Seismic images of structural variations along the deformation front of the Andaman-Sumatra subduction zone: Implications for rupture propagation and tsunamigenesis

- Variations in frontal thrust fault vergence along Sumatra-Andaman subduction zone.
- Landward vergence of frontal thrust associated with thick incoming sediment section.
- Creation of decollement at boundary between pelagic and thick turbiditic sediments.
- Decollement provides surface to accommodate coseismic displacement close to trench.
- Coseismic displacement near trench affects tsunamigenesis and intraplate seismicity.

Seven deep seismic reflection profiles cover the 3000 km-long subduction system from Andaman to Southern Sumatra, including zones that ruptured in 2004, 2007, and 2010. We find that (1) the frontal zone is characterized by a series of thrusts bounding folded blocks of sediments with preserved layering, showing a northward transition from dominantly seaward vergence of the frontal thrusts to dominantly landward vergence of the frontal thrusts, (2) the accretionary wedge is characterized by poor reflection of the seismic energy likely to be due to a high degree of faulting and compaction of the sediments, and (3) the oceanic crust is highly disturbed by faults and topographic reliefs along most of the margin. Landward vergence at the deformation front is associated with a thick incoming sediment section. The segment of the subduction zone where landward vergence is observed corresponds to an area with high near-trench slip during the December 2004 earthquake, the main tsunami source, and lies just west of the hypocenters of several intraplate events (Mw>7) in the years following the 2004 event.