The work conducted by the Geological Hazards and sedimentary Dynamics Laboratory is multidisciplinary and concentrating on fundamental and applied research in geotechnics, geophysics, geology, sedimentology and sedimentary hydrodynamics to determine the intensity (how ?), as well as the spatial (where ?) and temporal (when ?) occurrence of geological hazards associated with various external factors.
The seismic hazard at sea is significant, and essentially located around the active margins (subduction zones and transform faults) or tectonically re-activated passive ones.
- It is crucial to identify the slipped segments of active margins to determine the currently stable zones which might present a risk in the future. Accordingly, we study recent and paleo-seismology and image deep structure phenomena in order to constrain the factors generating the activation of margins. The societal issues involved are significant and this hazard may present a major direct risk for coastal populations and also an indirect risk through the formation of potential tsunami generated by an earthquake (Klingelhoefer et al., 2010) or density induced currents which may cause damage to underwater structures (Cattaneo et al., 2012). This process can also generate overpressure and destabilisation of superficial sediments (Sultan et al., 2009) and cause degassing of the products from this destabilisation into the water column. Conversely, the distribution of gas plumes in the water column may be a key to understanding the state of stress of an active margin (Dupré et al., 2015).
Gravitational hazards are the result of highly variably dimensioned processes affecting all types of continental margins (passive, convergent, transform and divergent) in very different geological and climatic contexts.
- The mechanisms of deformation and activation vary widely, depending on the physiographical zone, the environment and also susceptibility and triggering factors. Activation may generate large sediment slides or density currents whose location, volume and speed will determine the size of the destructive impact on underwater infrastructures and the generation of tsunamis affecting more widely the coastal zones. Gravitational hazards may be the consequence, the witness and indeed the cause of the activity of the other two hazards which are the fluid/gas hydrate hazard and the seismic hazard.
Gas hydrate/free gas systems are widespread throughout the world's oceans.
- In its solid (gas hydrate) or free form, the gases present in marine sediments may have a major impact on sedimentary deformation and on the quantity of greenhouse gas transferred from the sediments to the ocean and potentially towards the atmosphere. Gas hydrates are naturally stable when P/T conditions are favourable and they play a prominent role in storing gas in marine sediments, preventing free gas from escaping from the sediment to the ocean. The study of the stability of gas hydrates is a major challenge in attempting to assess earth-sea-atmosphere transfer and its associated risks.
The approach adopted for the study of these three hazards is based on four research axes :
Axis 1 - Observation, identification, hypothesis
- Identify active geological processes from the surface (landslides, flows, fluid escapes, ...) to the deep structure (e.g. seismogenic zone) through indirect measuring, e.g. acoustics, seismics, seismology...
- Formulate hypotheses on the formation of observed active geological processes.
Axis 2 - Defining the environment
- Provide quantitative data obtained through in situ measurements and direct (coring) and indirect observations in order to bridge the gap between the hypothesis and the model which may be verified through simulation (analogical and/or numerical).
Axis 3 - Numerical modelling of processes
- Develop numerical methods and tools capable of reproducing those processes provoking natural hazards, from the surface to the base of the crust.
Axis 4 - Chronology
- Identify the temporal variability of the identified processes, their repeatability or recurrence by studying their timeline and the stratigraphy ofassociated deposits and through the reconstruction of factors associated with sedimentary or tectonic deformation.