Indian Ocean Tectonic Breakup

Unusual Indian Ocean earthquakes hint at tectonic breakup

April 2012 quakes occurred away from plate edges, suggesting formation of a new boundary.

At least four faults within the Indo-Australian plate ruptured simultaneously in April 2012, resulting in two magnitude-8 earthquakes within two hours. (Red stars indicate the epicentres.)

Keith Koper, University of Utah Seismograph Stations

Geological stresses rending the Indo-Australian plate apart are likely to have caused the magnitude-8.6 and magnitude-8.2 quakes, which broke along numerous faults and unleashed aftershocks for 6 days afterwards, according to three papers published online today in Nature1–3.

Seismologists have suspected since the 1980s4 that the Indo-Australian plate may be breaking up. But the 11 April earthquakes represent “the most spectacular example” of that process in action, says Matthias Delescluse, a geophysicist at the Ecole Normale Supérieure in Paris and lead author of the first paper1. Worldwide, “it’s the clearest example of newly formed plate boundaries,” he says.

According to prevailing theories of plate tectonics, the Indo-Australian plate began to deform internally about 10 million years ago. As the plate moved northwards, the region near India crunched against the Eurasian plate, thrusting the Himalayas up and slowing India down. Most scientists think that the Australian portion forged ahead, creating twisting tensions that are splitting the plate apart in the Indian Ocean.

Delescluse and his team inferred the presence of these seismic stresses by modelling stress changes from shortly before the 2012 earthquakes. They found that two earlier earthquakes along the eastern plate boundary — the magnitude-9.1 tremor in 2004 that unleashed a massive tsunami across the Indian Ocean, and another quake in 2005 — probably triggered the 2012 event by adding to pent-up stresses in the plate’s middle region.

Gregory Beroza, a seismologist at Stanford University in Palo Alto, California, says that the model is a likely explanation. “The 2004 and 2005 earthquakes by themselves would not have caused this other earthquake. There had to be other stresses,” he says.

Slip-sliding away

Most large earthquakes occur when two plates collide at their boundaries, and one plate slides beneath the other. By contrast, when plates or portions of plates slip horizontally along a fault line, this usually results in smaller, ‘strike-slip’ earthquakes.

However, the first 11 April event defied expectations as the largest strike-slip earthquake on record, and one of the strongest to occur away from any conventional plate boundaries.

In the second study2, researchers found that the accumulated stresses spread over the plate’s interior broke free in the first 11 April event, resulting in one of the most complex fault patterns ever observed. Unlike most earthquakes that shake along a single fault, this one ruptured along four faults, one of which slipped as much as 20–30 metres.

“This earthquake, it was a ‘gee whiz’,” says study author Thorne Lay, a seismologist at the University of California, Santa Cruz.

Previous work had already identified multiple strike-slip faults for the magnitude-8.6 earthquake5, but no other study had analysed the slip amounts in such detail. Beroza says that Lay and his team “do a splendid job of picking apart this very important earthquake” in their paper.

Lasting impressions

Although much attention has focused on how the earthquakes played out, some researchers are also studying the after-effects of the giant tremor. In a third study3, scientists found that for six days following the event, earthquakes of magnitude 5.5 and greater occurred at almost five times their normal rate all around the world.

“Aftershocks are usually restricted to the immediate vicinity of a main shock,” says lead author Fred Pollitz, a geophysicist at the US Geological Survey in Menlo Park, California. He says that the 11 April example should challenge conventional definitions of how soon and how close aftershocks can occur to large earthquakes.

“Every earthquake is important to study, but this earthquake is rather unique,” says Hiroo Kanamori, a seismologist at the California Institute of Technology in Pasadena. With so many unusual characteristics to examine, the 11 April earthquake sequence may continue for some time to expand researchers’ ideas of how earthquakes can occur.




Pre-Historic Undersea Islands Found

Giant, Dinosaur-Age Islands Found in Deep Sea?

Odd rocks may have been from supercontinent Gondwana, research says.

A sonar image of the underwater land masses.
A sonar image shows the newfound deep-sea plateaus.

Photograph courtesy Joanne Whittaker

Richard A. Lovett

for National Geographic News

Published November 21, 2011

Giant, sunken pieces of an ancient continent from the time of the dinosaurs may have been discovered deep in the Indian Ocean, scientists say.

The two fragments, called microcontinents, are possibly leftovers from whenIndia, Antarctica, and Australia were part of a supercontinent known as Gondwana (see a map of Earth during this time.)

The plateaus, the combined size of West Virginia, have long been known to cartographers as the Batavia Seamount and the Gulden Draak—or Golden Dragon.

But not much else was known about the features, other than their location, about 1,000 miles (1,600 kilometers) west of Perth, Australia (map).

Surprising Deep-Sea Island Discovery

To fill in the gaps, an international team of scientists recently mapped the seabed and dredged samples from as deep as 8,200 feet (2,500 meters).

What the scientists found surprised them. Rather than the normal basalt rock of most seabeds, the scientists pulled up chunks of granite, gneiss, and sandstone—rocks normally found on continents.

Some samples even contained fossils, said team member Joanne Whittaker, a marine geophysicist at the University of Sydney in Australia.

“It’s quite clear that these two plateaus are little fragments of Gondwana left behind as India moved away from Australia,” Whittaker said.

(See “Undersea Mountain Photos: Brittlestar Swarm, More Found.”)

Ancient Continent Pieces Once Rolling Terrain?

Scientists initially thought the plateaus had flat tops, a sign that they’d been above sea level long enough to have been eroded into plains.

But, as mapping continued, it became clear the plateaus’ physical features were rolling, ranging in elevation from as little as 2,600 feet (1,000 meters) to 8,200 feet (2,500 meters) below the surface. That would mean the highest plateau rises up to about 15,000 feet (4,600 meters) above a surrounding abyss.

The fossils found in the fragments were marine bivalves, a type of mollusk—indicating that the life-forms had lived in shallow water, not on land.

The animals were also discovered in the deeper regions of the plateaus, not the highest peaks, which may have once been islands. “It’s difficult to tell,” Whittaker said. “But that’s certainly something we’ll be looking at.”

Whittaker and colleagues will also try to match the rock samples with rocks on a nearby geological feature, the underwater Western Australian margin, which may help “pin exactly where these little pieces [of Gondwana] came from,” she said.

Few details are known about how the breakup of Gondwana formed the Indian Ocean about 130 million years ago, she added. (See a prehistoric time line.)

Gondwana Breakup Still a Mystery

Some of the story of the breakup will never be told, since the Gondwana portion of what is now India later collided with Asia.

“In India, the equivalent rocks are probably now squashed beyond recognition somewhere in the Himalayas,” Whittaker said.

As for whether dinosaurs might have once roamed the two plateaus, that depends on whether the features ever extended above sea level, and if so, when.

“Who knows? Whittaker said. At the moment, “anything’s possible.”