Displays

Submarine landslides occur on continental margins globally and can have devastating consequences for marine habitats, offshore infrastructure and coastal communities due to potential tsunamigenic consequences. Evaluation of the magnitude and distribution of submarine landslides is central to marine and coastal hazard planning. Despite this, there are few studies that comprehensively quantify the occurrence of submarine landslides on a margin-wide scale.

We present the first margin-wide submarine landslide database along the eastern margin of New Zealand comprising >2200 landslide scars and associated mass-transport deposits. Analysis of submarine landslide distribution reveals 1) locations prone to mass-failure, 2) spatial patterns of landslide scale and occurrence, and 3) the potential preconditioning factors and triggers of mass wasting across different geologic settings.

Submarine landslides are widespread on the eastern margin of New Zealand, occurring in water depths from ~300 m to ~4,000 m. Landslide scars and mass transport deposits are more prevalent, and on average larger, on the active margin, compared the passive margin. We attribute higher concentrations of landslides on the active margin to the prevalence of deforming thrust ridges, related to active margin processes including oversteepening, faulting and seamount subduction. Higher sediment supply on the northernmost active margin is also likely to be a key preconditioning factor resulting in the concentration of large landslides in this region.

In general, submarine landslide scars are concentrated around canyon systems and close to canyon thalwegs. This suggests that not only does mass wasting play a major role in canyon evolution, but also that slope undercutting in canyons may be a fundamental preconditioning factor for slope failure.

Results of this study offer unique insights into the spatial distribution, magnitude and morphology of submarine landslides across different geologic settings, providing a better understanding of the causative factors for mass wasting in New Zealand and around the world.

How to cite: Watson, S., Mountjoy, J., and Crutchley, G.: Tectonic and geomorphic controls on the distribution of submarine landslides across active and passive margins, eastern New Zealand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3194, https://doi.org/10.5194/egusphere-egu2020-3194, 2020.

We investigate the sliding dynamics of two giant submarine landslides and their tsunamigenic capacity in the South China Sea (SCS) region: the Baiyun slide in the Pearl River Mouth Basin and the Brunei Slide in Northwest offshore Brunei. The two slides have comparable sizes with the estimated volumes of 1035 km³ for Baiyun Slide versus 1200 km³ for Brunei Slide and areas of 5500 km² versus 5300 km². Based on the available geophysical observations, we construct hypothetical scenarios for both slides. By treating the slides as translational mudflow, we are able to reproduce the observed run-out distribution of the Baiyun Slide. The sliding speeds of the failed material could reach 25~35 m/s in both slide events. Both slides could generate devastating tsunamis in the SCS although the tsunamigenic capacity of the Brunei Slide is significantly larger than the Baiyun Slide. Through a series of numerical experiments, we demonstrate that the steepness of the slope and initial water depth of the slides play the key role of determining their tsunamigenic capacity. The tsunami generated by the Baiyun Slide mainly affects the northern part of the SCS. Coastlines including the southern China, central Vietnam, western Philippines suffer the highest tsunami waves.  The tsunami waves generated by the Brunei Slide causes significant impact in northern coasts of Borneo Island, coasts of central and southern Vietnam and Palawan.

How to cite: Li, L., Qiu, Q., Shi, F., and Ma, G.: Insights into potential submarine landslide tsunamis in the South China Sea: A comparative study of the Baiyun Slide and the Brunei Slide, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12140, https://doi.org/10.5194/egusphere-egu2020-12140, 2020.

Outcrop examples of mass-transport complexes (MTCs) often show a complex internal fabric which reflects disaggregation, deformation and entrainment that occurred during transport and emplacement. This can include intense folding, included blocks of substratum and internal shear zones. Seismic reflection images often cannot properly image this internal fabric as the scale of such structure is usually below the effective resolution. This can limit seismic interpretation to characterising only the overall morphology of the deposits (the top and basal reflectors).

Seismic reflections are primarily generated by smooth, laterally continuous interfaces. Discontinuities at or below the scale of the seismic wavelength instead generate seismic diffractions (“diffraction hyperbolae” in unmigrated images). Diffractions are often ignored during seismic processing as they are generally lower in amplitude than reflections, though they do not suffer from the same lateral resolution limit as reflections so are potentially sensitive to smaller scale structure. We suggest that the discontinuous internal fabric of MTCs will generate a significant amount of diffraction energy relative to unfailed sediments.

The main goal of this study is to use diffraction imaging to image the small-scale, heterogeneous internal fabric of MTCs. We demonstrate this using two high-resolution, multi-channel 2-D marine seismic profiles (3.125 m CMP spacing, 500 m maximum offset) acquired in 2018 and 2019 as part of the INSIGHT project to investigate submarine geohazards in the Gulf of Cadiz. Profile 1 intersects the Marques de Pombal reverse fault and shows a series of stacked MTCs (~1 s TWTT from top to bottom) in the footwall, thought to be related to episodic fault activity. Profile 2 is located in the Portimão Bank area and contains two large MTCs thought to be related to the mobilisation of a salt diapir. The diffraction imaging method proceeds as i) dip-guided plane-wave destruction to separate reflected and diffracted wavefields; ii) velocity analysis by cascaded constant velocity migrations of the diffraction wavefield; iii) post-stack Kirchhoff time migration of the diffraction wavefield.

