Supplementary material to “Scientific Ocean Drilling Behind the Assessment of Geo-hazards From Submarine Slides”
Roger Urgeles and Angelo Camerlenghi, Departement d’Estratigrafia, Paleontologia i Geociéncies Marines, Universitat de Barcelona, Barcelona, Spain; Gemma Ercilla, Departamento de Geologia Marina i Oceanografia Física, Institut de Ciéncies del Mar, CSIC, Barcelona, Spain; Flavio Anselmetti, Geological Institute ETHZ, Zürich, Switzerland; Warner Brückmann, Leibniz-Institute for Marine Sciences, IFM-GEOMAR, Kiel, Germany; Miquel Canals, Departement d’Estratigrafia, Paleontologia i Geociéncies Marines, Universitat de Barcelona, Barcelona, Spain; Eulàlia Gràcia, Unitat de Tecnologia Marina, CSIC, Barcelona, Spain; Jacques Locat, Départament de Géologie et de Génie Géologique, Université Laval, Québec, Canada; Sebastian Krastel, DFG-Research Center Ocean Margins, University of Bremen, Bremen, Germany; Anders Solheim, International Centre for Geohazards/Norwegian Geotechnical Institute, Oslo
Citation:
Urgeles, R., A. Camerlenghi, G. Ercilla, F. Anselmetti, W. Brückmann, M. Canals, E. Gràcia, J. Locat, S. Krastel, and A. Solheim (2007), Scientific Ocean Drilling Behind the Assessment of Geo-hazards From Submarine Slides, Eos Trans. AGU, 88(17), 192.
[Full Article (pdf)]
Submarine slope instabilities are of significant interest from many aspects. They represent a major geohazard for offshore infra-structures (platforms, pipelines, cables and sub-sea installations) and may even cut back into onshore facilities (e.g. Longva et al., 2003; Sultan et al., 2004). In addition, they can create local, destructive tsunamis that pose a threat to coastal structures and population (e.g. Tappin et al., 2001; Lee et al., 2003). Submarine landslides can be used to make inferences regarding the hazard derived from earthquakes and volcanic activity, for instance, using slope failure back-analysis as a proxy for estimating paleo-seismicity. A relationship between stability of volcanic island flanks and volcanic eruptions may exist, though it is not yet well understood (Moore et al., 1994; Urgeles et al., 1999). Gas hydrate dissociation may play a role in initiating slope instabilities (Kenett et al., 2002), but large submarine landslides also have the potential of triggering dissociation and thereby releasing large quantities of methane to the atmosphere. Therefore submarine slope instabilities are also relevant to the understanding of natural climate changes. Submarine landslides are also a key element to understand sedimentary basin evolution. Detailed physical and sedimentological characterization of mass transport deposits has allowed the oil industry to better identify hydrocarbon reservoirs and seals (e.g. Weimer, 2004).
Despite all these implications, there is no known case of a medium to large size submarine mass movement that has been investigated to the point as to confirm the geometry and the in situ stresses and their evolution at the time of failure. This lack of knowledge is mainly caused by the fact that the stratigraphic horizons, in which failure occurred or along which the flows were deposited, are often not accessible by conventional coring techniques. An exception is the Storegga Slide on the Norwegian margin, studied extensively within the 'Ormen Lange Project' (Solheim et al., 2005) launched by the license partners of the Ormen Lange Gas Field. The lesson from this experience is that besides extensive geophysical surveys and seabed characterization, geotechnical boring and in situ measurements both in and outside the landslide body must be used to define and constrain the geotechnical parameters to a level that the uncertainty is limited and their lateral variability can be understood and, eventually, predicted. Drilling and in situ measuring is therefore considered as a fundamental tool to improve our understanding of submarine mass movements and to contribute to assess the potential occurrence of natural disasters.
