Katrin Huhn

Submarine mass movements and their consequences : 6th International Symposium

We present a short review of the state-of-the-art numerical tools that have been used for modeling landslide-generated waves. A comparative study is conducted on the physical properties of earthquake- and landslide-generated waves suggesting that both dispersion and nonlinearity effects may be neglected for the former waves whereas they may be considered for the latter ones. We introduce landslide tsunami models and group them into three classes: (1) models treating the moving mass as a fluid, (2) models estimating the initial water surface, and (3) models fed by the transient seafloor deformation. Selection of a particular model from the list of models introduced here depends on: (1) the dimensions of the source, (2) the available computing capacities, (3) availability of fine bathymetric grid, and (4) the purposes of the modeling.

A conceptual model of pore-space blockage in mixed sediments using a new numerical approach, with implications for sediment bed stabilization

In mixed sediment beds, erosion resistance can change relative to that of beds composed of a uniform sediment because of varying textural and/or other grain-size parameters, with effects on pore water flow that are difficult to quantify by means of analogue techniques. To overcome this difficulty, a three-dimensional numerical model was developed using a finite difference method (FDM) flow model coupled with a distinct element method (DEM) particle model. The main aim was to investigate, at a high spatial resolution, the physical processes occurring during the initiation of motion of single grains at the sediment–water interface and in the shallow subsurface of simplified sediment beds under different flow velocities. Increasing proportions of very fine sand (D50=0.08 mm) were mixed into a coarse sand matrix (D50=0.6 mm) to simulate mixed sediment beds, starting with a pure coarse sand bed in experiment 1 (0 wt% fines), and proceeding through experiment 2 (6.5 wt% fines), experiment 3 (10.5 wt% fines), and experiment 4 (28.7 wt% fines). All mixed beds were tested for their erosion behavior at predefined flow velocities varying in the range of U1-5=10–30 cm/s. The experiments show that, with increasing fine content, the smaller particles increasingly fill the spaces between the larger particles. As a consequence, pore water inflow into the sediment is increasingly blocked, i.e., there is a decrease in pore water flow velocity and, hence, in the flow momentum available to entrain particles. These findings are portrayed in a new conceptual model of enhanced sediment bed stabilization.

Submarine Slope Stability Assessment of the Central Mediterranean Continental Margin: The Gela Basin

This study investigates slope stability for a relatively small scale (5.7 km2, 0.6 km3), 8 kyr old landslide named Northern Twin Slide (NTS) at the slope of the Gela Basin in the Sicily Channel (central Mediterranean). The NTS is characterized by two prominent failure scars, forming two morphological steps of 110 and 70 m height. Geotechnical data from a drill core upslope the failure scar (GeoB14403) recovered sediments down to ∼52 m below seafloor (mbsf). The deposits show low over consolidation ratio (OCR = 0.24–0.4) and low internal friction angle (20–22°) around 28–45 mbsf, which suggests this mechanically weak interval may act as potential location of instability in a future failure event. Oedometer tests attest the sediments are highly under consolidated and the average overpressure ratio (λ*) is ∼0.7. Slope stability analyses carried out for different scenarios indicate that the slope is stable both under static undrained and drained conditions. A relatively small horizontal acceleration of 0.03–0.08 g induced by an earthquake may be sufficient to cause failure. We propose that moderate seismic triggers may have been responsible for the twin slide formation and could also cause mass wasting in the future.

Integrated Stratigraphic and Morphological Investigation of the Twin Slide Complex Offshore Southern Sicily

The Holocene Twin Slides form the most recent of recurrent mass wasting events along the NE portion of Gela Basin within the Sicily Channel, central Mediterranean Sea. Here, we present new evidence on the morphological evolution and stratigraphic context of this coeval slide complex based on deep-drilled sediment sequences providing a >100 ka paleo-oceanographic record. Both Northern (NTS) and Southern Twin Slide (STS) involve two failure stages, a debris avalanche and a translational slide, but are strongly affected by distinct preconditioning factors linked to the older and buried Father Slide. Core-acoustic correlations suggest that sliding occurred along sub-horizontal weak layers reflecting abrupt physical changes in lithology or mechanical properties. Our results show further that headwall failure predominantly took place along sub-vertical normal faults, partly through reactivation of buried Father Slide headscarps.

Do Embedded Volcanoclastic Layers Serve as Potential Glide Planes?: An Integrated Analysis from the Gela Basin Offshore Southern Sicily

The NE portion of the Gela Basin (Strait of Sicily) shows evidence of multiple mass wasting events of predominantly translational character. In this context, recent investigations implicate volcanoclastic layers as key stratigraphic surfaces acting as preferential planes of failure. We present an integrated analysis of a representative sedimentary transition from overlying homogeneous background sedimentation of silty clay to a volcanoclastic layer. A high-resolution CT scan and three drained direct-shear laboratory experiments from a 20 cm whole-round section (~28.2 mbsf) allow the delineation of the role of this volcanoclastic layer in the framework of slope stability and failure initiation. The mechanical results indicate a general strengthening of the material with increased volcanoclastic content. Tendency for failure is expected to be highest within the silty clay due to relatively lower shear strength and strain-weakening behaviour, which promotes progressive sediment failure. In contrast with recent findings, this suggests that volcanoclastic sediment would not act as a weak layer. However, the volcanoclastic layer exhibits significant mesoporosity (i.e., fracturing) and may therefore host large volumes of fluid. Temporarily undrained conditions, for example during seismic activity, could transiently elevate fluid pressures and thus reduce the material shear strength below that of the surrounding silty clay. Such a weak layer may preferentially form along the interface of fractured volcanoclastic material and relatively impermeable silty clay, where differences in material strengths are lowest.

