Abstract The consequences of subaqueous landslides have been at the forefront of societal conscience more than ever in the last few years, with devastating and fatal events in the Indonesian Archipelago making global news. The new research presented in this volume demonstrates the breadth of ongoing investigation into subaqueous landslides, and shows that while events like the recent ones can be devastating, they are smaller in scale than those Earth has experienced in the past. Understanding the spectrum of subaqueous landslide processes, and therefore the potential societal impact, requires research across all spatial and temporal scales. This volume delivers a compilation of state-of-the-art papers covering regional landslide databases, advanced techniques for in situ measurements, numerical modelling of processes and hazards.

Abstract Volcanic archipelagos are a source of numerous on- and offshore geohazards, including explosive eruptions and potentially tsunamigenic large-scale flank collapses. Fogo Island in the southern Cape Verdes is one of the most active volcanoes in the world, making it both prone to collapse (as evidenced by the c. 73 ka Monte Amarelo volcanic flank collapse), and a source of widely distributed tephra and volcanic material. The offshore distribution of the Monte Amarelo debris avalanche deposits and the surrounding volcaniclastic apron were previously mapped using only medium-resolution bathymetric data. Here, using recently acquired, higher-resolution acoustic data, we revisit Fogo's flank collapse and find evidence suggesting that the deposition of hummocky volcanic debris originating from the failed eastern flank most likely triggered the contemporaneous, multi-phase failure of pre-existing seafloor sediments. Additionally, we identify, for the first time, multiple mass-transport deposits in the southern part of the volcaniclastic apron of Fogo and Santiago based on the presence of acoustically chaotic deposits in parametric echo sounder data and volcaniclastic turbiditic sands in recovered cores. These preliminary findings indicate a long and complex history of instability on the southern slopes of Fogo and suggest that Fogo may have experienced multiple flank collapses.

Abstract Extensive evidence for submarine landslide failure is found along the east Australian continental margin. This paper assesses the sedimentological properties and models the failure event that created the Byron landslide scar, located on the SE Australian continental margin, c. 34 km off the coast of Byron Bay, New South Wales. Sedimentological analyses and dating (radiocarbon and biostratigraphic) were conducted on three gravity cores collected from within the Byron landslide scar. A paraconformity, identified in one of the three cores by a distinct colour change, was found to represent a distinct radiocarbon age gap of at least 25 ka and probably represents the detachment surface of the slide plane. The core-derived sediment properties for the Byron landslide scar were used to inform hydrodynamic modelling using the freely available numerical modelling software, Basilisk. Model results highlight the important role of sediment rheology on the tsunamigenic potential of the slide and on the resulting inundation along the east Australian coastline, therefore providing a greater understanding of the modern hazard posed by comparable future submarine landslide events for the east Australian coastline.

Abstract Sediment slumps are known to have generated important tsunamis such as the 1998 Papua New Guinea (PNG) and the 1929 Grand Banks events. Tsunami modellers commonly use solid blocks with short run-out distances to simulate these slumps. While such methods have the obvious advantage of being simple to use, they offer little or no insight into physical processes that drive the events. The importance of rotational slump motion to tsunamigenic potential is demonstrated in this study by employing a viscoplastic landslide model with Herschel–Bulkley rheology. A large number of simulations for different material properties and landslide configurations are carried out to link the slump's deformation, rheology, its translational and rotational kinematics, to its tsunami genesis. The yield strength of the slump is shown to be the primary material property that determines the tsunami genesis. This viscoplastic model is further employed to simulate the 1929 Grand Banks tsunami using updated geological source information. The results of this case study suggest that the viscoplastic model can be used to simulate complex slump-induced tsunami. The simulations of the 1929 Grand Banks event also indicate that a pure slump mechanism is more tsunamigenic than a corresponding translational landslide mechanism.

Abstract Palermo is a populous city situated on the northern coast of Sicily, bordered by the Tyrrhenian Sea. This central part of the Mediterranean Sea features dramatic bathymetry, numerous subaqueous landslides and is also the epicentre to many subaqueous earthquakes. As such, the region is an area prone to tsunamis. This investigation uses the Cornell Multi-Grid Coupled Tsunami (COMCOT) tsunami modelling package to simulate five near-field landslides, and five near-field earthquakes regarded as worst-case credible scenarios for Palermo. The seismic simulations produced waves on a very small scale, the largest being c. 5 cm at its maximum height, and none of the earthquake-generated tsunami waves produced any measurable inundation. The landslide simulations produced larger waves ranging from 1.9 to 12 m in maximum height, two of which resulted in inundation in areas surrounding the Port of Palermo. Sensitivity analysis identified that fault width and dislocation as well as landslide-specific gravity did have significant influence over maximum wave height, inundation and maximum run-up wave height. There are methodological issues limiting the extent to which this study forms a comprehensive tsunami hazard assessment of Palermo, such as gaps in bathymetric data, computational restrictions and lack of a probabilistic element. These issues are counteracted by the fact that this study does serve as a robust first step in identifying that landslides in the region may produce larger tsunami waves than earthquakes, and that the directionality of mass movement is critical in landslide-driven tsunami propagation in the southern Tyrrhenian region.

