Geomorphology of the 26 December 2004 Indian Ocean Earthquake Rupture Zone – Results from the First Seafloor Survey Onboard the HMS Scott, 2005

The 26 December 2004 Mw 9.3 Sumatra to Andaman Islands subduction zone earthquake was the second largest earthquake recorded. The tsunami generated by the earthquake caused the loss of ~300,000 lives and devastation of many Indian Ocean coastal communities. The Royal Navy's HMS Scott conducted a multibeam bathymetric survey over the rupture zone during January and February 2005. An area of ~40000 km2 was surveyed during the three week expedition using a 12 kHz, 361 beam hull-mounted Sass IV sonar. This study is the first time a high resolution deep-water seafloor survey has been carried out so soon after a major plate-boundary subduction zone earthquake. Existing geophysical data in the earthquake rupture zone are rare and therefore the general subduction zone structure is poorly known. Data were collected along 400km of the southern and intial part of the rupture zone offshore Sumatra, including the plate boundary and lower accretionary wedge, the southern termination of the earthquake rupture which represents a probable major segment boundary, and parts of the forearc basin and outer arc high system. Initial results reveal the geomorphology of the seafloor and clues to the tectonic and sedimentary processes controlling this morphology during periods of thousands to millions of years of plate convergence. The gradient of the seafloor from the plate boundary (deformation front) where the two plates collide across the accretionary wedge folds is very steep – rising ~4000 m over a distance of about 20 km in places. These steep slopes and continuing uplift due to collision augmented by earthquake shaking leads to extensive slope failure and submarine landsliding. The seafloor of the accretionary wedge folded ridges is therefore heavily eroded in places. At the deformation front, a few coherent landslides produce angular blocks which have slid onto the undeformed Indian plate. The youngest of these features has angular coherent blocks and a steep landslide headwall scarp. Sediment from these landslides and slumps is transported into neighbouring basins by short canyon and channel systems. No evidence was found of recent landslides large enough to contribute significantly to the tsunami generated in 2004. Fold ridges of the accretionary wedge are easily identified in the multibeam images. The youngest of these fold ridges at the deformation front where the plates meet are relatively uneroded and only a few hundred metres high. These ridges will have formed over thousands of years. The quality of the data allows small tectonic features to be identified at the base of these ridges. Some of these features may be formed by fault movement during earthquakes such as the event of 2004. The southern part of the HMS Scott survey overlaps with the rupture zone of the Mw 8.7 earthquake that occurred on March 28 2005, thus presenting the potential for "before" and "after" surveys to better understand how some of these structures may form and be modified during major plate boundary events