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Colloque organisé par Frédéric Dias et Stéphan Grilli du 19 au 24 octobre 2005Various aspects of the devastating tsunami of 26 December 2004 : earthquake, seafloor deformation, tsunami propagation, runup and inundation, data collected during the expedition organized soon after the tsunami.ParticipantsCharles J. Ammon, Jack Asavanant, Christin Ashamankas, James A. Austin, Christopher Baxter, Jon Copley, Frédéric Dias, Dany Dutykh, Donald Fisher, Stéphan Grilli, Tim J. Henstock, Baban Ingole, James Kirby, Tim Masterlark, David Mearns, Kathryn Moran, David Mosher, Stephen Saustrup, David R. Tappin, Paul Tyler&#160;Compte-renduSoon after the devastating tsunami of 26 December 2004, Kathryn Moran from the University of Rhode Island (USA) and David Tappin from the British Geological Survey (UK) organized a three-week expedition in the Indian Ocean. The main goal of the expedition was to image the seafloor with high resolution seismic reflection techniques in order to evaluate and interpret the size and character of the seafloor displacements that occurred during the 9.3M earthquake and to determine underwater landslide volumetric size and shape. Seafloor displacement features were “ground-truthed” to confirm that they occurred on December 26th by studying the seafloor megafauna using Remotely Operated Vehicle (ROV) imagery and sampling. For example, co-seismic displacements disrupt the pre-earthquake seafloor ecology. In the case of landslides, the slide scar areas expose biologically barren seafloor. The expedition involved twenty scientists from six different countries with different types of expertise: geophysics, geology, biology, visualization, hydrodynamics, computer science. The expedition took place in May 2005. While on board of the ship, it was decided to organize a workshop later on to discuss various issues concerning the analysis and the interpretation of the results. A proposal was sent to Fondation des Treilles and successfully funded.Most of the scientists who took part in the expedition (13 out of 20) were able to attend the workshop. In addition, the organizers invited the scientists who were supposed to take part in the expedition but could not make it because of the last minute change in the dates, and also some other scientists who could bring valuable insight in the analysis of the results. The first two days were devoted to general purpose talks on various aspects of the devastating tsunami of 26 December 2004 : earthquake, seafloor deformation, tsunami propagation, runup and inundation, data collected during the expedition. During the last two days, the participants provided summaries of the results on which everybody agreed, summaries of the results on which there was no complete agreement and directions for future research in the next months or even years.One of the original aspects of the expedition and of the workshop was to gather scientists from different backgrounds. For example the biologists provided precious help in order to estimate the date of some of the observed seafloor displacements. The fluid dynamicists helped the biologists by explaining the amplitude of currents induced by a propagating tsunami when the water depth is relatively small (typically less than 500 meters). The seismologists explained how the inversion of seismic data provides the space and time history of the seafloor deformation. The geophysicists were able to analyze the first kilometer of the seafloor and to provide a detailed map of the geological features in the surveyed areas.Concerning the observed landslides, the participants came to the conclusion that these landslides were not recent and occurred prior to the 2004 Sumatra earthquake. Concerning the ditch, a steep-sided linear depression at the foot of the deformation front, the participants concluded that it was recently formed. Indeed the ditch ROV dive shows several freshly deformed soft rock features and no marine life. It is important to point out that prior to the 2004 event, there had been very few observations made of the sea bottom in this part of the Indian Ocean. Therefore, while it was impossible to compare recent observations with older ones, there are now observations available for future comparisons in case a new tragic event occurs.At the end of the workshop, the scientists made a rather short list of the known results and a longer list of the results for which there is not enough evidence. Even if one now has a better understanding of what happened on December 26, 2004, there are still a lot of points that need clarification. New multi-disciplinary expeditions and workshops are necessary. Moreover, it must be emphasized that the area that was surveyed by the SEATOS expedition is only a tiny part of the whole area that was affected by the 2004 Sumatra/Andaman earthquake.