Natural marine sediments are heterogeneous with respect to sediment‐physical properties, and have a wide range in composition and structures. For many years, sediment‐physical characterization has relied primarily on laboratory experiments. However, the investigation of small‐(grain‐)scale sedimentary structures, which appear to control many sediment (re‐)depositional and emplacement mechanisms, requires new analytical methods. Here, we test high‐resolution X‐ray synchrotron micro‐tomography (μCT) to qualitatively and quantitatively investigate structural differences, in 3D, between two lithological end‐member types of marine sediments: a coarse‐grained, sandy sediment and a fine‐grained, silty‐clay sediment. Our results show clear compositional and structural differences between the two end‐members, as well as between samples taken from the same lithological unit. These differences can be attributed partly to different sediment types, that is, coarse‐versus fine‐grained sediments, but also reveal a dependency on the sedimentation regime. We find that pore space distribution is highly spatially variable, even down to a sub‐millimeter scale. Such high variability in porosity would be missed by standard geotechnical experiments, which only provide information averaged over far larger sediment samples. The identification of small‐(grain‐)scale changes in pore space, however, directly impacts sediment properties such as permeability, which in turn is crucial for the understanding of geological processes such as fluid flow and storage capacity of sediments and assessing hazards such as the preconditioning of submerged slopes to collapse. Our results therefore demonstrate the potential of μCT to investigate the internal structure of natural sediments, obtaining information that is not resolved or lost in data acquired through other analytical methods.

<p class="ThesisTitle2NoList"><span lang="EN-GB">Submarine landslides are common on all sediment bearing submarine slopes worldwide. They have the potential to damage expensive subsea infrastructure such as pipelines or telecommunication cables, and generate hazardous tsunamis.</span> <span lang="EN-GB">Numerous studies have shown that weak layers embedded within the slope stratigraphy play a crucial role in controlling the formation of submarine landslides; however, very little is known about their internal structure and composition. Although weak layers seem to be an essential pre-conditioning factor for slope failure, many questions remain unanswered, such as where with respect to weak layers do failure planes form: within the weak layer, above or below it? Previous studies usually relied on sedimentological and geotechnical sediment core and in-situ analyses to investigate weak layers. These analyses, however, do not provide insights into the internal structure of the sediments on a micro-scale level and hence, lack information needed to qualitatively and quantitatively investigate weak layers. </span></p> <p class="ThesisTitle2NoList"><span lang="EN-GB">Here, we present a new approach towards weak layer investigation that is based on high-resolution micro-Computed Tomography (µCT) imaging. µCT is used to visualise, and qualitatively and quantitatively investigate selected sediment samples taken from within weak layers and the background sediment of submarine landslides. Our results show clear compositional and structural differences between individual sub-units of the investigated weak layers, as well as the background sediment. These differences can be attributed partly to different sediment types, i.e. coarse- versus fine-grained sediments, but also reveal a dependency on the sedimentation regime. We find that pore space distribution is highly spatially variable and works on a sub-millimetre scale. Such high variability may be masked by standard bulk porosity measurements, which require larger (several centimetre) sediment samples and only provide information averaged over the entire sample. The identification of small-scale changes, however, appears to be crucial for the formation of weak layers. Our results therefore demonstrate the huge potential of µCT to investigate the internal structure of weak layers, obtaining information that is not resolved and lost in other analytical methods.</span></p>

When studying large landslides any data matters. In fact, in contrast to other scientific branches, data scarcity is a pressing issue especially for high altitude landslides. When large rockslides occur in steep valleys, long runouts can threaten buildings and infrastructures, even though the source area is well above the exposed elements. In such cases, usually no ground-truth monitoring data is available on site, which is needed to understand the causes and processes of the collapse. Seismic data from a widespread network of seismographs can help to partially fill gaps in the characterization of the above mentioned processes. The most straightforward information that can be inferred from ground motion recordings is the duration of the event – or at least the duration of the most intense and violent phase of the runout. By coupling the velocity of the event with some topographic data leading to the estimation of the detached volume and the deposit distribution, the user may gather sufficient information to produce a satisfactory numerical model through back analysis. In case the topographic data are characterized by high uncertainty and/or poor resolution, seismic records are particularly useful for ground-truthing because they represent an independent source of data. This study describes the modelling approach used to understand the dynamic of a 365,000 m3 rockslide in the Dolomites (UNESCO World Heritage, North-East Italy). The landslide detached from a steep slope located between 3100 and 2800 m a.s.l. and almost free fell for 600 m. Then it crashed and fragmented in a small rocky hanging valley of glacial origin, subsequently reaching 1400 m a.s.l. with a runout of approximately 2 km. For this area, pre-event and post-event DEMs were available but with different resolution, alignment and coverage. The comparison of pre- and post-event topography allowed the identification and quantification of erosional and depositional areas, the estimation of landslide volume and of the potential errors associated with this type of analysis. A DAN3D numerical model of the landslide was calibrated using both DEM of Difference (DoD) maps and seismic data. The ground motion records proved to be remarkably useful, as they ensured the reliability of the model notwithstanding the DoD maps intrinsic uncertainties. The seismic data provide a new layer of information in a swiss cheese model of reliability focused on reducing model equifinality and on increasing the overall robustness of the analysis. This finding is fundamental as the results of the back analysis may be used to model and test future scenarios, which can be used to support risk assessment and mitigation.

