Seafloor instabilities and sediment deformation processes: The need for integrated, multi-disciplinary investigations

Abstract

Please note that this is an author-produced PDF of an article accepted for publication following peer review. a Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site 2Highlights ► Sediment deformation and instability processes require integrated investigations ► Regional geology and environment control pre-conditioning factors ► There is paucity of high-quality geotechnical data and links with geophysics ► Challenges remain with triggers, pore pressure, landslide dynamics and weak layers ► Hazard and risk require event identification, recurrence frequency and magnitudes ACCEPTED MANUSCRIPT 5 to be addressed. A few examples of causative or preconditioning factors not addressed in this paper are permafrost and freeze-thaw cycles, large sediment input at deltas, large tidal fluctuations, static and cyclic liquefaction, or creep. Other types of instabilities, like rock falls, lie beyond the scope of this work. Offshore GeohazardsA geohazards is defined as a geological condition which represents -or has the potential to develop further into -a situation leading to damage or uncontrolled risk. Submarine landsliding is one of the most critical of offshore geohazards. Other examples include shallow sediment deformation phenomena (e.g., mud diapirism, gas chimneys, pockmarks, gas hydrates), but also shallow water flows, shallow gas accumulations, and seismicity ( Figure 1). Specific threats to society are the disappearance of valuable land near the shorelines, devastation of coastal areas by landslidegenerated tsunamis and the destruction of seafloor installations (e.g., communication cables, pipelines, templates). Assessing and mitigating offshore geohazards also implies estimating of risks, where probabilities associated with the different scenarios are difficult to quantify (Vanneste et al., 2011a). This becomes of primary importance as exploration and field development has developed rapidly in areas where geohazards may pose a risk since the 1990's, considering that both natural causes and human interferences may provide the ultimate trigger for instability.The results from large-scale national and international geosurvey efforts (e.g., MAREANO, Norway;MaGIC, Italy; ZEE project, Spain; UNCLOS, USA; NEREIDA, Canada) -in addition to the site surveying and reservoir imaging done by the offshore industry -to map the continental margins in ever increasing resolution, clearly indicate that recent mass movements with a variety of dimensions are common features on the seabed. More data were collected in the framework of dedicated slope stability investigation programmes, e.g., COSTA. In this paper, we discriminate between mass movements or slope instabilities on the one hand side and more local sediment deformation processes on the other side, as these phenomena may require different investigation methods due to A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 7 clays with extrem...

Dynamic growth of slip surfaces in catastrophic landslides

“…Thus, our dynamic analysis results in a slope failure length typical for many landslides [2–8,19]. It also shows that the static analysis underestimates this length by nearly a factor of two, which is in agreement with §5.…”

“…Various mechanisms such as earthquake-triggered liquefaction [27], methane-hydrate decomposition [8,28,29], excess pore pressure (i.e. pressure above hydrostatic) induced by rapid sedimentation [3,11] and local fluid fluxes [11,30] may cause the initial weak zone [23,24].…”

“…Quinn et al [16], for example, suggest τ r / τ p =0.013 for subaerial sensitive clays in Quebec area (Canada). Sensitivity of the sediment material appears to be a major factor in the deformation-softening process [5], and promotes strain localization and propagation of shear bands [8]. As a result, many landslides occurred in slopes composed of sensitive clays [16,60–62].…”

“…Landslide occurrence, morphology, distribution, scale and consequences have been discussed in detail [1–8]. This work considers slides caused by a shear band that develops below the sliding mass along the potential slip (rupture) surface in long slopes [1,5,9,10].…”

Geomorphological characteristics and variability of Holocene mass-transport complexes, St. Lawrence River Estuary, Canada

“…The common occurrence of submarine landslides in various geologic settings, on the present-day seafloor and in the sedimentary record of ancient basins, makes slope failure an important element for our understanding of sedimentary budget, remobilization and transport of sediments downslope, shaping of the seafloor and potential hazards to offshore infrastructure and coastal areas (Canals et al, 2004;Vanneste et al, 2014). However, compared with their onshore equivalents, submarine landslides or mass-transport complexes (MTCs) are more difficult to characterize from a geotechnical point of view because of their location in the marine realm.…”