The unmigrated profiles show that the MTC bodies do generate more internal diffractions than the surrounding unfailed sediments. We also observe large contributions of diffraction energy from the rugose top and base of the MTCs, the rugose top salt interface and from faults within the unfailed sediments. The migrated diffraction images reveal distinct internal structure, thought to represent rafted blocks, ramps and both extensional and compressional faulting. The envelope of the diffraction image is used as an overlay on the conventional reflection image to guide interpretation and highlight potential diffractors. This allows interpretation of thin MTCs and improved delineation of their lateral extent (runout) above conventional reflection images.

Diffraction imaging has previously been used to image heterogeneous geology such as fracture networks, channel systems and karst topography. Here we apply the technique to study the internal fabric of MTCs. The resulting images resolve small-scale internal structure that is not well resolved by conventional reflection images. Such structures can be used as kinematic indicators to constrain flow direction and emplacement dynamics, which inform the geohazard potential of future subaqueous mass-movements.

How to cite: Ford, J., Urgeles, R., Gràcia, E., and Camerlenghi, A.: Diffraction imaging to understand the internal fabric of mass-transport complexes from Gulf of Cadiz, south west Iberian Margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8826, https://doi.org/10.5194/egusphere-egu2020-8826, 2020.

Using a combination of geophysical and geotechnical data from the Storfjorden Trough Mouth Fan, off southern Svalbard, we investigate the role of glacial advances and retreats over the continental shelf on the hydrogeology of the continental margin. The results of compressibility and permeability tests are used together with margin stratigraphic models from seismic data, as input for numerical finite element models to understand focusing of interstitial fluids in glaciated continental margins. The modeled evolution of the Storfjorden TMF from 2.7 to 0.2 Ma shows that onset of glacial sedimentation (~1.5 Ma) had a significant role in developing aquicludes (tills) on the shelf that decreased the vertical fluid flow towards the sea floor and diverted it towards the slope. This model shows that prior to 220 ka, high overpressure ratio (λ~0.6) develops below the shelf edge and in the middle slope. A more detailed, high resolution model for the last 220 kyrs accounting for ice loading during Glacial Maxima shows that the less permeable glacigenic debris flows deposited during glacial maxima on the slope hinder fluid evacuation from the plumites. This effect in combination with the fluid flow focusing from the shelf allows high overpressure ratio (λ~0.7) to develop in the shallower-most plumite layers. These high overpressures likely persist to the Present and are critical in determining the onset of submarine slope failure. The safety factor of the upper continental slope is reduced by 60% due to the combination of high sedimentation rates, ice loading and focusing of fluids during Glacial Maxima with values of the factor of safety reaching 1.2 during the LGM and beginning of the last deglaciation.

How to cite: Llopart, J., Urgeles, R., Forsberg, C. F., Camerlenghi, A., Vanneste, M., and Rebesco, M.: Fluid flow and pore pressure development throughout the evolution of a trough mouth fan and implications for the slope stability, western Barents Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-163, https://doi.org/10.5194/egusphere-egu2020-163, 2020.

Earthquakes are a main trigger of subaqueous landslides and surficial sediment remobilization at ocean margins and lake basins. If the earthquake loading is insufficient to lead to sediment failure, the subsequent dewatering and inherent compaction may enhance the shear strength of sedimentary slopes, a process termed „seismic strengthening“, which is believed to be especially relevant for the upper 10s of meters. This mechanism has been suggested to explain the observed paucity of submarine landslides on active margins when compared to the short recurrence of strong earthquakes in such settings. However, only few field studies were dedicated on this topic and little is known about which settings are especially prone to seismic strengthening.

Here, we present geotechnical data from diatom-rich sedimentary slopes in Chilean lakes and at the Japan Trench margin. We use the overburden-normalized undrained shear strength as an indicator of consolidation state. In Chile, this data is derived from in-situ dynamic cone penetrometer measurements, whereas the Japan data is obtained by lab vane shear tests on sediment cores. Both settings show extremely elevated shear strength of about ~5-10 times higher than expected for normally-consolidated sediment in the upper meters of a sequence. Significant overconsolidation is confirmed by one-dimensional compression tests, providing overconsolidation ratios of ~2-8 (Chilean lakes) and 4-9 (Japan Trench). For each setting, the shear strength profiles of sites with different sedimentation rates show very similar trends when they are normalized over the sediment age instead of over overburden stress. As older sediments experienced more earthquakes, this apparent age-dependency may form a new argument supporting the hypothesis of seismic strengthening. Following previous lab experiments on mixtures of diatoms and clayey-silt, we postulate that a high susceptibility to seismic strengthening in both settings is caused by the abundance of diatom frustules which are typically characterized by a high particle interlocking and surface roughness. On the Japan Trench margin, biogenic opal forms ~15% in dry weight, and given the hollow structure of diatom frustules, we infer that diatoms take up a considerable space in the in-situ sediment texture. We conclude that seismically active margins with diatom-rich sediments have a reduced susceptibility to submarine landslide hazards.

How to cite: Moernaut, J., Wiemer, G., Wu, T.-W., Molenaar, A., Kopf, A., and Strasser, M.: Seismic strengthening of diatom-rich sediments: A comparison of slope sediments from Chilean lakes and the Japan Trench margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2578, https://doi.org/10.5194/egusphere-egu2020-2578, 2020.

