Morelia Urlaub

What causes large submarine landslides on low gradient (<2°) continental slopes with slow (∼0.15 m/kyr) sediment accumulation?

Submarine landslides can cause damaging tsunamis, the height of which scales up with the volume of the displaced mass. The largest underwater landslides are far bigger than any landslides on land, and these submarine megaslides tend to occur on open continental slopes with remarkably low gradients of less than 2°. For geohazard assessments it is essential to understand what preconditions and triggers slope failure on such low gradients. Previous work has suggested that generation of high excess pore pressure due to rapid sediment deposition plays a key role in such failures. However, submarine slope failure also occurs where sedimentation rates are low (<0.15 m/kyr), such as off northwest Africa. We use a fully coupled stress and fluid flow finite element model to test whether such low sedimentation rates can generate sufficient excess pore pressures to cause failure of a 2° slope. The sensitivity of overpressure generation and slope stability is assessed with respect to different sedimentation rates and patterns, sediment consolidation properties, and stratigraphic layer configurations. The simulations show that, in general, it is difficult to generate significant excess pore pressure if sediment accumulation is slow and the only pressure source. However, we identify a sediment compression behavior that can lead to submarine landslides in locations worldwide. Our results imply that compressibility is an important factor for the stability of low gradient continental slopes.

How Do ∼2° Slopes Fail in Areas of Slow Sedimentation? A Sensitivity Study on the Influence of Accumulation Rate and Permeability on Submarine Slope Stability

Overpressure generation due to rapid sediment deposition can result in low effective stresses within the sediment column. It has been proposed that these large overpressures are the main preconditioning factor for causing large-scale submarine slope failure on passive continental margins, such as those in the Gulf of Mexico and offshore Norway. The rate of overpressure generation depends on the sedimentation rate, sediment compressibility and permeability. The Gulf of Mexico and the Norwegian continental slope have experienced comparatively high sediment input, but large-scale slope failure also occurs in locations with very low sedimentation rates such as the Northwest African continental margin. Here we show results from 2D numerical modelling of a 2° continental slope subjected to deposition rates of 0.15 m/ka. These results do not indicate any evidence for significant overpressure or slope instability. We conclude that factors other than overpressure must be fundamental for initiating slope failure, at least in locations with low sedimentation rates.

Sea-level - induced seismicity and submarine landslide occurrence: Comment

Brothers et al. (2013) present modeling showing how rapid sea-level rise (SLR) could generate increases in stresses experienced by fault systems, which may lead to increased seismicity. This potential for linkage between rapid SLR and increased seismicity is an important result of widespread interest. However, a key assertion by Brothers et al. is that such increases in seismicity can then explain the “temporal coincidence between rapid late Pleistocene sea-level rise and large-scale slope failures.” The primary purpose of this Comment is to note that available age-dating of large submarine landslides (hereafter: slides) is not consistent with such a view. Once realistic uncertainties in slide ages are considered (Urlaub et al., 2013), there is either (1) no statistical relationship between rapid SLR and large slide frequency, or (2) the uncertainties in age-dates are too large for a correlation between rapid SLR and landslide timing to be recognized, even if a correlation existed. It is important to have a clear understanding of whether the frequency of large slides is (or is not) dependent on sea level. First, submarine slides can potentially generate damaging tsunamis. We need to understand whether there is an increased landslide-tsunami risk in the future as sea level rises. Second, these large slides are one of the major processes for moving sediment across Earth, and factors that precondition and trigger these prodigious failures are still poorly understood. Urlaub et al. (2013) provide the most recent and largest (n = 68) collection of ages for large (>1 km) submarine slides. Importantly, this study considered the uncertainties in these ages, which are mainly due to the position of samples relative to the slide deposit, and to vertical mixing of sediment by organisms (Urlaub et al., 2013, their figure 1). Urlaub et al. show that large slide ages can be described by a Poisson distribution that characterizes a temporally random process, and that peaks in slide frequency can be reproduced when ages are drawn from a temporally random distribution. The frequency distributions of ancient slides surrounding a basin margin, which disintegrate to form large turbidity currents, also appear to have a temporally random Poisson distribution (Clare et al., 2014).

Diatom ooze: Crucial for the generation of submarine mega-slides?

Numerous studies invoke weak layers to explain the occurrence of submarine mega-slides (>100 km3), in particular those on very gentle slopes (<3°). Failure conditions are thought to be met only within this layer, which is embedded between stable sediments. Although key to understanding failure mechanisms, little is known about the nature and composition of such weak layers, mainly because they are destroyed with the landslides. This study is the first to place detailed constraints on the weak layer for one of the submarine mega-slides that occurred on the nearly flat, subtropical, northwest African continental slopes. Integrating results from the Ocean Drilling Program with high-resolution seismic reflection data, we show that the failure surfaces traced into the undisturbed sedimentary sequence coincide with thin (<10 m) diatom ooze layers capped by clay. As diatom oozes are common on many continental margins, we suggest a new margin-independent failure mechanism to explain submarine mega-slides at low-gradient continental slopes globally. Diatom oozes are susceptible to building up excess pore fluid during burial due to their high compressibility and water content. If a low-permeable clay cap prevents upward drainage, excess pore pressures accumulate at the ooze-clay interface, causing the shearing resistance to increase at a lower rate than the shear stress until failure can occur. Changes in global climate affect the abundance of diatoms and thus formation of diatom oozes, thereby preconditioning the sediments for failure. However, the actual timing of failure is independent of environmental changes.

