Plate Tectonics of the 2004 Sumatra-Andaman Earthquake & Tsunami

The epicenter occured where the Indian Plate and the South-East Asian Plate are converging off the coast of Sumatra.

Elasticity of tectonic plates and their tendancy to want to return to an original shape dictated by the crystalline lattice structure of basalt minerals is the reason why earthquakes occur. As two plate boundaries press against each other, friction causes each to hold on to each other until they slip. The elastic nature of each plate springs it back to a state of equilibrium and in that millisecond of action, meters of distance are returned, and the shockwave can be heard all over the world. Geological Survey of India