Numerous studies indicate that tsunamis do not only occur in oceans but also in lakes. Lake Tsunamis are mainly caused by sublacustrine and subaerial mass movements that can be triggered by seismic or aseismic processes (Schnellmann et al. 2002, Strasser et al. 2007, Kremer at al. 2012, Hilbe and Anselmetti 2015). Such tsunamis can have devastating effects on the surrounding population and infrastructure.

To assess the tsunami hazard triggered by sublacustrine mass movements, the stability of the lake slopes needs to be examined. As a part of the SNSF funded SINERGIA project “Lake Tsunamis: Causes, Controls and Hazard”, we perform the slope stability analysis based on the comprehensive geotechnical in situ and laboratory dataset for the selected sites of Lake Lucerne, Central Switzerland.

During 2018-2019 dense geotechnical investigations were carried out along slope-perpendicular profiles at 10 sites where the slopes have failed in the past or are susceptible to failure and included more than 130 in-situ free-fall cone penetration tests with pore pressure measurement (CPTu) and laboratory analysis of 30 short sediment cores. Already existing reflection seismic dataset complements these data and provides the thickness of different sediment layers.

1D undrained, infinite slope stability analysis following Morgenstern and Price (1965) is used to define the Factor of Safety and critical conditions for deltaic and lateral slopes, where different triggers can be responsible for the failure. Based on the conducted analysis, static and dynamic stability together with critical failure conditions for different slopes in Lake Lucerne can be compared.

References:

Hilbe, M. and Anselmetti, F.S. (2015) Mass Movement-Induced Tsunami Hazard on Perialpine Lake Lucerne (Switzerland): Scenarios and Numerical Experiments. Pure and Applied Geophysics 172, 545-568.

Kremer, K., Simpson, G., Girardclos, S. (2012) Giant Lake Geneva tsunami in AD 563. Nature Geoscience 5, 756-757.

Morgenstern, N.R. and Price, V.E. (1965) Analysis of stability of general slip surfaces. Geotechnique 15(1): 79–93.

Schnellmann, M., Anselmetti, F.S., Giardini, D., McKenzie, J.A., Ward, S.N. (2002) Prehistoric earthquake history revealed by lacustrine slump deposits. Geology 30, 1131–1134.

Strasser, M., Stegmann, S., Bussmann, F., Anselmetti, F.S., Rick, B., Kopf, A. (2007) Quantifying subaqueous slope stability during seismic shaking: Lake Lucerne as model for ocean margins. Marine Geology 240, 77-97.

How to cite: Shynkarenko, A., Stegmann, S., Kremer, K., Bergamo, P., Imperatori, W., Lontsi, A. M., Kopf, A., and Fäh, D.: Stability assessment of submerged lateral and deltaic slopes in Lake Lucerne , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6979, https://doi.org/10.5194/egusphere-egu2020-6979, 2020.

When narrow continental shelves are stressed by extreme weather events, nearshore currents dominate the coastal circulation leading to complex flow patterns that can result in previously unforeseen cross-shelf exchange of water and sediment. Here we present a series of detailed studies carried out to investigate the nature of turbidity currents that impacted upon a submarine pipeline offshore Philippines, nearby tropical river deltas, after the landfall of intense typhoons. These rivers debouch into a shelf only a few hundreds of meters wide that is interrupted by steeper continental slopes carved by multiple submarine canyons. Turbidity currents were detected through regular pipeline monitoring, which showed lateral displacements and sea-floor erosion where the pipeline crosses some of these canyons. Seabed assessments indicated signatures of the occurrence of turbidity currents as opposed to landslides or ground motion due to earthquakes. Particularly, the submarine canyons were covered with regular sediment patterns that indicated the passage of deep-water turbulent flows, suggesting the local occurrence of turbidity currents. Meteorological data pointed at river floods and meteocean conditions, and associated fluvial sediment delivery and coastal sediment transport, as the most likely leading mechanisms for the triggering of turbidity currents. Hydrological modelling and related sediment transport calculations show these rivers were not capable to debouch into the sea with sediment concentrations high enough to generate hyperpycnal flows. Nevertheless, river plumes played an active role as source of sediment available on the shelf. Conversely, the role of the coastal circulation was found to be crucial for the triggering of turbidity currents. Our simulations show the development of exceptional rip currents (megarips) that flush out water and sediment from the inner shelf in the cross-shore direction towards the canyons’ heads, ultimately triggering turbidity currents into deep ocean waters. Such extreme nearshore circulations require the passage of intense typhoons in proximity to the trigger area inducing shore-normal incoming waves at peak conditions that in association with shoreline concavity at the river deltas favour the formation of erosional megarips, whose dynamics strongly depends on typhoon's approach latitude. The turbidity current modelling confirmed such an interpretation, matching field observations in the form of pipeline displacements. These evidences support our hypothesis that typhoon-induced megarip circulations could be responsible for the triggering of turbidity currents in submarine canyon systems offshore tropical river deltas. This newly identified mechanism has wide implications on the threatening of seafloor infrastructures and the assessment of frequency and duration of turbidity currents.

How to cite: Porcile, G., Bolla Pittaluga, M., Frascati, A., and Sequeiros, O.: A new mechanism for the triggering of turbidity currents offshore tropical river deltas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20168, https://doi.org/10.5194/egusphere-egu2020-20168, 2020.

