Geologic Cause
The mega-thrust earthquake formed when the Indian Plate subducted, or went under, the Sunda Plate. However, when the plate subducted it subducted the Burma plate which is a micro plate that is apart of the Sunda Plate. The earthquake's force triggered multiple tsunamis that played a large role in the destruction on December 26.
The Indian Plate has begun its descent into the mantle at the Sunda trench. The Sunda trench lies west of the earthquake's epicenter, and the trench is the surface expression of the plate interface between the Australia and India plates which are to the southwest of the trench. The Burma and Sunda plates are situated in the northeast. "The interface between the two plates results in a large fault, called an interplate thrust or mega-thrust. This fault lies below the southwestern part of Sumatra and the Andaman Islands. Where the interplate thrust intersects the sea floor is marked by the Sunda trench that can be traced along an arc from Burma in the north Java in the south." (USGS)