Recent large earthquakes have prompted studies to reevaluate seismicity and rupture propagation behavior along the world's major subduction margins. Our study area covers the entire fore arc from northwest of northern Sumatra to west of Simeulue Island, the southern portion of the 2004 tsunamigenic earthquake rupture zone. The accretionary prism width is up to ~180 km, water depths between ~4.5 km near the Sunda Trench and <1 km on fore-arc high. The wedge consists of a steep outer slope (5–12°), a plateau ~100–120 km wide with anticlinal folds spaced 2–15 km apart, and a steep inner slope adjacent to the Aceh Basin. Analysis of seismic profiles and bathymetry reveal three main structural zones consistent along-strike, from the trench landward: (1) predominantly landward vergent folds, (2) mixed vergent folds, and (3) predominantly seaward vergent folds. This paper uses those zones to propose a geometry of an underlying rigid backstop. This backstop is seaward dipping and extends from under the Aceh Basin to beneath the mixed vergence zone. A dynamic backstop possibly exists seaward of the rigid backstop and is responsible for the steep slope of the outer prism. Indurated accreted sediments form the landward vergence zone. Along with the possible dynamic backstop beneath the outer wedge, and the rigid backstop in the inner wedge, all behave as a solid block coseismically. This block allows great earthquake rupture to propagate farther seaward toward the Sunda Trench, with resultant hazardous tsunamigenic potential.