Submarine Slope Instability Research Articles

Coupling Submarine Slope Stability and Wellbore Stability Analysis with Natural Gas Hydrate Drilling and Production in Submarine Slope Strata in the South China Sea

This research explores the geomechanical challenges associated with gas hydrate extraction in submarine slope zones, a setting posing a high risk of significant geological calamities. We investigate slope and wellbore deformations driven by hydrate decomposition within a subsea environment. Utilizing Abaqus, a fluid-solid-thermal multi-field coupling model for gas hydrate reservoirs was created. Hydrate decomposition during drilling is minimal, resulting in minor formation deformation near the wellbore. However, a year of hydrate production caused a maximum displacement of up to 7 m in the wellbore and formation, highlighting the risk of submarine landslides. This indicates the need for meticulous surveillance of formation subsidence and wellhead equipment displacement. In the aftermath of a hydrate-induced submarine landslide, both the hydrate layer and the overlying strata descend together, inflicting considerable damage on the formation and wellbore. Our study presents a holistic examination of the interplay between environmental geomechanics risks and engineering structure risks for submarine slope instability and wellbore stability during hydrate development, providing crucial insights for enhancing safety measures in hydrate drilling and production, and ensuring wellbore stability.

Recent Technological and Methodological Advances for the Investigation of Submarine Landslides

Submarine landslides have attracted widespread attention, with the continuous development of ocean engineering. Due to the recent developments of in-situ investigation and modelling techniques of submarine landslides, significant improvements were achieved in the evolution studies on submarine landslides. The general characteristics of typical submarine landslides in the world are analyzed. Based on this, three stages of submarine landslide disaster evolution are proposed, namely, the submarine slope instability evolution stage, the large deformation landslide movement stage, and the stage of submarine landslide deposition. Given these three stages, the evolution process of submarine landslide disaster is revealed from the perspectives of in-situ investigation techniques, physical simulation, and numerical simulation methods, respectively. For long-term investigation of submarine landslides, an in-situ monitoring system with long-term service and multi-parameter collaborative observation deserves to be developed. The mechanism of submarine landslide evolution and the early warning factors need to be further studied by physical modelling experiments. The whole process of the numerical simulation of submarine landslides, from seabed instability to large deformation sliding to the impact on marine structures, and economizing the computational costs of models by advanced techniques such as parallel processing and GPU-accelerators, are the key development directions in numerical simulation. The current research deficiencies and future development directions in the subject of submarine landslides are proposed to provide a useful reference for the prediction and early warning of submarine landslide disasters.

Ultimate lateral pressures exerted on buried pipelines by the initiation of submarine landslides

Submarine slope instabilities are considered one of the major threats for offshore buried pipelines. This paper presents a novel method to evaluate the ultimate pressure acting on a buried pipeline during the liquefaction of an inclined seabed. Small-scale model tests with pipes buried at three different embedment ratios have been conducted at an enhanced centrifugal acceleration condition. A high-speed, high-resolution imaging system was developed to quantify the soil displacement field of the soil body and to visualize the development of the liquefied zone. The measured lateral pressures were compared with the hybrid approach proposed for the landslide–pipeline interaction in clay-rich material by Randolph and White (2012) and Sahdi et al. (2014). The hybrid approach is proved to be able to predict later pressures induced by the movement of (partially) liquefied sand on buried pipelines. It is found that the fluid inertia (fluid dynamics) component plays an important role when the non-Newtonian Reynolds number >~2 or the shear strain rate > 4.5 × 10−2 sec−1.

Transient Groundwater Flow Through a Coastal Confined Aquifer and Its Impact on Nearshore Submarine Slope Instability

AbstractRare in situ pore pressure and temperature data sets collected over more than one decade in a submarine landslide‐prone area off Nice (Southeast France) have been used to characterize a transient groundwater exchange between a coastal confined aquifer and the nearshore shallow‐water submarine shelf. The near‐coast study zone is highly exposed to geohazards and remains sadly famous for the 1979 catastrophic tsunamigenic submarine landslide with several casualties and substantial material damage. Measured pore pressure buildup does not appear to be synchronized with either the discharge of the nearby Var River or with precipitation in the Nice area. Unexpectedly, pore pressure fluctuations synchronizes well with seabed temperature trends indicating that the nearshore submarine shelf is mainly impacted by long smooth seasonal variations. We used the pore pressure and temperature data to describe the general transport mechanisms of pore waters and show that pore pressure diffusion is predominantly responsible for the near‐seabed pore pressure buildup. Based on our results and previously published geotechnical data, slope stability calculations reveal that groundwater exchange has a major impact on slope instability and shear zone formation in a prone‐to‐failure area. Minor earthquake‐induced ground accelerations seem to contribute to the nearshore slope instability by decreasing its factor of safety.

