A long break-up: Sumatra quakes part of epic shift

Planet Earth may be 4.5 billion years old, but that doesn't mean she can't serve up a shattering surprise now and again.

Such was the case earlier this year when two massive earthquakes erupted beneath the Indian Ocean, far from the usual danger zones. Now scientists say the seafloor ruptures are part of a long suspected, yet never before observed, event: the slow-motion splitting of a vast tectonic plate.

The first of the April 11 quakes, a magnitude 8.7, was 20 times more powerful than California's long anticipated "big one" and tore a complex network of faults deep in the ocean floor.

The violence also triggered unusually large aftershocks thousands of miles away, including four off North America's western coast. "It was jaw-dropping," said Thorn Lay, a professor of Earth and planetary sciences at the University of California, Santa Cruz. "It was like nothing we'd ever seen."

Monumental forces

At first, Lay wondered whether the computer code he used to analyze was wrong. Eventually, he and other scientists realized that they had documented the break-up of the Indo-Australian plate into two pieces, an epic process that began about 50 million years ago and will continue for tens of millions more. They reported their findings online in the journal Nature.

Most great earthquakes occur along plate borders, where one plate dives beneath the adjoining plate and sinks deep into Earth's mantle, a process called subduction. The April 11 quakes, however, occurred in the middle of the plate and involved a number strike-slip faults, meaning the ground on one side of the fault moves horizontally past ground on the other side.

'The most powerful'

As the Indo-Australian plate continues to slide northwest, the western portion of the plate, where India is, has been grinding against and underneath Asia. But the eastern part, which contains Australia, moves without the same obstruction. That difference creates squeezing pressure in the area where the quakes occurred.

The study authors said that the quake is likely to teach seismologists about the physics of earthquakes and that over time, the cracks will eventually coalesce into a single fissure. "This is part of the messy business of breaking up a plate," said University of Utah seismologist Keith Koper, senior author of one of the studies. "Most likely it will take thousands of similar large quakes for that to happen."

The quake, which was unanticipated by seismologists, was also notable for triggering powerful aftershocks thousands of miles away. While major quakes have been known to trigger aftershocks at great distance, they are usually less than 5.5 in magnitude. The April quake triggered 11 aftershocks that measured 5.5 or greater and were felt 6,000 to 12,000 miles away.