be the slope gradient that produce different instability features, ranging from gravity-driven failure (simple and complex slides) to gravity-flow (debris and grain flows).

Analysis of resedimented carbonate and terrigenous units in the middle Turonian to early Coniacian southern Provence Basin (southeastern France) permits us to interpret them as evolutionary gravity flow deposits. The carbonate units evolve from proximal megabreccias to thick debris flows and distal megaturbidites. The terrigenous units evolve from proximal deltaic foreset conglomerates to distal high- and low density turbidites, and locally to slides of huge olistoliths. The submarine mass movements are believed to originate from earthquakes and associated tsunamis linked to transtensive tectonics that controlled the opening of the southern Provence Basin at that time.

Sea-floor (multibeam bathymetry, multibeam-derived backscattering, and side-scan sonar data) and subsea-floor (very-high-resolution seismic reflection profiles) imagery has been obtained in a series of surveys from the BIG’95 debris flow source area and deposit in the Ebro continental slope and rise, Northwestern Mediterranean sea. Side-scan sonar imagery provides valuable information on the morphology of the headwall scar, bathymetry and backscattering are useful for imaging the deposit, while seismics show the internal structure and relationship with the underlying sediments. Together with cores obtained in the area, the existing data set for the BIG’95 is a powerful tool for the study of the morphology and dynamics of debris flows and for their further modelisation.

Submarine prodelta instabilities appear as a sequence of rotational block slumps, restricted to the Upper Holocene silty clay deposits (unit A). The individual slump blocks are about 80-150m long and 5-15m in thickness. Slip zones are associated with the deeper Lower Holocene and/or methane gas-charged sediments (unit B). Mud flows appear towards the steeper prodelta slopes. Calculation of slope stability with the Normalised-Soil-Parameter (NSP) method of the normally consolidated prodelta sediments (Lower Holocene unit B), indicates that instabilities could be induced by earthquake ground accelerations from 0.13-0.14 g. Somewhat higher accelerations (0.27-0.30 g) are able to initiate mass failures in overconsolidated prodelta sediments (Upper Holocene unit A). These estimations are in accordance with the observed sediment mass wasting processes. Since the regional expected peak ground acceleration range between 0.10-0.15 g. the prodelta slopes in W. Greece are highly unstable.

Very High Resolution 3D seismic data acquired on the Adriatic shelf image the internal geometry of the late-Holocene mud wedge in a transitional zone between shore-parallel undulated features and small-scale mud relief elongated perpendicular to the bathymetry. The 3D seismic approach applied to Very High Resolution studies allowed to image the internal structure of these features possibly related to either sediment failure or transport by current and to enhance the comprehension of geological problems of key importance.

Very High Resolution 3D seismic data collected on the Adriatic shelf define the smallscale geometric complexity of late Holocene deposits. Three groups of units have been distinguished from deeper to shallower: the Transgressive System Tract (TST), a condensed interval at the base of the Highstand System Tract (HST) and the progradational units of the HST. The HST deposits show large-scale shore-parallel undulations and shore-normal reliefs. The genesis of these features seems to be related to a combination of two mechanisms: 1) sediment deformation in relation with the condensed section and 2) depositional process related to shore parallel currents that increasingly interact with topography.

A detailed marine survey, in Antikyra bay, in the northern margin of the Corinth Gulf graben in Greece, was carried out to examine the distribution and dispersion of bauxite “red-mud” tailings which have been discharged since 1970 on the shelf at a depth of 100 m. The ‘red-mud’ tailings are transported to the basin floor by turbidity currents is a depth of about 800 m and at a distance of up to 17 km from the source. Over a period of 14 years, ten (10) turbidity flow events have occurred. The turbidity flows form small scattered sheet-like deposits. The deposits are usually lobe shaped, between 0.6 and 4 cm thick, and with an aerial coverage from 1-106 to 12.6106 m2. The turbidites overlap each other and cover a total area of 48 km2.