Contourite deposits are generated by the interplay between deepwater bottom-currents, sediment supply and seafloor topography. The Gulf of Cadiz, in the Southwest Iberian margin, is a famous example of extensive contourite deposition driven by the Mediterranean Outflow Water (MOW), which exits the Strait of Gibraltar, flows northward following the coastline and distributes the sediments coming from the Guadalquivir and Guadiana rivers. The MOW and related contourite deposits affect the stability of the SW Iberian margin in several ways: on one hand it increases the sedimentation rate, favoring the development of excess pore pressure, while on the other hand, by depositing sand it allows pore water pressure to dissipate, potentially increasing the stability of the slope.

In the Gulf of Cadiz, grain size distribution of contourite deposits is influenced by the seafloor morphology, which splits the MOW in different branches, and by the alternation of glacial and interglacial periods that affected the MOW hydrodynamic regimes. Fine clay packages alternates with clean sand formations according to the capacity of transport of the bottom-current in a specific area. Generally speaking, coarser deposits are found in the areas of higher MOW flow energy, such as in the shallower part of the slope or in the area closer to the Strait of Gibraltar, while at higher water depths the sedimentation shifts to progressively finer grain sizes as the MOW gets weaker. Previous works show that at present-day the MOW flows at a maximum depth of 1400 m, while during glacial periods the bottom-current could have reached higher depths.

In this study we derived the different maximum depths at which the MOW flowed by analyzing the distribution of sands at different depths along the Alentejo basin slope, in the Northern sector of the Gulf of Cadiz.

Here we show how changes in sand distribution along slope, within the stratigraphic units deposited between the Neogene and the present day, are driven by glacial – interglacial period alternation that influenced the hydrodynamic regime of the MOW.

By deriving the depositional history of sand in the Alentejo basin, we are able to correlate directly the influence that climatic cycles had on the MOW activity. Furthermore, by interpreting new multi-channel seismic profiles we have been able to derive a detailed facies characterization of the uppermost part of the Gulf of Cadiz.

An accurate definition of sand distribution along slope plays an important role in evaluating the stability of the slope itself, e.g. to understand if the sediments may be subjected to excess pore pressure generation. As sand distribution is a direct function of the bottom-current transport capacity, the ultimate goal of this study is to understand how climate variations can affect the stability of submarine slope by depositing contourite-related sand.

How to cite: Mencaroni, D., Urgeles, R., Ford, J., Llopart, J., Sànchez Serra, C., Calahorrano, A., Brito, P., Lo Iacono, C., Bartolomè, R., Gràcia, E., Rebesco, M., Camerlenghi, A., and Bellwald, B.: How deep does sand deposits in the Alentejo basin (Gulf of Cadiz) reach? Evaluating slope stability from bottom-current activities through time, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18904, https://doi.org/10.5194/egusphere-egu2020-18904, 2020.

The Gulf of Lions (GoL) is a passive margin of about 200 km long and 70 km wide with main sediment supply from the Rhone River supplying Alpine sediments to the Rhone delta. Submarine landslides in the GoL are widespread from the upper slope to the deep basin, within the canyon flanks and in the interfluves of major canyons. The two main submarine landslides present in the GoL are the Eastern Rhône Interfluve Slide (ERIS) and an unnamed slide complex on the western side of the Petit Rhone Canyon. Their resulting mass transport deposits (MTDs), the Rhone Eastern MTD (REMTD) and the Rhone Western MTD (RWMTD) have previously been described in detail in several studies. However, due to the lack of high-resolution multidisciplinary datasets, such as high-resolution seismic, sediment cores, and in-situ geotechnical measurements, a detailed analysis of weak layers and preconditioning factors was never performed. Here, we present a suite of a multidisciplinary dataset; particularly very high-resolution deep-towed multichannel seismic data acquired using Ifremer’s in-house acquisition system SYSIF (SYstème SIsmique de Fond) to assess seafloor instabilities in the GoL. The objectives of this study are twofold and aimed at 1) using deep-towed multichannel seismic data to capture the internal structure of the mass-wasting products previously imaged as seismically transparent or chaotic intervals in conventional seismic data; 2) using multidisciplinary dataset to analyse the basal surfaces of slope failures in the GoL. For the first time, the newly-acquired SYSIF data show in unprecedented detail the internal structure of mass-transport deposit along with small-scale slope failures. We present here an example of a failure that consists of slide blocks, folded and faulted strata with remnant stratigraphy previously associated with a transparent or chaotic facies in the conventional reflection seismic data. The combination of deep-towed seismic and sedimentological data, as well as in-situ measurements allowed us to analyse and characterize the nature of the basal surface of the slope failures in greater detail. We show that the basal surfaces of the recurring slope failures mainly consist of fine-grained clay-rich sediments as compared to turbiditic sequences typical of Rhone turbiditic system. Such observations suggest that greater degree of lithological heterogeneity in sedimentary strata promotes slope failure in the GoL, most likely related to higher liquefaction potential of coarser-grained material, excess pore pressure and possibly resulting variation in sediment strength.

How to cite: Badhani, S., Cattaneo, A., Colin, F., Marsset, B., Urgeles, R., Leroux, E., Dennielou, B., Rabineau, M., and Droz, L.: Imaging seafloor instabilities using very high-resolution deep-towed multichannel seismic data in the Gulf of Lions (NW Mediterranean), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19658, https://doi.org/10.5194/egusphere-egu2020-19658, 2020.

