Subduction system variability across the segment boundary of the 2004/2005 Sumatra megathrust earthquakes

Subduction zone earthquakes are known to create segmented patches of co-seismic rupture along-strike of a margin. Offshore Sumatra, repeated rupture occurred within segments bounded by permanent barriers, whose origin however is still not fully understood. In this study we image the structural variations across the rupture segment boundary between the Mw 9.1 December 26, 2004 and the Mw 8.6 March 28, 2005 Sumatra earthquakes. A set of collocated reflection and wide-angle seismic profiles are available on both sides of the segment boundary, located offshore Simeulue Island. We present the results of the seismic tomography modeling of wide-angle ocean bottom data, enhanced with MCS data and gravity modeling for the southern 2005 segment of the margin and compare it to the published model for the 2004 northern segment. Our study reveals principal differences in the structure of the subduction system north and south of the segment boundary, attributed to the subduction of 96°E fracture zone. The key differences include a change in the crustal thickness of the oceanic plate, a decrease in the amount of sediment in the trench as well as variations in the morphology and volume of the accretionary prism. These differences suggest that the 96°E fracture zone acts as an efficient barrier in the trench parallel sediment transport, as well as a divider between oceanic crustal blocks of different structure. The variability of seismic behavior is caused by the distinct changes in the morphology of the subduction complex across the boundary related to the difference in the sediment supply.