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Sumatra is an active (Andean) continental margin that would be linked by land to SE Asia if sea level fell by as little as 50 m. Present-day tectonic processes are controlled by three major fault systems, the most obvious of which is the subduction thrust which crops out in the Sunda Trench. The trench curves very little in the 800 km between Enggano and Nias, i.e. off central Sumatra (Fig. 2.1), but is markedly convex towards the Indian Ocean both further north and further south. Water depths of more than 6000 m are reached in the south but the maximum in the north may be less than 5000 m. The difference is usually, and convincingly, attributed to the presence on the Indian Ocean plate of the Nicobar Fan, consisting of sediments, derived ultimately from erosion of the Himalayas, which increase steadily in thickness towards the north (e.g. Hamilton 1979). Continuing subduction is attested by a Wadati-Benioff Zone (WBZ) that extends to depths of the order of 200 km (e.g. Newcomb & McCann 1987) and by volcanic activity in the Barisan mountains, the peaks of which generally lie within a few tens of kilometres of the coast. The change, of more than 45Ð, in the trend of the trench between 96ÐE and 97ÐE (the 'Nias Elbow') may have been initiated by subduction of the 2 km high Investigator Ridge (Investigator Fracture Zone), which trends approximately north-south at about 98ÐE. Sieh & Natawidjaja (2000) defined a 'Central Domain' of mainland Sumatra between