A Fluid Pulse on the Hikurangi Subduction Margin: Evidence From a Heat Flux Transect Across the Upper Limit of Gas Hydrate Stability: Fluid Pulse on the Hikurangi Margin

A transect of seafloor heat probe measurements on the Hikurangi Margin shows a significant increase of thermal gradients upslope of the updip limit of gas hydrate stability at the seafloor. We interpret these anomalously high thermal gradients as evidence for a fluid pulse leading to advective heat flux, while endothermic cooling from gas hydrate dissociation depresses temperatures in the hydrate stability field. Previous studies predict a seamount on the subducting Pacific Plate to cause significant overpressure beneath our study area, which may be the source of the fluid pulse. Double-bottom simulating reflections are present in our study area and likely caused by uplift based on gas hydrate phase boundary considerations, although we cannot exclude a thermogenic origin. We suggest that uplift may be associated with the leading edge of the subducting seamount. Our results provide further evidence for the transient nature of fluid expulsion in subduction zones.

Interrelationship Between Sediment Fabric, Pore Volume Variations as Indicator for Pore Pressure Changes, and Sediment Shear Strength

The physical characterization of sediments forms the basis for slope stability analysis. The shear strength of water-saturated sediments is a function of sediment properties and pore pressure conditions. A reduction in strength, e.g., as a result of transient pore pressure changes, can cause the collapse of the sediment matrix and subsequently slope sediment fails. As failure processes in the sediment elude from direct observation, matrix deformation processes, sediment grain interactions, and the associated stresses can be examined by numerical simulations. The major aim of this study was to evaluate the effects of varied sediment fabric on pore volume changes and sediment strength in sheared samples. For all sediment fabrics, an inversely proportional relationship between strength and porosity was observed. A controlling effect of grain shape on the maximum friction coefficient value was demonstrated.

Landslide Frequency and Failure Mechanisms at NE Gela Basin (Strait of Sicily)

Despite intense research by both academia and industry, the parameters controlling slope stability at continental margins are often speculated upon. Lack of core recovery and age control on failed sediments prevent the assessment of failure timing/frequency and the role of pre-failure architecture as shaped by paleoenvironmental changes. This study uses an integrated chronological framework from two boreholes and complementary ultra-high-resolution acoustic profiling in order to assess (1) the frequency of submarine landsliding at the continental margin of NE Gela Basin and (2) the associated mechanisms of failure. Accurate age control was achieved through absolute radiocarbon dating and indirect dating relying on isotope stratigraphic and micropaleontological reconstructions. A total of nine major slope failure events have been recognized that occurred within the last 87 kyr (~10 kyr return frequency), though there is evidence for additional syn-depositional, small-scaled transport processes of lower volume. Preferential failure involves translational movement of mudflows along sub-horizontal surfaces that are induced by sedimentological changes relating to pre-failure stratal architecture. Along with sequence-stratigraphic boundaries reflecting paleoenvironmental fluctuations, recovered core material suggests that intercalated volcaniclastic layers are key to the basal confinement and lateral movement of these events in the study area. Another major predisposing factor is given by rapid loading of fine-grained homogenous strata and successive generation of excess pore pressure, as expressed by several fluid escape structures. Recurrent failure, however, requires repeated generation of favorable conditions and seismic activity, though low if compared to many other Mediterranean settings, is shown to represent a legitimate trigger mechanism.

A computational investigation of the interstitial flow induced by a variably thick blanket of very fine sand covering a coarse sand bed

Blanketed sediment beds can have different bed mobility characteristics relative to those of beds composed of uniform grain-size distribution. Most of the processes that affect bed mobility act in the direct vicinity of the bed or even within the bed itself. To simulate the general conditions of analogue experiments, a high-resolution three-dimensional numerical ‘flume tank’ model was developed using a coupled finite difference method flow model and a discrete element method particle model. The method was applied to investigate the physical processes within blanketed sediment beds under the influence of varying flow velocities. Four suites of simulations, in which a matrix of uniform large grains (600 μm) was blanketed by variably thick layers of small particles (80 μm; blanket layer thickness approx. 80, 350, 500 and 700 μm), were carried out. All beds were subjected to five predefined flow velocities (U1–5=10–30 cm/s). The fluid profiles, relative particle distances and porosity changes within the bed were determined for each configuration. The data show that, as the thickness of the blanket layer increases, increasingly more small particles accumulate in the indentations between the larger particles closest to the surface. This results in decreased porosity and reduced flow into the bed. In addition, with increasing blanket layer thickness, an increasingly larger number of smaller particles are forced into the pore spaces between the larger particles, causing further reduction in porosity. This ultimately causes the interstitial flow, which would normally allow entrainment of particles in the deeper parts of the bed, to decrease to such an extent that the bed is stabilized.

Quantifying the Key Role of Slope Material Peak Strength – Using Discrete Element Simulations

This study investigates how progressive oversteepening and fault kinematics impact on slope failure initiation and subsequent landsliding along subsiding basin flanks using 2D DEM simulations. We use large assemblages of granular particles to simulate the deformation behaviour of slope sediments with varying peak strength. Sediments with high peak strength deform preferentially on major faults and produce a stepped topography and a stable slope in the long-term. Mass failures in these sediments occur as large, compact slides of short run out. In contrast, slopes with lower peak strength deform diffusely and present large numbers of faults that fail frequently and maintain the slope at its critical angle of inclination. The resulting slope topography is smoother and laterally more elongated. These differences in mass movements are governed by (i) characteristic fault patterns, and (ii) repeated oversteepening during ongoing basin subsidence, which is an important prerequisite for failure initiation. Our experiments indicate quantitatively that the failure distribution, dimension, and transport mechanism, as well as the recurrence rate of landslides are essentially controlled by the peak strength of the failed material.