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Colloque organisé par James Austin (University of Texas at Austin, USA) et Frédéric Dias (CMLA, ENS Cachan, France) du 6 au 11 avril 2011ParticipantsJames A. Austin, Nathan Bangs, Kai Berglar, Frédéric Dias, Yusuf S. Djajadihardja, Denys Dutykh, Donald M. Fisher, Dieter Franke, Chris Goldfinger, SeanP.S. Gulick, Timothy J. Henstock, Heidrun Kopp, John McCloskey, Lisa McNeill, Haryadi Permana, Jean-Claude Sibuet, Kerry Sieh, Satish Singh, Frederik Tilmann, Kelin Wang--Compte-renduThe December 2004 mega-earthquake on the margin of Sumatra produced an enormous tsunami which led to the loss of more than a quarter of a million lives around the Indian Ocean basin. Subsequent large earthquakes on this margin have shown that the danger to people and property in Indonesia and surrounding countries is far from over. But their occurrence has also fostered a tremendous amount of research, both onshore, on Sumatra and adjacent islands, and offshore. A prominent international group of scientists representing these ongoing investigations met to assess the state of knowledge, to discuss possible new research directions, and to extend existing international collaborations, in cooperation with representatives from the Indonesian government.The first two days of the workshop were given over to individual oral presentations on diverse topics, to bring the group up to speed on the current state of knowledge; these talks generated discussion on results and new goals and objectives:1. Neotectonic Studies/Approaches K. Sieh: “Variable seismic behavior of the Sunda megathrust revealed by Sumatran corals and GPS array” C. Goldfinger et al.: “Temporal clustering, energy-state proxy, and recurrence of Holocene paleo-earthquakes in the region of the 2004 Sumatra-Andaman earthquake” H. Permana et al.: “General structural pattern inferred from bathymetry data of Aceh forearc and Aceh outer arc Ridge, Sumatra, Indonesia” Y. Djajadihardja: “Bathymetric map of the deep sea trench, accretionary prism and forearc basin of the western Sumatra water compiled from several cruises after the great Aceh earthquake 26 December 2004” Sibuet et al.: “The Great Sumatra-Andaman earthquake and tsunami: where are we?”2. Tsunami Modeling Studies D. Dutykh: “Energy of tsunami waves” F. Dias: “Influence of sedimentary layering on tsunami generation”3. Accretionary Wedge Mechanics/Subduction Processes D. Fisher: “Analysis of fore arc morphology along accretionary and erosive convergent margins in the context of critical wedge mechanics” K. Wang: “4-D view of great subduction earthquakes” J. McCloskey: “Controls on rupture nucleation, propagation and termination: constraining Monte Carlo simulations” H. Kopp: “The Eastern Sunda margin: subduction processes off Java, Lombok and Sumba” F. Tilman: “Seismic experiments along the Sumatran margin: variations in seismicity patterns between post-seismic, creeping, and ‘seismic gap’ segments” Bangs et al.: “Conjugate thrust geometry of Sumatra accretionary wedge frontal thrusts on line SUMUT 19 and implications for initial fault strength”4. Seismic Reflection Studies D. Franke and K. Berglar: “BGR studies on the Sumatra margin: Oblique subduction, forearc evolution, segmentation” S. Singh et al.: “Marine study of the 2004 Great Sumatra Earthquake and Sumatra seismic gap region” T. Henstock et al.: “Deep forearc structure of the Sumatran margin and links to along-strike segmentation” L. McNeill et al.: “Shallow forearc structure of the Sunda-Sumatran margin: Along-strike segmentation and expressions of deeper structure and seismogenic behaviour” S. Gulick et al.: “Evidence of thick and consolidated sediments in the 2004 earthquake region: Implications for updip rupture and margin structure” J. Austin et al.: “Structural and stratigraphic styles in the Aceh (forearc) basin”The final two days of the workshop, with the exception of a group field trip to view the nearby Gorges du Verdon one afternoon, were given over to two breakout group discussions on: 1) resubmission of a new, data-rich version of an existing proposal to the Integrated Ocean Drilling Program (IODP), to begin to assess the geologic impact of the 2004 mega-earthquake within its ~1, 200-km long rupture zone, and 2) preparation of an IODP pre-proposal to use scientific ocean drilling to help ongoing onshore and offshore geological and geophysical studies to assess seismic risk in the generally acknowledged portion of the Sunda mega-thrust which has not yet broken, the so-called Mentawai gap. Each group reviewed extensive existing data, identified potential sites for future IODP drilling, and began to put together supporting text and illustrations for these proposals, which will be due at IODP on 1 October 2010.In a final plenary session, the entire group reviewed and was very satisfied with the progress made on the two proposals in question, and also discussed the need for a follow-up workshop in Indonesia, likely in Bandung, Java, in the spring of 2011. Some funding for such a workshop is already in hand, and additional sources of support were identified.The group also decided that the co-convenors should write up a short summary of the meeting for EOS, the weekly newsletter of the American Geophysical Union.