http://www.washingtonpost.com/wp-dyn/articles/A27886-2004Dec26.html
This latest earthquake apparently broke along a 600-mile section of the Sumatran "subduction zone," starting just north of where Sieh does his research. A subduction zone is a plate boundary where a slab of the Earth's crust surges downward beneath another slab.
"I worry about my segment of the subduction zone," he said. "My section of the subduction zone is still locked, as far as I know."
Along the curving western coast of the Indonesian archipelago, the piece of crust known as the India plate is sinking beneath another expanse of crust called the Burma plate. This process of subduction isn't smooth. It happens violently, joltingly, sometimes here and sometimes there, occasionally prefigured by a less powerful quake (a 7.7 magnitude event occurred in the same area near Sumatra two years ago), but usually without any obvious hint that a disaster is in the offing.
The theory of plate tectonics has done for geology what Charles Darwin's theory of evolution did for biology. It provides geology with a comprehensive theory that explains "how the Earth works." The theory was formulated in the 1960s and 1970s as new information was obtained about the nature of the ocean floor, Earth's ancient magnetism, the distribution of volcanoes and earthquakes, the flow of heat from Earth's interior, and the worldwide distribution of plant and animal fossils.
Graphic of the World's plates at the link.....