From October 25 to 27 2006, the workshop 'Scientific ocean drilling behind the assessment of geo-hazards from submarine slides' was held in Barcelona, Catalonia, Spain. The workshop grouped 50 scientists and representatives of private companies, mainly from the Europe, representing a wide spectrum of disciplines such as geophysics, stratigraphy, sedimentology, palaeoceanography, marine geotechnology, geotechnical engineering and tsunami modeling (full details of participants and program can be found in http://www.geohazards.no/IGCP511/). The workshop is one of the European Science Foundation (ESF; http://www.esf.org) Workshops on Marine Research Drilling (Magellan Workshop Series) whose aim is to stimulate the development of innovative scientific ocean drilling proposals and hence, to ensure the effective exploitation of research opportunities. The workshop was co-funded by the European Consortium for Ocean Drilling (ECORD), Consejo Superior de Investigaciones Cientificas (CSIC), the Faculty of Geology of the University of Barcelona (UB), the UNESCO-IGCP Project 511 (Submarine Mass Movements and their Consequences), and the Spanish Ministry of Education and Science (MEC).
The workshop participants recognized that so far geohazards have been only tackled by scientific drilling as a complementary goal, and clearly more investigations are needed to be able to fully understand and improve our predictive capability about marine geo-hazards and submarine slope instabilities in particular. The workshop participants identified the following questions that can only be answered through drilling:
What is the frequency of submarine landslides?
The assessment of natural hazards and related risk requires information of the recurrence intervals between events. Only drilling and appropriate high-resolution stratigraphic and geo-chronologic tools can establish the frequency of submarine slope failures. To date, only a few mega events, such as the Storegga Slide off Norway, and the strata involved in these events have been dated with sufficient accuracy. Many medium- and small-sized submarine slides have been mapped/imaged in detail, but accurate dating is still missing. In addition, the frequencies need to be linked to the intensity or the magnitude of the failure event which requires establishing a link between a signature and a potential source.
How the tsunamigenic potential of past and future submarine failures can be assessed?
The tsunamigenic potential of submarine landslides is a function of a number of parameters in addition to those related to environmental settings (e.g. water depth). The most critical ones relate to the pre-failure, failure and the early post-failure behavior of the failing mass. For the failure stage, confirming the morpho-stratigraphy will only be completed with drilling to validate failure conditions in the former or potential starting zone. In many cases, actual mobilized volumes will also be assessed by morhpo-stratigraphic characterization in the debris, which will also require drilling to obtain clues on the rheological behavior of the failed mass. Only after this is known the actual tsunamigenic potential can be evaluated using still-to-be-improved advanced modeling.
Do precursory phenomena of slope failure exist?
In order to improve our predictive capability we need to determine, which transient signs might indicate imminent slope instability. For this, it is clearly necessary to improve sea-floor observations and perform long-term monitoring of slopes, where slope failure might occur in a relatively short term. Transients in physical parameters that were deemed important are mainly pore pressure, temperature and slope deformation. The geochemistry of pore fluids, in combination with a better understanding of fluid flow patterns, may also be used to monitor failure development. IODP Proposal 685-Full (Ligurian Margin Borehole Observatory) that plans to instrument three holes with a variety of tools, including a seismometer, pore pressure and strain sensors, is indeed a first step towards monitoring the stability of submarine slopes.
Can we monitor seafloor gravitational movements such as creep?
Monitoring is also essential to quantitatively describe how slow deformation of slopes occurs. Seismic reflection profiling has supplied contradictory evidence of distorted slope strata that has been interpreted to result, in some instances, from slow sediment deformation. Drilling and monitoring these slopes can provide the clues that allow identifying these features as sediment deformation or not. In the case these features are still active, monitoring of hole tilt and pore pressure will provide information on the rates of deformation. The noise generated by failure processes can also trigger various sound frequencies that may be captured by in situ geophones and used to monitor slope activity.
What makes up weak layers in continental slope sediments?
In many occasions submarine landslides are identified to be rooted in one or several stratigraphic levels. These levels most probably represent a ìweak layerî that had a fundamental role in landslide initiation, as well as in determining the volume and geometry of the failed mass. In glaciated margins, where contrasting sedimentary materials are present, weak layers have been identified in contouritic deposits formed during interglacial periods. It is not clear, however, what determines the occurrence of weak layers in non-glaciated margins or which processes during the sedimentation, burial or early diagenesis tends to form them. It is also important to note that the ìweak layersî may not be weak in general, but only under certain conditions, such as during earthquake loading.