Numerous subaqueous landslides exhibit spreading failure morphologies which are typically characterized by repetitive patterns of parallel ridges and troughs oriented perpendicular to the direction of movement. Whilst these spreading failures are commonly attributed to (i) downslope removal of material causing unloading of the temporary stable slope or (ii) significant loss of shear strength of the substratum allowing blocks of overlying sediment to detach and slide downslope, their movement rates and potential triggers remain poorly constrained. Spreading appears to be a dominant failure mechanism within the Tuaheni Landslide Complex (TLC) on the Hikurangi Subduction Margin off the coast of Gisborne, New Zealand. A combination of swath bathymetric, 2D and 3D seismic data, drilling investigations and laboratory experiments on sediments recovered from the TLC indicate that this geomorphology has been generated by translational failure. Failure could occur through episodic cycles of movement-arrest in response to either elevated pore fluid pressures or undrained loading during earthquakes. We developed numerical models that integrate this unique data set to explore the processes that lead to spreading failure and determine how large shear strains can be accommodated without accelerating to catastrophic failure. The results provide a novel approach that demonstrates how seafloor morphology can, in part, be controlled by the underlying failure processes

How to cite: Urlaub, M., Carey, J., Crutchley, G., and Mountjoy, J.: Why is the Tuaheni Landslide Complex a spreading failure?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18710, https://doi.org/10.5194/egusphere-egu2020-18710, 2020.

We investigate the tectonically active Northern Sicily Continental margin focusing on the neotectonics affecting the Offshore of Termini (Southern Tyrrhenian Sea) by using high-resolution seismic and multibeam data. The sedimentary succession along the North Sicilian Continental Margin (NSCM) represents the marine prolongation of those outcropping along the Northern Sicily coastal belt. The NSCM has been originated as a consequence of a complex interaction of compressional events, crustal thinning, and strike-slip faulting. E–W, NW–SE, and NE–SW trending, both extensional and compressional faults, with a local strike-slip component, exerted control on the morphology of the present-day shelf and coastal areas during the Pleistocene. During the Quaternary,  the tectonic as well as depositional events have strongly shaped the margin forming the actual complex geomorphic setting of the margin. We present the main results of a high resolution survey that allow to identify several features (e.g. Mass Transport Deposits and pockmarks) linked to gravitational mass movement and fluids escape processes strongly controlled by the tectonics affecting the NSCM. All over the study area, we mapped inside the Late Quaternary depositional sequence repeated and variously distributed MTDs, characterised by transparent/chaotic seismic facies, interbedded to hemipelagic deposits, with seismic facies showing subparallel seismic reflectors of the transgressive and high stand systems tracts. We infer that this MTDs have been seismically induced by earthquakes.  We estimate the recurrence times of earthquakes, by using an elaborate age-model that considers a constant sedimentation rate for the last 11.5 My, between 680 and 2200 years.

How to cite: Zizzo, E., Sulli, A., Spatola, D., Gorini, C., and Gasparo Morticelli, M.: Late Quaternary Tectonics vs Sedimentation history of the offshore Termini in a seismically active segment of the Northern Sicily Continental margin (Southern Tyrrhenian Sea), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5841, https://doi.org/10.5194/egusphere-egu2020-5841, 2020.

Submarine landslides are common features occurring on the flanks of seamounts. Often triggered by earthquakes or volcanic activity, such landslides are potential generators of tsunamis that constitute a dire geohazard for coastal communities. Understanding the recurrence history and geomorphology of seamount-flanking landslides and their link to seismic triggers is crucial for tsunami hazard assessment. This work aims at revealing the recurrence history of the landslides on the Gorringe Bank and their role on regional geohazards. Morphologically, the Gorringe Bank is the largest submarine seamount in Europe, with a length of circa 180 km and a height of 5000 m. It is linked to NW-directed thrusting which led to the exhumation of upper mantle lithologies in this major bathymetric structure. Numerous landslide scars are identified on both its northern and southern flanks, yet there is limited evidence of their presence and morphology on modern bathymetric data. A wealth of 2D seismic reflection profiles from offshore Southwest Portugal is here used to analyse the occurrence, timing and morphology of landslides complexes on the northwestern flank of the Gorringe Bank. A widespread frontal landslide complex of approximate Upper Miocene age is present along the whole flank, likely associated with the main phase of uplift. However, the recurrence of expressive submarine landslides in the Plio-Quarternary sequence is generally focused towards the northern segment of the Gorringe Bank. The geographical correlation between the areas of higher landslide number and clusters of seismicity epicentres suggest a close link between the two. This has direct implications for the assessment of landslide-prone locations on the seamount and to regional tsunami hazard models applicable to the Iberian and Northern African margins.

This work is supported by the FCT funded project MAGICLAND - MArine Geo-hazards InduCed by underwater LANDslides in the SW Iberian Margin (Ref: PTDC/CTA-GEO/30381/2017),

How to cite: Gamboa, D., Omira, R., Terrinha, P., and Piedade, A.: Submarine landslide recurrence and links to seismicity on the Gorringe Bank Seamount, Offshore Portugal, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9031, https://doi.org/10.5194/egusphere-egu2020-9031, 2020.

Submarine mass-failures are recognized worldwide as a potential source of marine geo-hazards. They can compromise the safety and integrity of seafloor and subsurface infrastructures through destroying offshore installations or triggering potential tsunamis. This applies to the SW Iberia Margin, where the occurrence of damaging and tsunamigenic underwater landslides were evidenced in various research works.

