Earthquake 'the equivalent of million atomic bombs'

SCIENTISTS describe the devastating earthquake off the island of Sumatra as a "megathrust" - a grade reserved for the most powerful shifts in the Earth's crust and the equivalent of one million atomic bombs.

The term does not 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.

Sumatra is one of the most earthquake-prone places in the world, sitting atop one of the handful of sites where several plates of the planet's crust overlap and grind. Colossal pressures build up over decades, only to release in a snap.

"These subduction zones are where all the world's biggest earthquakes are produced," said geologist Kerry Sieh of the California Institute of Technology. "Sunday was one of the biggest earthquakes in the region in the past 200 years."

How powerful? By some estimates, it was equal to detonating a million atomic bombs.

Sieh and other scientists said it probably jolted the planet's rotation. "It causes the planet to wobble a little bit, but it's not going to turn Earth upside down," Sieh said.

Researchers also speculated on the extent to which the jolt might have changed Sumatra's coastline. Extensive damage and flooding was preventing investigators from immediately reaching the scene.

Beneath the ocean, the flexible edges of the crustal plates might have shifted vertically by as much as 60 feet relative to each other. But even that kind of displacement would lift or lower the Sumatran coast by only a few feet or less, they said, and sea levels would not change dramatically.

But inland, ground levels in northern Sumatra might have changed noticeably in places, Sieh said.

"As the block of land on top of a subduction zone lurches out west toward the Indian Ocean, you expect that area behind it to sink," he said.

Seismologists said the epicentre of Sunday's quake was more than 5.5 miles below the Indian Ocean off the west coast of Sumatra and about 150 miles south of the city of Bandah Aceh on the island's northern tip.