Tectonic evolution of strike-slip zones on continental margins and their impact on the development of submarine landslides (Storegga Slide, northeast Atlantic)

AbstractSubmarine landslides have affected the mid-Norwegian margin since the Last Glacial Maximum. However, the role of tectonic movements, and most especially fault reactivation, in generating landslides offshore Norway is largely unconstrained. This study uses high-quality three-dimensional seismic and borehole data to understand how landslide development is controlled by faults propagating within the uplifted south Modgunn arch. Variance and structural maps above the south Modgunn arch show that: (1) local scarps of recurrent landslides were formed close to the largest faults, and mainly above strike-slip faults; (2) distinct periods of fault generation were associated with tectonic events, such as the breakup of the northeast Atlantic Ocean, and those events forming the south Modgunn arch; and (3) important fluid-flow features coincide with faults and sill intrusions. In total, 177 faults were analyzed to demonstrate that fault throw values vary from 10 ms to 115 ms two-way traveltime (8 m to 92 m). We propose that the long-term activity of faults in the study area has contributed to fluid migration, weakened post-breakup strata, and controlled the development of submarine slope instability. In particular, strike-slip faults coincide with the locations of several Quaternary landslide scars near the modern seafloor. Similar processes to those documented in Norway may explain the onset of large-scale landslides on other continental margins.

On the potential of gassy marine soils triggering submarine slope instabilities

The development of debris flows and turbidity currents in the course of a submarine slope failure event can cause major damage in offshore infrastructure. Additionally, the tsunamogenic potential of large slope failures at continental margins poses a direct threat to coastal communities. Therefore, the trigger mechanisms of submarine slope failures have been thoroughly investigated in the past. However, the influence of free gas in the sediment, which has been observed close to several slide events, remains unexplained. In order to evaluate the potential of gassy marine soils to precondition or trigger slope failure the mechanical behaviour of gassy soils is assessed based on an extensive literature review. It is found that gas-induced excess pore pressures can lead to liquefaction failure in sands, while cohesive, gassy soils show a less conclusive response. Hence, fine-grained soils and approaches to implement the gas impact into relevant existing constitutive soil models are assessed in greater detail. Concludingly, based on the predominant boundary conditions in failure prone regions at the continental margins, free gas occurrence can be defined as a preconditioning factor rather than as a definite trigger mechanism.

Thermo-haline circulations in subsea confined aquifers produce saline, steady-state deep submarine groundwater discharge

Deep Submarine Groundwater Discharge (DSGD) is a ubiquitous and highly significant phenomenon, yet it remains poorly understood. This work proposes a simple theoretical basis for steady-state DSGD from a confined aquifer. The simple theory is tested against numerical modeling (FEFLOW) of a case study of DSGD offshore northern Israel (eastern Mediterranean Sea). Modeling further investigates the hydrogeological setting that enables DSGD and the parameters that affect it – conductivity of the confining unit, head on land, seawater salinity and seawater temperature. The main findings are thus: steady-state DSGD eminates far offshore from a confined aquifer exposed by a submarine canyon, even under moderate heads. It is driven by a thermohaline circulation cell that forms around an intrinsic transition zone between salty, cold seawater and fresh, warm terrestrial groundwater. The circulation is driven by tilted isopycnals and consists of seawater entering the confined aquifer at the exposed section offshore, mixing with terrestrial groundwater within the aquifer, and seeping back out to the sea as saline warm water. The circulation mode and the character of the DSGD depend on the conductivity of the confining unit and on the hydraulic head. The seepage velocity is linearly related to the temperature and salinity gradients that develop in the confined aquifer during the thermohaline circulation. These new insights have potentially important implications for coastal hydrology, seawater chemistry, biogeochemistry, and submarine slope instability.