This work assesses the geotechnical properties of the marine sediments forming the slopes of the SW Iberia Margin and provides implications to the marine geohazard in the region. Taking advantage of the availability of the cores from previous projects and expeditions (i.e. CONDRIBER, and IODP Expedition-339), we perform conventional Triaxial laboratory tests. These tests allow determining the in-situ shear strength and stress deformation properties, pre-and post-rupture of the undisturbed sediments.

Furthermore, we present, through a landslide case-study in the SW Iberia Margin, a sensitivity analysis of the marine geohazards (sliding mass drag forces and tsunamigenesis) to the geotechnical properties of the marine sediments. We demonstrate that the geotechnical analysis is crucial for an accurate modelling of the submarine mass movements, their impact on offshore installations, and their induced tsunamis.

This work was financed by national funds through FCT—Portuguese Foundation for Science and Technology, I.P., under the framework of the project MAGICLAND – Marine Geo-hazards Induced by Underwater Landslides in the SW Iberian Margin (PTDC/ CTA-GEO/30381/2017).

How to cite: Piedade, A., Santos, N., Lemos, L., Roque, C., Quinta-Ferreira, M., Ramalho, I., and Omira, R.: Geotechnical properties of marine sediments in the SW Iberia Margin – Implications to marine geohazards., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11327, https://doi.org/10.5194/egusphere-egu2020-11327, 2020.

Subaqueous mass failures that comprise slides, slumps and debris flows are a major process that transport sediments from the continental shelf and upper slope to the deep basins (both oceans and lacustrine settings). They are often viewed together with other natural hazards such as earthquakes, and can have serious socioeconomic consequences. It is increasingly important to understand the relationship between mass failures and climate-driven factors such as changes in water-level. Despite extensive marine investigations on this topic world-wide, the relationship between changes in water-level and mass failures is still highly disputed. This is due largely to the significant uncertainties in age dating and different potential triggers and preconditioning factors of mass failure events from different geological settings. Here, we present a 70 kyr-long record of mass failure from the Dead Sea Basin center (ICDP Core 5017-1). This sedimentary sequence has been dated in high accuracy (±0.6 kyr) and has similar responses to climate forcing. Moreover, the mass failure record is interpreted to be controlled by a single trigger mechanism (i.e. seismicity).
Based on the recent detailed study on the sedimentological signature of seismic shaking in the Dead Sea center, these seismogenic mass failures (seismites) account only for a part of the whole seismites catalog, suggesting that mass failure follows only part of seismic shaking irrespective of intensities of the shaking. This is evidenced by the common absence of mass failures following the in situ developed and preserved seismites (e.g., the in situ folded layer and intraclast breccias layer) which represent different intensities of seismic shaking. This feature implies that some non-seismic factor(s) must have preconditioned for the seismogenic mass failures in the Dead Sea center.
Our observations reveal decoupling between change in sedimentation rates and occurrence probability of these seismogenic mass failures, thus suggesting that a change in sedimentation rate is not the preconditioning factor for the failure events. While 79% of seismogenic mass failure events occurred during lake-level rise/drop in contrast to 21% events occurred in the quiescent intervals between. Our dataset implies that seismogenic mass failures can occur at any lake-level state, but are more likely to occur during lake-level rise/drop due to the instability of the basin margins. In addition, the seismogenic mass failures occurred more frequently during glacials (characterized by highstand and high-amplitude lake-level changes) than during interglacials, as a result of the morphologic characteristics of the lake margin slopes and different lithologies (e.g. halite) influences which are both connected to the glacial-interglacial lake-level changes.

How to cite: Lu, Y., Agnon, A., Marco, S., Bookman, R., Waldmann, N., Wetzler, N., Alsop, I., Moernaut, J., Strasser, M., and Hubert-Ferrari, A.: Sharp changes in lake-levels preconditioning seismogenic mass failures in the Dead Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14056, https://doi.org/10.5194/egusphere-egu2020-14056, 2020.

Dynamic Cone Penetration test (CPTu) is a cost and time efficient way of collecting in situ geotechnical parameters (i.e. cone tip resistance, sleeve friction and total pore pressure) of near-surface marine sediments for submarine slope stability analysis. Conventional-established correlation for geotechnical parameters estimation from CPTu are built on static CPTu data, requiring correction of dynamic CPTu sounding records mainly due to strain-rate effects (i.e. increasing of soil resistance due to the increase of applied strain-rate). Empirical correlations have been proposed to overcome this issue, nevertheless, their application requires the quantification of correlation’s coefficients for which no general regression has been derived yet, arising strong uncertainty in data conversion and consequently geotechnical parameter prediction. Moreover, dynamic CPTu parameters are also uncertain due to their inherent variability and instrument precision. In this framework, this study proposes a multivariable Bayesian back-analysis for probabilistic conversion of dynamic CPTu parameters into static CPTu profile. Inherent variability of soil properties, instrument measurements error and uncertainty introduced by correlations used are modeled as random variables and updated within a Bayesian framework. Equivalent samples are randomly generated from established proposal distributions and integrated with parameter’s prior knowledge through a hybrid Markov-chain MonteCarlo procedure. The proposed approach is tested for 20 dynamic CPTu tests, characterized by different impact velocities, performed at Trondheim Fjord. The method applied aims to provide an improvement of strain rate correction with respect traditional data conversion. Preliminary results well match with ones computed from back-calculation employing both static and dynamic CPTu profiles. Results should be further validated for mechanical soil delineation and geotechnical parameters prediction from CPTu.