Evidence of the Zanclean megaflood in the eastern Mediterranean Basin

The Messinian salinity crisis (MSC) - the most abrupt, global-scale environmental change since the end of the Cretaceous – is widely associated with partial desiccation of the Mediterranean Sea. A major open question is the way normal marine conditions were abruptly restored at the end of the MSC. Here we use geological and geophysical data to identify an extensive, buried and chaotic sedimentary body deposited in the western Ionian Basin after the massive Messinian salts and before the Plio-Quaternary open-marine sedimentary sequence. We show that this body is consistent with the passage of a megaflood from the western to the eastern Mediterranean Sea via a south-eastern Sicilian gateway. Our findings provide evidence for a large amplitude drawdown in the Ionian Basin during the MSC, support the scenario of a Mediterranean-wide catastrophic flood at the end of the MSC, and suggest that the identified sedimentary body is the largest known megaflood deposit on Earth.

Gravitational collapse of Mount Etna’s southeastern flank

Gravitational collapse of Mount Etna’s SE flank: New seafloor geodetic data capture active displacement of underwater volcanic flank. The southeastern flank of Etna volcano slides into the Ionian Sea at rates of centimeters per year. The prevailing understanding is that pressurization of the magmatic system, and not gravitational forces, controls flank movement, although this has also been proposed. So far, it has not been possible to separate between these processes, because no data on offshore deformation were available until we conducted the first long-term seafloor displacement monitoring campaign from April 2016 until July 2017. Unprecedented seafloor geodetic data reveal a >4-cm slip along the offshore extension of a fault related to flank kinematics during one 8-day-long event in May 2017, while displacement on land peaked at ~4 cm at the coast. As deformation increases away from the magmatic system, the bulk of Mount Etna’s present continuous deformation must be driven by gravity while being further destabilized by magma dynamics. We cannot exclude flank movement to evolve into catastrophic collapse, implying that Etna’s flank movement poses a much greater hazard than previously thought. The hazard of flank collapse might be underestimated at other coastal and ocean island volcanoes, where the dynamics of submerged flanks are unknown.

A consistent global approach for morphometric characterisation of subaqueous landslides

Abstract Landslides are common in aquatic settings worldwide, from lakes and coastal environments to the deep sea. Fast-moving, large-volume landslides can potentially trigger destructive tsunamis. Landslides damage and disrupt global communication links and other critical marine infrastructure. Landslide deposits act as foci for localized, but important, deep-seafloor biological communities. Under burial, landslide deposits play an important role in a successful petroleum system. While the broad importance of understanding subaqueous landslide processes is evident, a number of important scientific questions have yet to receive the needed attention. Collecting quantitative data is a critical step to addressing questions surrounding subaqueous landslides. Quantitative metrics of subaqueous landslides are routinely recorded, but which ones, and how they are defined, depends on the end-user focus. Differences in focus can inhibit communication of knowledge between communities, and complicate comparative analysis. This study outlines an approach specifically for consistent measurement of subaqueous landslide morphometrics to be used in the design of a broader, global open-source, peer-curated database. Examples from different settings illustrate how the approach can be applied, as well as the difficulties encountered when analysing different landslides and data types. Standardizing data collection for subaqueous landslides should result in more accurate geohazard predictions and resource estimation.

A Numerical Investigation of Sediment Destructuring as a Potential Globally Widespread Trigger for Large Submarine Landslides on Low Gradients

Submarine landslides on open continental slopes can be far larger than any slope failure on land and occur in locations worldwide on gradients of <2°. Significantly elevated pore pressure is necessary to overcome the sediment’s shearing resistance on such remarkably low gradients, but the processes causing such overpressure generation are contentious, especially in areas with slow sedimentation rates. Here we propose that the progressive loss of interparticle bonding and fabric could cause such high excess pore pressure. Slow sedimentation may favour the formation of a structural framework in the sediment that is load-bearing until yield stress is reached. The bonds then break down, causing an abrupt porosity decrease and consequently overpressure as pore fluid cannot escape sufficiently rapidly. To test this hypothesis, we implement such a loss of structure into a 2D fully coupled stress-fluid flow Finite Element model of a submerged low angle slope, and simulate consolidation due to slow sedimentation. The results suggest that destructuring could indeed be a critical process for submarine slope stability.

Seismogenic faults, landslides, and associated tsunamis off southern Italy - Cruise No. M86/2 - December 27, 2011 - January 17, 2012 - Cartagena (Spain) - Brindisi (Italy)

The continental margins of southern Italy are located along converging plate boundaries, which are affected by intense seismicity and volcanic activity. Most of the coastal areas experienced severe earthquakes, landslides, and tsunamis in historical and/or modern times. The most prominent example is the Messina earthquake of Dec. 28, 1908 (Ms=7.3; 80,000 casualties), which was characterized by the worst tsunami Italy experienced in the historical time (~2000 casualties). It is, however, still unclear, whether this tsunami was triggered by a sudden vertical movement along a major fault during the earthquake or as a result of a giant marine slide initiated by the earthquake. The recurrence rates of major landslides and therefore the risk associated with landslides is also unknown. Based on detailed bathymetric data sets collected by Italian colleagues in the frame of the MaGIC Project (Marine Geohazards along the Italian Coast), we collected seismic data (2D and 3D) and gravity cores in three working areas (The Messina Straits, off Eastern Sicily, the Gioia Basin). A dense grid of new 2D-seismic data in the Messina Straits will allow to map fault patterns in great detail. One interesting outcome in this context is the identification of a set of normal faults striking in an EW-direction, which is almost perpendicular to the previously postulated faults. This EW-striking faults seem to be active. The area off eastern Sicily is characterized by numerous landslides and a complex deformation pattern. A 3D-seismic data set has been collected during the cruise using the so called P-cable in order to investigate these deformation patterns in detail. The new data will be the basis for a risk assessment in the working areas.