Susceptibility assessment of regional submarine landslides triggered by seismic actions

Submarine landslide and even its evolution to high-speed sliding masses can seriously threaten the safety of ocean engineering. Earthquakes are the most frequent trigger factors for landslides, and thus, seismic instability evaluation of submarine slopes has become a hot topic. However, the mechanism of submarine slope failures triggered by earthquakes is enormously complex. Although certain satisfactory results have been achieved in specific slopes, submarine landslides are still difficult to monitor and control, meaning that prevention-oriented research remains the mainstream. Regional slide predictive analyses of vast submarine slopes should be emphasized, but relevant research is notably rare. In this paper, a regional assessment methodology for submarine slope instabilities under seismic loads is proposed, and susceptibility mapping of landslides is established. First, considering the quasi-static bidirectional seismic action and the marine soil strength-weakening effect, an infinite slope sliding mode is used to establish the stability evaluation formula for a multiple soil-layer slope. Second, according to water depth data and GIS techniques, slope gradients are investigated, and the safety factors of submarine slopes located at specific stations are acquired. Furthermore, discrete stations are taken as sample points, and susceptibility mapping of submarine landslides is achieved using the inverse distance weighted and GIS techniques. Finally, regional seismic instability assessments of submarine slopes are conducted in the northeastern South China Sea, and various factors that influence submarine landslides are briefly discussed. This study offers an important basis for the development of marine resources and the risk assessments in ocean engineering.

The Distinct Elemental Analysis of the Microstructural Evolution of a Methane Hydrate Specimen under Cyclic Loading Conditions

Submarine slope instability may be triggered by earthquakes and tsunamis. Methane hydrate sediments (MHS) are commonly buried under submarine slopes. Submarine slides would probably be triggered once the MHS are damaged under cyclic loading conditions. For this reason, it is essential to research the mechanical response of MHSs under dynamic loading conditions. In this study, a series of drained cyclic biaxial compressive tests with constant stress amplitudes were numerically carried out with the distinct element method (DEM). The cyclic loading number decreased as the hydrate saturation (Sh) increased when the MHS were damaged. The failure mode of the MHS was shown to be dependent on the dynamic stress amplitude and hydrate saturation. The microstructure of MHS during the cyclic loading shear process was also analyzed. The results can help us to understand the mechanical behavior of MHS during the cyclic loading process and develop micromechanical-based constitutive models.

Large-scale seafloor stability evaluation of the northern continental slope of South China Sea

With the continuous expansion of energy demand, the deep-water continental slope in the northern South China Sea has become one of the significant offshore oil and gas exploration regions. The frequent occurrence of marine geological hazards in this region, especially submarine landslides, can cause serious damage to engineering facilities. However, there have been few studies on the stability of the northern continental slope of the South China Sea; these studies mainly focused on a specific submarine slope or small-range evaluation, resulting in a lack of large-scale and quantitative understanding. Hence, considering the variation in the physical and mechanical properties of marine soils with depth, formulas for calculating the safety factor of submarine slopes by an infinite sliding model are established, and the factors affecting slope stability such as soil properties, slope gradient and horizontal seismic action are systematically investigated. Using GIS techniques, the terrain slope gradients and a historical seismic database of the northern South China Sea are obtained. Combined with soil mechanical parameters, a regional stability evaluation of the northern continental slope is carried out. Furthermore, the distribution of risk zones is given. On the whole, under strong seismic action, large-scale submarine slope instability occurs and must be highly considered when assessing risk. This achievement is of great significance to engineering sites, route selection and engineering risk assessment.

Centrifuge modelling of submarine landslides due to static liquefaction

Sand erosion and scouring caused by waves and marine currents result in gradual increase of local seabed inclination and formation of slopes around hydraulic structures and offshore foundations. During this process, shear stresses in the soil body increase monotonically which may lead to static liquefaction and damage of the adjacent offshore infrastructure. This paper presents the details of a newly developed static liquefaction triggering actuator to be used at an enhanced gravity condition in a geotechnical centrifuge. This actuator simulates the steeping process of submarine sand layers due to scouring and enables the investigation of failure mechanisms in submerged slopes. The details of the centrifuge test set-up designed and constructed to simulate the process of triggering static liquefaction in loose sand layers are presented. Furthermore, the performance of the novel integrated model preparation facility using sand fluidization is explained. The set-up was used to conduct several centrifuge tests at four different slope steepening rates to investigate the slope steepening rate effects. Moreover, the effect of viscosity of the submerging pore fluid on the behaviour of the slopes at the onset of failure is investigated. The Coriolis effect on loose saturated sand samples during increase of g-level is examined as well. Results show that the built-up of pore pressure due to local shear deformations can be detected and considered as one of the triggering mechanisms of this kind of submarine slope instabilities.

