The relationship of the seismic source and subduction zone structure for the 2004 December 26 Sumatra-Andaman earthquake

Studies of the 2004 December 26 Sumatra-Andaman earthquake indicate that the rupture and subsequent down-dip slip of the Mw 9.3 event propagated with variable speed as it progressed northwards. The transitions between different behaviour for the source correspond to changes in the physical properties of the subducted slab along the Sumatra-Andaman arc revealed by multi-wavespeed seismic tomography. The transition from rapid slip to dominantly slower slip near 6.5° N and the end of the main zone of tsunamigenic source at 10° N lies in each case just to the south of localised features where changes in the physical properties of the slab are indicated from the ratio of shear wavespeed to bulk-sound speed; these features are associated mainly with bulk-modulus variations. The changing morphology of the subduction zone is associated with changes in physical properties, that are manifested in the tomographic images and have modified the character of the slip along the 1500 km long arc segment that ruptured during this great earthquake.