Abstract Although lake tsunamis constitute a rare peril, they have repeatedly occurred in peri-alpine lakes in the past. There are several documented historical examples of landslide-triggered tsunamis in Swiss lakes. However, fundamental information and workflows to rapidly quantify the lacustrine tsunami hazard for multiple lakes are missing so far. The fact that the shorelines of major peri-alpine lakes are densely populated underlines the need for a hazard assessment. Detailed assessments require high-resolution geophysical, geotechnical and sedimentological data, and considerable computation time. Due to the involved data acquisition and calculation efforts, such assessments are mainly conducted as detailed case studies for single lakes. We present a workflow for a rapid first-order estimation of the landslide-triggered tsunami hazard along the shores of peri-alpine lakes. A crucial step is the identification of potential tsunami sources. Unstable slopes are mapped automatically, based on parameters that are derived from past studies. Such parameters include the bathymetry and derived parameters, and type, characteristics and thickness of the sediments. Wave amplitudes are estimated with existing predictive equations, based on the constructed maps of potentially unstable slopes. The results may be used for focusing more detailed, lake-specific tsunami-hazard assessments in respective areas.

Abstract Characterized by an active margin to the west, passive margins to the east and north, and numerous fjords and estuaries, the seafloor of Canada is prone to subaqueous landslides. The Geological Survey of Canada (GSC) facilitates government response in times of crisis by providing timely and concise information to Canadians, and informs the strategies to address natural hazards. Thus, the GSC is conducting a national assessment of the subaqueous landslide hazard. This paper reviews dozens of major subaqueous mass movement deposits with an emphasis on recent publications and summarizes the attempt to produce a national database. The types range from ephemeral turbidity current deposits to very large deposits (>100 km 3 ). To date, 1266 deposits are identified with many more expected as mapping progresses. This work is important as it will feed into the larger national tsunami strategy, and is a step forward for the national government to manage the risk. Canada is among the first countries to enter its entire database using the consistent morphometric characterization recommended by members of the UNESCO IGCP-640 (S4SLIDE) Community.

Abstract The subduction of large and heterogeneous mass-transport deposits (MTDs) is discussed to modify the structure and physical state of the plate boundary and therewith exert an influence on seismicity in convergent margins. Understanding which subduction-zone architectures and structural boundary conditions favour the subduction of MTDs, primarily deposited in oceanic trenches, is therefore highly significant. We use bathymetric and seismic reflection data from modern convergent margins to show that a large landslide volume and long runout, in concert with thin trench sediments, increase the chances for an MTD to become subducted. In regions where the plate boundary develops within the upper plate or at its base (non-accretionary margins), and in little-sedimented trenches (sediment thickness <2 km), an MTD has the highest potential to become subducted, particularly when characterized by a long runout. On the contrary, in the case of a heavily sedimented trench (sediment thickness >4 km) and short runout, an MTD will only be subducted if the thickness of subducting sediments is higher than the thickness of sediments under the MTD. The results allow identification of convergent margins where MTDs are preferentially subducted and thus potentially alter plate-boundary seismicity.

Abstract Subaqueous mass-transport deposits (MTDs) can be important elements in hydrocarbon systems, forming potential reservoirs or seals. Most research has targeted outcrops or moderately to deeply buried MTDs and, therefore, the petrophysical properties of near-seafloor MTDs, and their influence in the trapping and release of shallow fluids, is poorly studied. Here, we investigate shallow MTDs in Lake Villarrica (Chile) by combining sub-bottom profiles, free-fall penetrometer data, pore pressure dissipation tests and geotechnical properties of sediment cores. Low undrained shear strength under a surficial MTD indicates underconsolidation caused by sudden loading and rapid sealing. Larger, buried MTDs show acoustic signatures of free gas at their base, indicating effective sealing. This is supported by degassing core gaps just below MTDs and by excess pore pressure ratios c . 30–70% within MTDs. Acoustic windows below rafted blocks suggest local fluid escape. MTDs exhibit elevated undrained shear strength and reduced porosity compared to surrounding sediments, but are comparable to upslope source sequences. This suggests that MTD sealing capacity in Villarrica relates to the apparently overconsolidated nature of the slope sequence, leaving a minor role for shear densification. This study shows that shallow MTDs can form a relatively rapid seal for fluid migration, locally degraded by rafted blocks.