Geohazard assessment related to submarine instabilities in Bjørnafjorden, Norway

Abstract This paper presents the geohazard assessment for a proposed bridge across Bjørnafjorden in western Norway. The fjord is c. 5 km wide with a maximum depth of 550 m at the proposed bridge crossing. The main geohazards of concern are submarine slope instabilities. To identify locations of instability, their susceptibility to failure, and their potential runout distances, we performed the following analyses: (1) static and pseudo-static limit equilibrium analyses for the entire fjord crossing area; (2) 1D seismic slope stability sensitivity analyses for different slope angles and soil depths; (3) 2D static and pseudo-static finite element analyses for selected profiles; (4) back-analysis of a palaeolandslide; and (5) quasi-2D and quasi-3D landslide dynamic simulations calibrated using results from the back-analysis. The workflow progresses from simplified to more advanced analyses focusing on the most critical locations. The results show that the soils in many locations of the fjord are potentially unstable and could be the loci of landslides and debris flows. The evidence of numerous palaeosubmarine landslides identified on geophysical records reinforces this conclusion. However, the landslide triggers and timing are currently unknown. This paper demonstrates the need for comprehensive and multidisciplinary geohazard analyses for any infrastructure projects conducted in fjords.

Lava delta deformation as a proxy for submarine slope instability

The instability of lava deltas is a recurrent phenomenon affecting volcanic islands, which can potentially cause secondary events such as littoral explosions (due to interactions between hot lava and seawater) and tsunamis. It has been shown that Interferometric Synthetic Aperture Radar (InSAR) is a powerful technique to forecast the collapse of newly emplaced lava deltas. This work goes further, demonstrating that the monitoring of lava deltas is a successful strategy by which to observe the long-term deformation of subaerial–submarine landslide systems on unstable volcanic flanks. In this paper, displacement measurements derived from Synthetic Aperture Radar (SAR) imagery were used to detect lava delta instability at Stromboli volcano (Italy). Recent flank eruptions (2002–2003, 2007 and 2014) affected the Sciara del Fuoco (SdF) depression, created a “stacked” lava delta, which overlies a pre-existing scar produced by a submarine–subaerial tsunamigenic landslide that occurred on 30 December 2002. Space-borne X-band COSMO-SkyMED (CSK) and C-band SENTINEL-1A (SNT) SAR data collected between February 2010 and October 2016 were processed using the SqueeSAR algorithm. The obtained ground displacement maps revealed the differential ground motion of the lava delta in both CSK and SNT datasets, identifying a stable area (characterized by less than 2 mm/y in both datasets) within the northern sector of the SdF and an unstable area (characterized by velocity fields on the order of 30 mm/y and 160 mm/y in the CSK and SNT datasets, respectively) in the central sector of the SdF. The slope stability of the offshore part of the SdF, as reconstructed based on a recently performed multibeam bathymetric survey, was evaluated using a 3D Limit Equilibrium Method (LEM). In all the simulations, Factor of Safety (F) values between 0.9 and 1.1 always characterized the submarine slope between the coastline and −250 m a.s.l. The critical surfaces for all the search volumes corresponded to the 30 December 2002 landslide, which involved the lava delta and its surrounding areas. InSAR data provided the post-effusive deformation field after the 2007 and 2014 flank eruptions, whereas LEM results highlighted that the accumulation of lava flows on the prone-to-failure SdF submarine slope is the main cause of the detected lava delta deformation. Lava delta instability, measured also at Pico Island (Azores) and Kilauea volcano (Hawaii), is evidence of the broader spectrum of instability phenomena that take place in the coastal or submarine area of the flanks of the volcanoes. At Kilauea, past lava deltas have moved faster than the surrounding slope and the recorded movements relate only to the collapses of the deltas themselves, producing rapid mass wasting near the coasts. In contrast, at Stromboli and Pico, lava deltas move at the same velocity as the surrounding slope. In these cases, the displacement at lava deltas can be considered as a proxy for the deformation of submarine slides. There are very few studies dealing with lava delta deformation, thus, the analysis presented in this work will benefit the monitoring of submarine slopes in other prone-to-failure coastal or island volcanic systems which have the potential to generate tsunamis.

