Running in a rough north-south line on the seabed of the Andaman Sea is the boundary between two tectonic plates, the Burma Plate and the Sunda Plate. These plates  are believed to have formerly been part of the larger Eurasian Plate, but were formed when transform fault activity intensified as the Indian Plate began its substantive collision with the Eurasian continent. As a result, a back-arc basin center was created, which began to form the marginal basin which would become the Andaman Sea, the current stages of which commenced approximately 3-4 million years ago . The boundary between two major tectonic plates results in high seismic activity in the region . Numerous earthquakes have been recorded, and at least six, in 1797, 1833, 1861, 2004, 2005, and 2007, had the magnitude of 8.4 or higher. On 26 December 2004, a large portion of the boundary between the Burma plate and the Indo-Australian plate slipped, causing the 2004 Indian Ocean earthquake. This megathrust earthquake had a magnitude of 9.3. Between 1,300 and 1,600 kilometers of the boundary underwent thrust faulting and shifted by about 20 meters, with the sea floor being uplifted several meters. This rise in the sea floor generated a massive tsunami with an estimated height of 28 meters  that killed approximately 280,000 people along the coast of the Indian Ocean. The initial quake was followed by a series of aftershocks along the arc of the Andaman and Nicobar Islands. The entire event severely damaged the fishing infrastructure.:40-42
Question: What caused the 2004 Indian Ocean earthquake?
Answer: plate slipped