Abstract Three-dimensional (3D) seismic data reveal the complex interplay between the surface topography of a c. 4405 km 3 mass transport deposit (MTD) and overlying sedimentary packages over approximately the last two million years. The data image part of the Pleistocene to recent shelf to slope to basin-floor Giant Foresets Formation in offshore western New Zealand. The MTD created substantive topographic relief and rugosity at the contemporaneous seabed, formed by the presence of a shallow basal detachment surface, and very large (up to 200 m high) intact slide blocks, respectively. Sediments were initially deflected away from high-relief MTD topography and confined in low areas. With time, the MTD was progressively healed by a series of broadly offset-stacked and increasingly unconfined packages comprised of many channel bodies and their distributary complexes. Positive topography formed by the channels and their distributary complexes further modified the seafloor and influenced the location of subsequent sediment deposition. Channel sinuosity increased over time, interpreted as the result of topographic healing and reduced seafloor gradients. The rate of sediment supply is likely to have been non-uniform, reflecting tectonic pulses across the region. Sediments were routed into deep water via slope-confined channels that originated shortly before emplacement of the MTD.

Abstract Contourite drifts are sediment deposits formed by ocean bottom currents on continental slopes worldwide. Although it has become increasingly apparent that contourites are often prone to slope failure, the physical controls on slope instability remain unclear. This study presents high-resolution sedimentological, geochemical and geotechnical analyses of sediments to better understand the physical controls on slope failure that occurred within a sheeted contourite drift within the Faroe–Shetland Channel. We aim to identify and characterize the failure plane of the late Quaternary landslide (the AFEN Slide), and explain its location within the sheeted drift stratigraphy. The analyses reveal abrupt lithological contrasts characterized by distinct changes in physical, geochemical and geotechnical properties. Our findings indicate that the AFEN Slide likely initiated along a distinct lithological interface, between overlying sandy contouritic sediments and softer underlying mud-rich sediments. These lithological contrasts are interpreted to relate to climatically controlled variations in sediment input and bottom current intensity. Similar lithological contrasts are likely to be common within contourite drifts at many other oceanic gateways worldwide; hence our findings are likely to apply more widely. As we demonstrate here, recognition of such contrasts requires multi-disciplinary data over the depth range of stratigraphy that is potentially prone to slope failure.

Abstract The Makassar Strait is an important oceanic gateway, through which the main branch of the Indonesian Throughflow (ITF) transports water from the Pacific to the Indian Ocean. This study identifies a number of moderate (>10 km 3 ) to giant (up to 650 km 3 ) mass transport deposits within the Makassar North Basin Pleistocene–Recent section. The majority of submarine landslides that formed these deposits originated from the Mahakam pro-delta, with the largest skewed to the south. We see clear evidence for ocean-current erosion, lateral transport and contourite deposition across the upper slope. This suggests that the ITF is acting as an along-slope conveyor belt, transporting sediment to the south of the delta, where rapid sedimentation rates and slope oversteepening results in recurring submarine landslides. A frequency for the >100 km 3 failures is tentatively proposed at 0.5 Ma, with smaller events occurring at least every 160 ka. This area is therefore potentially prone to tsunamis generated from these submarine landslides. We identify a disparity between historical fault rupture-triggered tsunamis (located along the Palu-Koro fault zone) and the distribution of mass transport deposits in the subsurface. If these newly identified mass failures are tsunamigenic, they may represent a previously overlooked hazard in the region.

Abstract Coarse-grained deltas are often characterized by steep foreslopes (often more than 10°) that are traversed by delta-front channels. The channels thus erode into relatively steeply inclined bedding. In this context, the slopes flanking the channels can be steeper than the friction angle since they include a component of dip related to the delta-front slope as well as the channel-related erosion slope. In this study, part of the Busu River delta (Papua New Guinea) was imaged using a high-resolution multibeam bathymetry survey over an area where the angle of the slopes flanking the channels locally reaches 50°. A detailed analysis of the delta slope morphology has revealed an additional source of instability due to erosion within the main channels. In some places, erosion cuts into the channel flank forming a local knickpoint inclined in a direction approaching that of the bed dip. The cut can then initiate breaching or static liquefaction failure from that point up to the crest of the interfluve resulting in a V-shaped gully.

