"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 quake in the world since 1964, she said. That year, a major earthquake hit Alaska's Prince William Sound.

Sunday's quake struck at 7.59 a.m. (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.

"These big earthquakes, when they occur in shallow water, ... basically slosh the ocean floor ... and it's as if you're rocking water in the bathtub and that wave can travel basically throughout the ocean," USGS geophysicist Bruce Presgrave told the BBC.

He said there had been no signs of the impending quake.

"Unfortunately, we are not able to predict earthquakes at this time and one of the big reasons is typically these big earthquakes occur with no warning, no foreshock activity or anything like that," he added.

As the Earth moves and its plates hit each other, it breaks in one place and pressure builds up in a different area, Martinez said. When that pressure increases, another earthquake occurs.

"About 1,000 km of the Andaman thrust (or faultline) broke, which is a huge area," Martinez said. "This doesn't occur that often. To have a break along that long of a faultline ... is more unusual."

The magnitude of the quake increased the likelihood of aftershocks -- the tremors that follow as minor readjustments occur along the fault after the main shock.

"I would say it makes aftershocks more likely and it makes them more likely to be large," Presgrave said. "We have already seen one aftershock of magnitude 7.3 and there have been a large number of aftershocks in the magnitude 5-6 range."

Martinez said aftershocks are usually in more or less the same area.

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