Submarine landslides pose a hazard to coastal communities as they can generate powerful tsunamis, and threaten critical offshore infrastructure such as seafloor cable networks that underpin global communications. Such events can be orders of magnitude larger than their onshore equivalents. Despite the hazard they pose, many aspects of submarine landslides remain poorly understood, such as why they fail on low angle (<2°), seemingly stable slopes. Many studies have proposed that failure on low slope angles, and the large areal extent of submarine landslides, may be controlled by the presence of laterally-extensive weak layers embedded within the slope stratigraphy, which precondition slopes to failure. Little remains known, however, about the characteristics and processes that control and form weak layers. We conducted a comprehensive review of published submarine landslide studies that examine failure planes and apparent weak layers associated with historical and ancient submarine landslides. Based on a new global landslide catalogue that comprises 64 case studies, this review aims to investigate the types of sediment that form weak layers and to understand the controls on their global variability. Existing classification schemes are based on mechanical process(es), and do not readily enable a diagnosis of weak layers from unfailed sediments. Here, a new and complementary classification of weak layers based on lithology is introduced. This classification enables weak layer recognition from sediment cores (including those sampling unfailed sediments), and allows us to attribute failure mechanisms to different environmental settings where distinct types of weak layers are more likely. The results show that failure planes usually form in the vicinity of an interface between distinct lithologies that together comprise a weak layer. The weak layers of 22 of the 64 case studies were related to characteristic sediment sequences within the slope stratigraphy, of which 19 were classified based on direct measurements from sediment cores and in-situ measurements: 16 weak layers were classified as siliciclastic, four as volcaniclastic, and two as fossiliferous sediment sequences. Only three submarine landsides were related to clay-dominated weak layers. In addition, failure along lithological contrasts was inferred for six case studies. Based on global depositional models likely locations of these different types of weak layer can be inferred. These include oceanic gateways where long-term circulation can create distinct permeability interfaces within siliciclastic sequences, areas of high productivity where biogenic sediments may dominate, and regions that experience widespread ash fall from volcanic eruptions. We highlight that many submarine landslide studies have historically not collected sediment cores that characterise weak layers within intact sedimentary sequences and instead have focused on characterising the slope failure deposit. As weak layers can collapse or become heavily modified during failure, there is a widespread omission of key information required for geotechnical analysis to determine where and why certain slopes are predisposed to failure. We conclude by highlighting the need to combine detailed geotechnical measurements with sedimentological and geophysical analyses including grain-scale observations (e.g. micro-Computed Tomography 3D imagery), and emphasise the importance of a uniform workflow that will allow for a better comparison between individual studies.

Abstract We have successfully constructed and tested a new, portable, Hybrid Lister‐Outrigger (HyLO) probe designed to measure geothermal gradients in submarine environments. The lightweight, low‐cost probe is 1–3 m long and contains 4–12 semiconductor temperature sensors that have a temperature resolution of 0.002°C, a sample rate of <2 s, and a maximum working depth of ~2,100 m below sea level (mbsl). Probe endurance is continuous via ship power to water depths of ~700 mbsl or up to ~1 week on batteries in depths >500 mbsl. Data are saved on solid‐state disks, transferred directly to the ship during deployment via a data cable, or transmitted via Bluetooth when the probe is at the sea surface. The probe contains an accelerometer to measure tilt, internal pressure, temperature, and humidity gauges. Key advantages of this probe include (1) near‐real‐time temperature measurements and data transfer; (2) a low‐cost, transportable, and lightweight design; (3) easy and rapid two‐point attachment to a gravity corer, (4) short (3–5 min) thermal response times; (5) high temporal/spatial resolution; and (6) longer deployment endurance compared to traditional methods. We successfully tested the probe both in lakes and during sea trials in May 2019 offshore Montserrat during the R/V Meteor Cruise 154/2. Probe‐measured thermal gradients were consistent with seafloor ocean‐drilling temperature measurements. Ongoing probe improvements include the addition of real‐time bottom‐camera feeds and long‐term (6–12 months) deployment for monitoring.

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.

Latest advances in topographic data acquisition techniques have greatly enhanced the possibility to analyse landscapes in order to understand the processes that shaped them. High-resolution Digital Elevation Models (DEMs), such as LiDAR-derived ones, provide detailed topographic information. In particular, if multi-temporal DEMs are available, it is possible to carry out a detailed geomorphic change detection analysis. This analysis may provide information about the dynamics of large landslides and may thus, be useful for landslide risk assessments. However, LiDAR-derived DEMs are mostly available only as post-event surveys. The technique is relatively recent, and local or national authorities only started widespread surveys in the last decade. Therefore, it is of a certain interest to analyse the effectiveness of DEMs derived from technical cartography to produce reliable volumetric estimates related to large landslides. This study evaluates the use of a multi-source DEM of Difference (DoD) analysis for the investigation of a large landslide –Le Laste–, which occurred on November 12, 2014 on Mount Antelao (eastern Italian Alps). The landslide initiated as a 365,000 m3 rockslide close to the summit of the mountain and transformed into a debris avalanche during its runout. The comparison of pre- and post-event DEMs allowed for the identification and quantification of erosion and deposition areas, and for the estimation of landslide volume. A sound back-analysis of the landslide with the 3D numerical model DAN3D was based on this comparison and on seismic records of the event. These seismic records proved to be remarkably useful, as they allowed for the calibration of the simulated landslide velocity. This ensured the reliability of the model notwithstanding the topographic datasets, intrinsic uncertainties. We found that using a pre-event DEM derived from technical cartography tends to slightly overestimate the volume with respect to the use of the more accurate LiDAR-derived DEM. In recent years, the landslide risk around Mt. Antelao has been increasing alongside the ever-growing population and human activities in the area. Sediment accumulations produced by the Le Laste landslide significantly amplified the debris flow hazard by providing new sediment sources. Therefore, it is crucial to delineate the distribution of this material to enable an adequate debris flow hazard assessment. The material properties derived from the back-analysis of the Le Laste landslide can be used to simulate the runout of possible future events, and to generate reliable hazard zone maps, which are necessary for effective risk mitigation.