From CNN:
Scientists describe Sunday’s devastating earthquake off the island of Sumatra as a “megathrust” — a grade reserved for the most powerful shifts in the Earth’s crust.
The term doesn’t entirely capture the awesome power of the fourth largest earthquake since 1900, or the tsunami catastrophes it spawned for coastal areas around the Indian Ocean.
Despite its awesome power, the quake itself was not much of a surprise, scientists said Monday.
More here.