Seismic image of the segment boundary of the Sumatra Dec. 2004 and March 2005 megathrust earthquakes

Two mega-thrust events occurred off northern Sumatra on December 26th 2004 (Mw=9.1-9.3) that produced a devastating tsunami, and on March 28th 2005 (Mw=8.6). Seismological investigations, GPS measurements, as well as in-situ and remote observation of vertical motion on the fore-arc islands show both, an abrupt southern termination of the large December 2004 rupture and a sharp northern termination of the rupture zone of the March 2005 mega-thrust. With newly acquired marine geophysical data (reflection and refraction seismics, magnetics, gravity and bathymetry) for the first time the boundary has been imaged. From wide-angle/refraction seismic data it becomes clear that there is an abrupt arc parallel depth change of 3 kilometres within 40 kilometres in the oceanic crust SW of Simeulue Island. The change in depth obtained in wide angle reflections corresponds with a change in depth and also the reflectivity of the oceanic crust reflection in MCS data. We interpret that the abrupt depth change originates from a ramp or tear in the subducted oceanic crust. The discontinuity in the oceanic crust likely trends NNE and is east of a continuation of an extinct fracture zone on the subducting Indo-Australian plate, indicating a pervasive lower plate control on margin structure, particularly its segmentation. The tear may be the reason for rupture propagation termination of the great December 26th 2004 and March 28th 2005 Sumatra Andaman earthquakes. At the ramp or tear, strain is decoupled resulting in a tectonic boundary for earthquake propagation.