Considering the wide spectrum of geological environments where slope instabilities occur the workshop participants identified at least two hypotheses and models that could be tested through drilling:
Focusing of fluids and lateral transfer of stresses
Two-dimensional modeling of the New Jersey margin suggests that lateral fluid flow in permeable beds under differential overburden stress produces fluid pressures that approach the lithostatic stress where overburden is thin. This transfer of pressure may cause slope failure initiation at the base of the continental slope (Dugan and Flemings, 2000). IODP expedition 308 (Expedition 308 Scientists, 2005) was the first attempt to test a hydrogeologic model by which pore fluids are advected laterally under certain loading and stratigraphic conditions. The model implies that pore pressure is transferred to zones of lower overburden over high permeability sediments, thus with an important effect on slope stability. Similar models of pore fluid advection might occur due to glacial loading of permeable sediments (e.g. Storegga Slide or Antarctic Peninsula margins), and thus, it seems more and more important that we validate this model and further investigate for which margin architectures and stratigraphic settings it is applicable.
Methane emissions during rapid climatic changes and submarines slides
The Clathrate Gun Hypothesis (Kenneth et al., 2000) states that methane emissions from gas hydrates dissociation induced by bottom water warming occur primarily via the emplacement of submarine slides. The un-roofing of buried hydrate-bearing sediments by submarine slides enhances methane emissions from the seafloor by decreasing instantaneously the confining pressure. The carbon isotope chemistry and the assemblages of benthic calcareous foraminifera living close to paleo-slide heads could therefore be used as a local proxy of massive paleo-methane seeps. Other biological and microbiological indicators can probably be used for this purpose as well.
Workshop concluding remarks
Scientific drilling through IODP has focused to date on fundamental scientific problems, while marine geo-hazards are shorter term issues. If scientific drilling is willing to address them it may have to adjust its goals to be able to support a more applied research field. In the opinion of the workshop participants, there is a need of an explicit reference to geohazards in the next revised version of the IODP Initial Science Plan. Drilling proposals should address mega-landslides as well as small- to medium-sized submarine landslides. Mega-landslides are less frequent but trigger catastrophic consequences both in terms of geohazards and of sedimentary basin evolution. The understanding of the mechanics of a mega-landslide might require a multi-expedition effort and require a large amount of site survey data. Smaller landslides occur with a frequency that might be close to the frequency of natural hazards considered in the determination of the risk (500 years). More than one small- to medium-sized submarine landslide can be addressed effectively in one single drilling expedition. A multi-platform approach could be required by the necessity to perform stratigraphic drilling, geotechnical drilling, and installation of borehole and seafloor observatories.
For appropriate post-cruise geotechnical analyses, it will be essential that the drilling vessel is able to obtain high-quality undisturbed geotechnical samples. Current drilling techniques on board the JOIDES Resolution, however, do not fulfill this requirement. To overcome this, two possible solutions were considered: (1) to develop a new generation of thin-walled corers or (2) to explore the possibilities for chartering a geotechnical survey vessel through Mission-Specific-Platforms (MSP). Long term monitoring of key parameters in boreholes (e.g. pore pressure, hole inclination or ambient acoustic noise) can utilize the well-established CORK technology or developments in progress based on Cone Penetration Testing (CPT) technology (Moran et al., 2006). For off-site monitoring, a link with existing initiatives promoting seafloor observatories was recognized as very important.
Drilling to understand submarine slope instability was perceived as highly multidisciplinary. Drilling proposals should address the scientific questions outlined above in a variety of geological environments. The teams of proponents should include experts on tsunami and deep sea observatories, as well as geotechnical engineers. Paleoceanography will also have an important role. The ideas raised during the Barcelona workshop will find a broader forum next summer 2007, during the planned IODP-MI Geologic Hazards Workshop (http://www.iodp.org/geohazards/). The broader spectrum of submarine geohazards that this workshop will challenge will eventually lead to important initiatives such as multiple expeditions or a mission proposal.
References
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Expedition 308 Scientists, 2005. Overpressure and fluid flow processes in the deepwater Gulf of Mexico: slope stability, seeps, and shallow-water flow. IODP Prel. Rept., 308. doi:10:2204/iodp.pr.308.2005
Kennett, J.P., Cannariato, K.G., Hendy, I.L., and R.J. Behl, 2000. Carbon Isotopic Evidence for Methane Hydrate Instability During Quaternary Interstadials. Science, 288, 128-133.
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