  The 26 December 2004 Sumatra earthquake (Mw = 9.1) initiated around 30 km
depth and ruptured 1300 km of the Indo-Australian Sunda plate boundary. During
the Sumatra OBS (ocean bottom seismometer) survey, a wide angle seismic profile
was acquired across the epicentral region. A seismic velocity model was
obtained from combined travel time tomography and forward modeling. Together
with reflection seismic data from the SeaCause II cruise, the deep structure of
the source region of the great earthquake is revealed. Four to five kilometers
of sediments overlie the oceanic crust at the trench, and the subducting slab
can be imaged down to a depth of 35 km. We find a crystalline backstop 120 km
from the trench axis, below the fore arc basin. A high velocity zone at the
lower landward limit of the raycovered domain, at 22 km depth, marks a shallow
continental Moho, 170 km from the trench. The deep structure obtained from the
seismic data was used to construct a thermal model of the fore arc in order to
predict the limits of the seismogenic zone along the plate boundary fault.
Assuming 100C-150C as its updip limit, the seismogenic zone is predicted to
begin 530 km from the trench. The downdip limit of the 2004 rupture as inferred
from aftershocks is within the 350C 450C temperature range, but this limit is
210-250 km from the trench axis and is much deeper than the fore arc Moho. The
deeper part of the rupture occurred along the contact between the mantle wedge
and the downgoing plate.
