Morphology and structure of the accretionary prism offshore North Sumatra, Indonesia and offshore Kodiak Island, USA : a comparison to seek a link between prism formation and hazard potential
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Sumatra and Kodiak Islands experienced recent megathrust earthquakes with devastating tsunami; recurrence of large earthquakes is predicted. Studies of the accretionary prism offshore of northern Sumatra, 1-7°N and 92-97°E, reveal a steep outer slope (5-12°), a plateau ~100-120 km wide, and a steep inner slope adjacent to the Aceh Basin. Three primary structural zones are consistent along strike where a predominantly landward vergence zone exists from the deformation front for a distance ~70 km landward. An extended landward vergence zone is not common; for northern Sumatra, a seaward dipping rigid backstop may be the reason, which assists subsequent younger accreted sediment to form the observed zone. The prism toe region shows prominent mass failures presumably related to activation of thrust faults and/or the shaking in response to the 2004 Sumatra-Andaman earthquake (Mw 9.1). These seafloor changes suggest that the 2004 rupture energy reached near the accretionary prism toe. The rigid backstop in the inner wedge together with the suggested dynamic backstop within the outer wedge, and the consolidated sediment on the outer slope form a rigid block dynamically, which together allows earthquake rupture to propagate under it and farther seaward toward the Sunda Trench, resulting in enhanced tsunami potential. Along the Aleutian Trench offshore of Kodiak Island, 145-155° W and 55-58° N, exist a distinct horizon, associated with the onset of the Surveyor Fan sedimentation along which the preferred zone forms. Most if not all of the sediment beneath this horizon seemed subducted, smoothing the high relief of the subducting plate. Subduction of large-buried seamounts begins with creation of a proto-thrust zone seaward of the existing deformation front. As a seamount reaches the deformation front, steepening of the prism toe occurs by formation of out-of-sequence thrusts. Upon further subduction, a deformation front jump occurs where the outer limit of proto-thrust zone becomes the new deformation front. This study contributes insights to other subduction zones with similar characteristics such as thick incoming sediment, subducting seamounts, and/or recent megathrust events. This study also underlines the need to establish fundamental time series data sets for mitigation efforts in hazard-prone areas.