How to cite: Collico, S., Stegmann, S., Kopf, A., Arroyo, M., and Devincenzi, M.: Bayesian back analysis for dynamic CPTu strain-rate correction., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19799, https://doi.org/10.5194/egusphere-egu2020-19799, 2020.

The 1929 Grand Banks submarine landslide was triggered by a M_w 7.2 strike slip earthquake on the southwestern Grand Banks of Newfoundland. Studies following the event by several decades were the first to recognize that slope failure can cause tsunamis. These studies identified St. Pierre Slope as the main failure area and showed widespread, shallow (<25 m-thick), translational and possibly retrogressive sediment failures occurred predominately in >1700 m water depth (mwd). It seems unlikely this style of failure in deep water generated a tsunami that had >13 m of run-up along the coast of Newfoundland. The objective of this study is to identify possible alternative tsunami source mechanisms and pre-conditioning factors that may have led to sediment instability. These objectives are addressed using a comprehensive data set of multiscale 2D seismic reflection, multibeam swath bathymetry and laboratory geomechanical test data. Results show numerous reflection offsets within the Quaternary section of the slope underneath modern seafloor escarpments (750-2300 mwd).  These offsets appear down to 550 m below seafloor (mbsf) and are interpreted as low angle (~17°), planar-normal faults of <100 m-high vertical and ~330 m of horizontal displacement. The faults are interpreted as part of a massive (~560 km³) complex slump with evidence for multiple décollements (250 mbsf & 400-550 mbsf) and slumping in at least two directions. Infinite slope stability analysis using peak ground acceleration (PGA) indicates that a combination of earthquake loading and the presence of geomechanical weak layers are needed to explain the slope failure. At St. Pierre Slope, the analysis of sediment cores shows that geomechanical weak layers form as a consequence of underconsolidation in connection with excess pore pressures that are related to: 1) high sedimentation rates, 2) instantaneous deposition of mass transport deposits (MTD’s) and sandy turbidites, and 3) the presence of gas. The décollements of the slump are associated with MTD’s and sediment waves that likely form weak layers. The layers of sediment waves are assumed to consist of sorted silts or fine sands and are therefore likely to be susceptible to excess pore pressure development during earthquake loading. Excess pore pressure development results in reduced effective stress and higher potential for instability. It is interpreted, therefore, that the 1929 earthquake triggered displacement of a 550 m-thick slump with ~100 m of vertical seafloor displacement. This instantaneous displacement of the slump in 750 mwd with a seafloor volume displacement of 70 to 130 km² is likely a more effective source for tsunami generation than the translational, shallow (<25 m) failures in deeper water.

How to cite: Schulten, I., Mosher, D., Piper, D., Krastel, S., and MacKillop, K.: Sediment failure of St. Pierre Slope: new insights of failure mechanisms and slope instability due to the 1929 Grand Banks Earthquake , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15909, https://doi.org/10.5194/egusphere-egu2020-15909, 2020.

It is hypothesized that submarine transport of sediments down a continental slope induce physical disintegration of pristine (non-broken) foraminifera shells, and thus mass transport deposits should include a significant percentage of fragmented shells. To validate this hypothesis, we studied two gravity-cores from the Eastern-Mediterranean continental slope, offshore Israel: AM113 sampled within a landslide lobe at 848 m water depth, and AM015 located away from a landslide at 1080 m. At least one interval, ~0.5 m thick, of massive sediments hosting mud-clasds (i.e. debrite) was identified within each core. The timing of these debrites, based on biostratigraphy, oxygen isotopes and total organic carbon data, predates sapropel S1 in both cores and is contemporaneous (AM113) or slightly predates (AM015) the most recent deglaciation.

We found a noticeable increase in benthic and planktic foraminiferal shells fragmentation through the last deglaciation and up to the base of S1. This strongly-fragmented sequence is located in the debrite of AM113 but it is overlaying the debrite of AM015. Accordingly, we suggest two possible mechanisms for the increased fragmentation of foraminiferal shells in both cores: Sediment transport and turbulence related to submarine mass-transport events, or geochemical changes in the lower water column properties at the transition from MIS-2 to the Holocene.

How to cite: Katz, O., Ashkenazi, L., Sultan-Levi, S., Abramovich, S., Almogi-Labin, A., and Hyams-Kaphzan, O.: Characterization of recent deep-sea debrites in the Eastern Mediterranean based on foraminiferal taphonomy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7973, https://doi.org/10.5194/egusphere-egu2020-7973, 2020.

Earthquake-induced submarine slope destabilization is known to cause debris flows and turbidity currents. These also interact with currents caused by tsunami and seiches resulting in deposits with specific sedimentological characteristics, turbidite-homogenites being a common example. Data on the deep-sea hydrodynamic events following earthquakes are, however, limited. An instrumented frame deployed at the seafloor in the Sea of Marmara Central Basin recorded some of the consequences of a magnitude 5.8 earthquake that occurred Sept 26, 2019 at 10-12 km depth without causing any significant tsunami. The instrumentation comprises a Digiquartz® pressure sensor recording at 5 s interval and a 1.9-2 MHz Doppler recording current meter set 1.5 m above the seafloor and recording at 1-hour interval. The device was deployed at 1184 m depth on the floor of the basin near the outlet of a canyon, 5 km from the epicenter. Chirp sediment sounder profiles indicate a depositional fan or lobe is present at this location. The passing of the seismic wave was recorded by the pressure sensor, but little other perturbation is recorded until 25 minutes later when the instrument, probably hit by a mud flow, tilts by 65° in about 15 seconds. Over the following 10 hours the tilted instrument records bursts of current of variable directions. The last burst appears to be the strongest with velocities in the 20-50 cm/s range, causing enough erosion to free the device from the mud and allowing the buoyancy attached to the upper part of the frame to straighten it back to its normal operation position. Then, the current, flowing down along the canyon axis, progressively decays to background level (≈2 cm/s) in 8 hours. Doppler signal backscatter strength is a proxy for turbidity, sensitive to sand-size suspended particles. Signal strength increased to high values during the event (max -7.6 dB from a background value of -40dB) and decayed over the next three days. These observations show that even a moderate earthquake can trigger a complex response involving mud flows and turbidity currents. We infer simultaneous slope failures at various locations may produce complex current patterns and cause build-up of kinetic energy over several hours.

