Shallow subsurface morphotectonics of the NW Sumatra subduction system using an integrated seismic imaging technique
Résumé
In subduction zones, shallow subsurface structures are the manifestation of the plate interactions at depth. However, significant water depths, rough bathymetry and presence of heavily deformed accretionary wedge materials hamper imaging of the near-surface features to a great extent using conventional imaging techniques. In this study, we show results using an integrated processing technique to a multichannel seismic data set acquired in 2006 from the northwestern offshore Sumatra. We start with first downward continuing the 12-km-long surface streamer data to the seafloor, followed by a high-resolution traveltime tomography of refracted phases to determine a detailed velocity-depth model of subsurface, which in turns, is used for pre-stack depth migration in order to delineate the shallow subsurface structures beneath the trench, subduction front and outer accretionary wedge. Our velocity-depth model and the depth migrated image depict variation of sediment properties across the front and structures of uppermost sedimentary sequence with an unprecedented high resolution providing the precise location of the frontal and conjugate thrusts, highly folded sedimentary sequences, which in turns describe their relationship with the top of the subducting plate and factors that control rupture propagation to the trench. Furthermore, we estimate the porosity distribution across the front, where we find a 12 and 18 per cent decrease in porosity beneath the deformation front and the inner accretionary plateau at 500 m below the seafloor, respectively, which we interpret to be due to the compaction. A significant decrease in porosity at the plate interface below 5-6-km thick sediments near the deformation front would increase the coupling, leading to the rupture propagation up to the trench, uplifting 4.5 km water and producing large tsunami.
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