Abstract In lakes, landslides can be studied in high resolution due to their accessibility and limited size. Here, we investigate mass-transport deposits in glacigenic Wörthersee (Eastern European Alps) by integration of seismic, sediment core and multibeam bathymetric data. Two outstanding landslide events were revealed: the first occurred in the Late Glacial, leading to multiple deposits of up to 15 m thickness; they consist of sandy turbidites and mudclast conglomerates, which are overlain by a 2.5 m thick megaturbidite. The extensive, likely earthquake-triggered failure linked to this event was preconditioned by rapid sedimentation of fine-grained glaciolacustrine sediments and associated build-up of excess pore pressure. The second event was presumably triggered by a major earthquake (M w ≈7) in AD 1348 and comprises a mass-transport complex and several landslides, which led to a c. 30 cm thick turbidite. In total, 62 landslides are imaged in the multibeam map, 6 of which are most likely human-induced. Some of these show horseshoe-type compressional ridges and frontal breaching, whereas others exhibit an extensive zone of rafted blocks. We attribute these morphological differences to four main factors: (1) slope gradient and changes therein; (2) preconditioning of the impacted zone; (3) volume of remobilized sediment; and (4) type of impactor.

Abstract Submarine landslides are common at glaciated continental margins. The onset of large-scale landslides coincides with the initiation of Northern Hemisphere glaciations in the Quaternary. This implies that processes related to glacial cycling provide favourable conditions for submarine landslides at high-latitude margins. Potential processes include glacial deposition patterns and enhanced seismicity. It is also possible that advances and retreats of ice sheets, a highly dynamic process in geological terms, makes slopes discernible to failure by modifying the stress regime. Here, we quantify this effect using 2D finite element modelling of a glaciated continental margin. Different model runs investigate the pore-pressure development in homogeneous, as well as layered, slopes during glaciation when loaded by an ice stream with one or more ice advances. Ice streams cause significant variations in excess pore pressure in the very shallow sediment sequences at the continental shelf. However, lateral fluid flow is not efficient enough to increase pore pressures significantly at the slope, where large-scale submarine slides are observed. Hence, while ice-sheet dynamics appear to favour the occurrence of shallow slides close to the shelf edge, ice sheets seem to be irrelevant for the generation of large-scale submarine landslides at the continental slope.

Abstract The sheared-passive margin offshore Durban (South Africa) is characterized by a narrow continental shelf and steep slope hosting numerous submarine canyons. Supply of sediment to the margin is predominantly terrigenous, dominated by discharge from several short but fast-flowing rivers. International Ocean Discovery Program Expedition 361 provides a unique opportunity to investigate the role of sea-level fluctuations on the sedimentation patterns and slope instability along the South African margin. We analysed >300 sediment samples and downcore variations in P -wave, magnetic susceptibility, bioturbation intensity and bulk density from site U1474, as well as regional seismic reflection profiles to: (1) document an increase in sand input since the Mid-Pliocene; (2) associate this change to a drop in sea-level and extension of subaerial drainage systems towards the shelf-edge; (3) demonstrate that slope instability has played a key role in the evolution of the South Africa margin facing the Natal Valley. Furthermore, we highlight how the widespread occurrence of failure events reflects the tectonic control on the morphology of the shelf and slope, as well as bottom-current scour and instability of fan complexes. This information is important to improve hazard assessment in a populated coastal region with growing offshore hydrocarbon activities.

Abstract Submarine slope failures pose a direct threat to seafloor installations and coastal communities. Here, we evaluate the influence of free gas on the soil's shear strength and submarine slope failures in areas with gassy soils based on an extensive literature review. We identify two potential destabilization mechanisms: gas bubbles in the pore space lead to a reduced shear strength of the soil and/or gas induces excess pore pressures that consequently reduce the effective stress in the soil. Our evaluation of the reported mechanical and hydraulic behaviour of gassy sediments indicates that the unfavourable impact of entrapped gas on a soil's shearing resistance is not sufficient to trigger large-scale slope failures. Liquefaction failure due to high gas pressures is, however, a viable scenario in coarse-grained soils. Transferring the gas influence on the soil mechanical behaviour to constitutive models is identified as the most important prerequisite for a successful future analysis of slope stability.

Abstract The development of overpressure in continental margins is typically evaluated with hydrogeological models. Such approaches are used to both identify fluid flow patterns and to evaluate the development of high pore pressures within layers with particular physical properties that may promote slope instability. In some instances, these models are defined with sediment properties based on facies characterization and proxy values of porosity; permeability or compressibility are derived from the existing literature as direct measurements are rarely available. This study uses finite-element models to quantify the differences in computed overpressure generated by fine-grained hemipelagic sediments from the Gulf of Cadiz, offshore Martinique and the Gulf of Mexico, and their consequences in terms of submarine slope stability. By comparing our simulation results with in situ pore pressure data measured in the Gulf of Mexico, we demonstrate that physical properties measured on volcanic-influenced hemipelagic sediments underestimate the computed stability of a submarine slope. Physical properties measured on sediments from the study area are key to improving the reliability and accuracy of overpressure models, and when that information is unavailable, literature data from samples with similar lithologies, composition and depositional settings enable better assessment of the overpressure role as a pre-conditioning factor in submarine landslide initiation.