The mechanism of shallow submarine landslides triggered by storm surge

Submarine landslides triggered by storm surge have a major impact on human life by destabilizing offshore installations and generating tsunamis. Based on disaster-related marine geology, this paper analyzes the characteristics of storm surge load and response of seabed soil under the action of storm surge, and then examines mechanisms of submarine landslides triggered by storm surge. Important advances in this field are summarized and a scope for future studies is suggested. Current studies show that there are three load forms of storm surge: storm tide, storm wave, and storm current. Among these, the storm wave is the main factor in submarine slope instability. Under storm surge load, there is a series of changes in seabed soil, such as pressure consolidation, liquefied rheology, and mass transport. Therefore, two types of theory can be used to explain the mechanism of submarine landslide, liquefaction failure, and shear failure. Future studies in this field may focus on the mechanism of soil–water interaction under storm surge, establishing the critical condition of slope failure and failure modes of submarine landslides, and improving techniques of field monitoring and testing.

Hazard analysis of seismic submarine slope instability

To assess the risk associated with a submarine landslide, one must estimate the probability of slope failure and its consequences. This paper proposes a procedure to estimate the probability of earthquake-induced submarine slope failure (hazard) based on probabilistic seismic hazard analyses, ground response analyses and advanced laboratory tests. The outcomes from these analyses are treated in a probabilistic framework, with analytical simulations using mathematical techniques such as the first-order reliability method, Monte Carlo simulation and Bayesian updating. Fragility curves of slope failure during the earthquake (co-seismic) and after the earthquake (post-seismic) were developed in this study, and were shown to provide a clear and well-organized procedure to estimate the annual failure probability of a submarine slope under earthquake loading.

The assessment of submarine slope instability in Baiyun Sag using gray clustering method

It is a significant task to evaluate the seafloor stability for the exploitation of marine resources and the construction of marine structures. This paper analyzes the impact factors of submarine slope instability using geological and geophysical data of the Baiyun Sag in the Pearl River Mouth Basin and builds the evaluation index system of submarine slope instability. The weight of each index is calculated by analytic hierarchy process method to weaken subjective effect. Gray clustering method is applied to estimate the risk of submarine slope instability in Baiyun Sag qualitatively and quantitatively. The assessment standard is divided into five grades, and the whitening function of each gray cluster is determined by the assessment scheme. The gray evaluation weight vector of each indicator is calculated by the table-dispatching method. The gray assessment value is estimated by making the gray assessment weight vector single value, and the gray grade is determined according to the maximum principle. The evaluation results show quantitatively that the risk grade of submarine slope instability in the Baiyun Sag is medium, and the gray clustering method is reasonable and feasible.

Submarine Slope Failure Primed and Triggered by Bottom Water Warming in Oceanic Hydrate-Bearing Deposits

Many submarine slope failures in hydrate-bearing sedimentary deposits might be directly triggered, or at least primed, by gas hydrate dissociation. It has been reported that during the past 55 years (1955–2010) the 0–2000 m layer of oceans worldwide has been warmed by 0.09 °C because of global warming. This raises the following scientific concern: if warming of the bottom water of deep oceans continues, it would dissociate natural gas hydrates and could eventually trigger massive slope failures. The present study explored the submarine slope instability of oceanic gas hydrate-bearing deposits subjected to bottom water warming. One-dimensional coupled thermal-hydraulic-mechanical (T-H-M) finite difference analyses were performed to capture the underlying physical processes initiated by bottom water warming, which includes thermal conduction through sediments, thermal dissociation of gas hydrates, excess pore pressure generation, pressure diffusion, and hydrate dissociation against depressurization. The temperature rise at the seafloor due to bottom water warming is found to create an excess pore pressure that is sufficiently large to reduce the stability of a slope in some cases. Parametric study results suggest that a slope becomes more susceptible to failure with increases in thermal diffusivity and hydrate saturation and decreases in pressure diffusivity, gas saturation, and water depth. Bottom water warming can be further explored to gain a better understanding of the past methane hydrate destabilization events on Earth, assuming that more reliable geological data is available.