How to cite: Henry, P., Özeren, M. S., Yakupoğlu, N., Çakir, Z., de Saint-Léger, E., Desprez de Gésincourt, O., Tengberg, A., Chevalier, C., Papoutsellis, C., Postacıoğlu, N., Dogan, U., and Çağatay, M. N.: Slow build-up of turbid currents triggered by a moderate earthquake in the Sea of Marmara, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10289, https://doi.org/10.5194/egusphere-egu2020-10289, 2020.

To better understand earthquake reoccurrence and hazard, records of ancient earthquakes spanning millennia are routinely generated using seabed turbidite deposits, inferred to be synchronously triggered by strong ground motions over large geographic areas. However, the use of turbidites for paleoseismology is underpinned by untested hypotheses due to the dearth of verified earthquake-triggered turbidite deposits produced by well-characterised earthquakes. The aim of a RV Tangaroa voyage in October 2019 was to use the unique opportunity provided by the widespread and documented occurrences of the 2016 Mw7.8 Kaikōura earthquake-triggered turbidite to determine whether synchronous turbidite deposition can be reconstructed from the sedimentary record alone. Our presentation will summarise insights gleaned from precision short cores and high-resolution sub-bottom profiles collected along and across the axis of submarine canyons that preserve turbidite deposits triggered by the Kaikōura earthquake. Planned detailed laboratory characterisation of the turbidites will include high-resolution core imaging, texture, densitometry, down-core physical properties and geochemical characterisation combined with radioisotope-derived chronology. Through repeat coring at historical sites and future coring campaigns we hope to also quantify the impact of biological mixing on the Kaikōura event deposit to determine its likely preservation potential in the geological record. Our results have the potential to provide the first robust test of turbidite paleoseismology.

How to cite: Orpin, A., Howarth, J., Maier, K., Nodder, S., and Strachan, L.: Testing the veracity of turbidite paleoseismology using the Kaikōura earthquake-triggered turbidite, New Zealand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21034, https://doi.org/10.5194/egusphere-egu2020-21034, 2020.

Turbidity currents form many of the largest sediment accumulations, longest channels, and deepest canyons on our planet. These seabed sediment avalanches can be very (> 10 m/s) fast, runout for hundreds of kilometres, and break seabed cables that now form the backbone of the internet and global data transfer. It was once thought that detailed monitoring of turbidity currents in action was impractical, ensuring these flows were relatively poorly understood. However, a series of recent projects have used new approaches and technology to show how these flows can be measured in shallow water (< 2 km) settings, such as Monterey Canyon and Canadian fjords, where flows ran out for < ~50 km and had speeds of up to 8 m/s. Here we present initial results from an ambitious project to measure active flows that runout for >1,000 km to form a major submarine fan in the deep ocean. The project studies the Congo submarine canyon-channel system that extends for ~1,100 km from the mouth of the Congo River, offshore West Africa. Monitoring in 2010 at a single site in the upper Congo Canyon had previously shown that flows are active for ~30% of the time, and reach speeds of up to 3 m/s. In this new project, direct flow monitoring at 11 sites are being combined with detailed time-lapse mapping and coring of flow deposits, through a series of 4 or 5 major research cruises from 2019 to 2023. Here we present initial results from the first of these cruises (JC187) in August-to-October 2019, which placed 11 moorings with sensors at water depths of 1.6 to 5.5 km. The presentation will initially focus on the geomorphology of the channel system, and how it varies down-slope and through time. For example, it is apparent that a landslide partly blocked one location in the upper canyon in the last 20 years, causing meander bend cut-off and sediment ponding. The talk will then discuss models for how submarine channel bends evolve, and the implications for channel deposits. Recent work in sandy submarine channels suggests that they can be dominated by very fast-moving knickpoints (waterfall like features). However, the much muddier Congo channel displays well-developed meander bend bars for which cores are available. We therefore start to show how muddy deep-sea channels may differ in significant ways from their sandier cousins in shallow water. 

How to cite: Talling, P., de Silva Jacinto, R., Baker, M., Pope, E., Heijnen, M., Hage, S., Heerema, C., Simmons, S., McGhee, C., Ruffell, S., Hasenhündl, M., Apprioual, R., Ferrant, A., Urlaub, M., Cartigny, M., Clare, M., Parsons, D., Dennielou, B., Gaillot, A., and Peirce, C.: First direct monitoring and time-lapse mapping starts to reveal how a large submarine fan works, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2407, https://doi.org/10.5194/egusphere-egu2020-2407, 2020.