Sumatran quakes in April were part of tectonic plate breakup

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

Such was the case on April 11 when two massive earthquakes erupted beneath the Indian Ocean off the coast of the Indonesian island of Sumatra, 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 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 Thorne Lay, a professor of Earth and planetary sciences at UC Santa Cruz. "It was like nothing we'd ever seen."

At first, Lay wondered whether the computer code he used to analyze earthquakes was wrong. Eventually, he and other scientists realized that they had documented the breakup of the Indo-Australian plate into two pieces, an epic process that began roughly 50 million years ago and will continue for tens of millions more. Lay and other scientists reported their findings online Wednesday 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 of strike-slip faults, meaning the ground on one side of the fault moves horizontally past ground on the other side.

Scientists say the 8.7 main shock broke four faults. The quake lasted 2 minutes and 40 seconds — most last just seconds — and was followed by a second main shock, of magnitude 8.2, two hours later.

Unlike the magnitude 9.1 temblor that struck in the same region on Dec. 26, 2004, and created a deadly tsunami, the April 11 quakes did not cause similar destruction. That's because horizontally moving strike-slip faults do not induce the massive, vertical displacement of water that thrust faults do on the borders of plates.

The type of interplate faults involved in the Sumatran quakes are the result of monumental forces, some of which drove the land mass of India into Asia millions of years ago and lifted the Himalayan Mountains. 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 portion of the plate, which contains Australia, keeps on moving without the same obstruction. That difference creates squeezing pressure in the area where the quakes occurred.

La Times