Two extensive mass-failure deposits originated during the late-Quaternary sea level rise on the eastern Tyrrhenian margin. The deposits that failed had markedly different architectures: offshore Cape Licosa, a shelf-margin low-stand wedge failed along its basal downlap surface; in Paola slope basin, extensive failure on the upper slope involved a few-m-thick mud drape and older consolidated units. Regardless of their geometric differences, both failures occurred close to an interval of accelerated lateQuaternary sea-level rise (ca.13.8 cal. kyr BP). This evidence suggests that rapid drowning of unconsolidated sediment resulted in increased water load; enhanced pore pressure played a role in favoring failure.

In this paper sedimentological, geotechnical, and age data from 7 piston cores of the continental slope and rise off the Ebro margin, North-western Mediterranean, are presented. The cores were obtained within and nearby an area that has undergone a major instability event occurred about 10 ka, known as the BIG’95. They show, at least, three distinct units, which are identified in relation to such event, namely a postlandslide, landslide and pre-landslide unit. Each one of these units shows distinct sedimentological and geotechnical characteristics interpreted in terms of depositional processes and consolidation history. The sedimentological and geotechnical data allows to infer that the BIG’95 is the result of a retrogressive slide and that the location of the channel levee complexes probably had a fundamental role in triggering the landslide, as well as controlling the location of the failure surface.

Free gas is evident on high resolution seismic profiles near the 1996 Finneidfjord submarine landslide, Norway. A bright reflector at about 6 m sub-seabed depth on intact terrain adjacent to the slide coincides with the initial failure surface. We hypothesise that the bright reflector comprises free gas collecting in relatively sandy layers, and that the free gas could have contributed to the generation of excess pore pressures and the initiation of the submarine landslide. Preliminary measurement of the bright layer reflection coefficient from seismic sections, and interpretation of the available geotechnical data, support this hypothesis.

The Pointe-du-Fort deposits have been previously interpreted as the result of a single sedimentation event triggered by the 1663 M_s ~7 Charlevoix earthquake. It has been proposed that these deposits represent the spread of a failed mass coming from the south fjord wall. This paper presents multibeam, seismic and CAT-scan imagery evidences revealing that the Pointe-du-Fort deposits are the result of a multiphase build-up which includes many episodes of erosion and sedimentation by debris flows. These processes were induced by bottom slope erosion caused by the passage of a major debris flow coming upstream from the surveyed area, which was previously triggered by the 1663 M_s ~7 earthquake.

Extensive submarine mass movements have been mapped in the upper reaches of the Saguenay Fjord, Québec, Canada. Our analysis indicates that they were most likely triggered by the 1663 earthquake which shook a large area in Eastern Canada. Signs of instabilities have been observed at various places and although the triggering mechanism is the same, the types of mass movements differ largely including slides, spreads and flow failures in the soft Holocene sediments. Seismic and morphological investigations have revealed the presence of a fault -like deformation possibly linked to terrestrial fractures suggesting that the epicentre of the 1663 earthquake may be located in the area.

The submarine flow-slide of Pointe-du-Fort is situated on the south shore of the Saguenay Fjord, near the mouth of the Baie des Ha!Ha!, Quebec, Canada. About 1.5 million m3 of material, constituted of clayey silt rhythmites and thin sand layers, were involves in the slide. Surface of rupture would have reach a till contact. Seismic surveys over the displaced mass revealed a multiphase accumulation of the debris. Stratigraphic position of the debris link the event to the 1663 (Ms~7) earthquake. The slide was approximately dated to be over 260 years old (from present day) using sedimentation rates.

The 1996 Finneidfjord slide was a combined submarine/subaerial, retrogressive flow/ quick clay slide that mobilized 1 mill, m3 of sediments and killed four people. Interpretation of the morphology of the submarine slide scar and the slide deposits combined with eyewitness observations suggest that the initial failure occurred on the steepest slope of the foreshore platform. Detachment along a weak layer within silty clay caused unloading, oversteepening and erosion and triggered retrogressive sliding. Outrunner blocks from the slide occur up to 1.6 km outside the main debris lobe. Excess hydrostatic pressure related to meteorological and anthropogenic influence is thought to have triggered the slide.