"We've just updated it to 8.9 magnitude. That makes it the fifth largest earthquake since 1900," said Julie Martinez, geophysicist for the US Geological Survey's Earthquake Hazards Program in Golden, Colorado.

It was the largest earthquake in the world since 1964, she said.

That year, an earthquake struck Alaska's Prince William Sound.

Sunday's quake, first struck at 7.59am (0059 GMT) off the coast of Aceh province on the northern Indonesian island of Sumatra and appeared to swing north into the Andaman islands in the Indian Ocean.

It triggered a tsunami that killed hundreds in Sri Lanka, Thailand, Indonesia and India.

"About 1000 kilometres of the Andaman thrust (or fault line) broke, which is a huge area," Martinez said. "This doesn't occur that often. To have a break along that long of a fault line, that is more unusual."

Pressure
 
As the earth moves and its plates hit each other, the earth breaks in one area and pressure is built up in a different area, Martinez said. When that pressure builds up, another earthquake occurs, she said.

"Because of the size of this quake, you will see more quakes in a larger area because the break or the fault line is larger"

Julie Martinez, US Geological Survey's Earthquake Hazards Programme

The quakes that follow, or aftershocks, are minor readjustments along the fault after the main shock or quake occurs, Martinez explained.

"Usually, aftershocks are in more or less the same area," she said.

"Because of the size of this quake, you will see more quakes in a larger area because the break or the fault line is larger."