A new model for the evolution of La Réunion volcanic complex from complete marine geophysical surveys

Results from recent marine geophysical surveys offer a new perspective for characterizing the evolution processes of volcanic islands. In 2006, cruises FOREVER and ERODER 1 investigated the submarine flanks and the surrounding abyssal plain of La Réunion (Indian Ocean) to obtain for the first time a complete geophysical survey of the area. Combined analyses of these data reveal major differences in the evolution of the two emerged volcanoes, Piton des Neiges and Piton de la Fournaise. We show that debris avalanche deposits extend on the abyssal plain only offshore the active Piton de la Fournaise volcano attesting the occurrence of large flank‐collapse events. The absence of such deposits offshore Piton des Neiges and the presence of compressive structures within the sedimentary unit below the edifice support a mechanism of slow deformation of this volcano, such as sliding or spreading. The slow deformation of Piton des Neiges has led to numerous secondary submarine slope instabilities and favored some unconfined turbidity flows which generated large sediment waves running downward all around the island. This study proposes a new model using the most complete marine data set available: slow deformation controls the evolution of Piton des Neiges whereas Piton de la Fournaise (formed on the flanks of a pre‐existing edifice) experienced catastrophic, large flank‐collapse events.

Recent structures in the Alboran Ridge and Yusuf fault zones based on swath bathymetry and sub-bottom profiling: evidence of active tectonics

The seafloor of the Alboran Sea in the western Mediterranean is disrupted by deformations resulting from convergence between the African and Eurasian plates. Based on a compilation of existing and new multibeam bathymetry data and high-resolution seismic profiles, our main objective was to characterize the most recent structures in the central sector, which depicts an abrupt morphology and was chosen to investigate how active tectonic processes are shaping the seafloor. The Alboran Ridge is the most prominent feature in the Alboran Sea (>130 km in length), and a key element in the Gibraltar Arc System. Recent uplift and deformation in this ridge have been caused by sub-vertical, strike-slip and reverse faults with associated folding in the most recent sediments, their trend shifting progressively from SW–NE to WNW–ESE towards the Yusuf Lineament. Present-day transtensive deformation induces faulting and subsidence in the Yusuf pull-apart basin. The Alboran Ridge and Yusuf fault zones are connected, and both constitute a wide zone of deformation reaching tens of kilometres in width and showing a complex geometry, including different active fault segments and in-relay folds. These findings demonstrate that Recent deformation is more heterogeneously distributed than commonly considered. A narrow SSW–NNE zone with folding and reverse faulting cuts across the western end of the Alboran Ridge and concentrates most of the upper crustal seismicity in the region. This zone of deformation defines a seismogenic, left-lateral fault zone connected to the south with the Al Hoceima seismic swarm, and representing a potential seismic hazard. Newly detected buried and active submarine slides along the Alboran Ridge and the Yusuf Lineament are clear signs of submarine slope instability in this seismically active region.

Geomorphologic features related to gravitational collapse: Submarine landsliding to lateral spreading on a Late Miocene–Quaternary slope (SE Crete, eastern Mediterranean)

Detailed geological mapping, complemented by sedimentary facies analysis, photomosaics and topographic measurements (height, width) allowed the investigation of the geomorphological features related to the Late Miocene–Quaternary gravitational collapse of a palaeoslope located in SE Crete (eastern Mediterranean). In the study area, carbonate megablocks indicative of submarine landsliding during the Late Miocene alternate with collapse features more typical of subaerial settings; the latter generated after a major event of tectonic uplift initiated in Crete during the Early-mid Pliocene. Submarine features typically show basal shear zones, rather than planes, generated in near-seafloor strata deformed in ductile form during the gravitational collapse of the megablocks. The lithology of failed carbonate strata differs from that of their basal shear surfaces, a characteristic providing a reliable estimate for the degree and styles of basal deformation during submarine slope instability. Styles of submarine collapse include, by order of magnitude; (i) lateral spreading of fractured segments of fan cones and carbonate sheet flows, eventually transported 100s of metres downslope; (ii) aperture of ravines and chasms in gravitationally unstable fan cones and boulder conglomerates; (iii) gliding of megablocks over a ductile basal layer through a distance of up to several kilometres; and (iv) rolling of subcircular blocks, often within a debris-flow matrix in fan cones and deltas, or embedded in slope siliciclastic strata. This work highlights the existence of prominent 2–10 m basal shear zones in strata underneath the larger megablocks deposited on marine slope strata. Basal shear zones comprise a melange of reworked conglomerates and breccia clasts from overlying megablocks, large ripped blocks of rock and faulted near-seafloor strata, at places showing remnant beds and sand injection features. Consequently, the outcrop data show an average 5:1 ratio between the maximum observed thickness of megablocks and the thickness of basal shear zones ( R), a value of similar magnitude to published examples from offshore landslides.