Intraplate quakes signal tectonic breakup

By Alexandra Witze: Two giant earthquakes in the eastern Indian Ocean have shown geologists that breaking up is easy to do — for tectonic plates, that is.

The pair of quakes hit on April 11, startling seismologists with their size (magnitudes 8.6 and 8.2) and location (hundreds of kilometers from the active zone that spawned the deadly 2004 magnitude 9.1 earthquake and tsunami). Now, three studies reveal that the April quakes were an indication that one great slab of Earth’s crust is slowly fracturing into two.

The work, reported online September 26 in Nature, confirms that seismic risk remains high in the area.

“You’d be nuts to think it was all over in offshore Sumatra,” says Kerry Sieh, a seismologist at the Earth Observatory of Singapore who was not involved in the new research.

The bigger April quake leapt straight into the record books. It was the largest earthquake ever recorded in the middle of a tectonic plate, rather than at a plate’s edges where most quakes happen. It was also the largest earthquake recorded along a strike-slip fault, in which two chunks of Earth’s crust slide past each other horizontally, like along California’s San Andreas. And it was the most complex strike-slip rupture ever seen, breaking along at least four separate faults interlaced like a geological lattice.

Add together the 2004 killer Sumatra quake, two nearby great quakes in 2005 and 2007, and these April Indian Ocean quakes, says Sieh, and “you get the greatest release of seismic energy anywhere on Earth in the past half-century.”

Blame it on the massive Indo-Australian crustal plate, which stretches from the Himalayas in the north to well below Australia in the south. You can think of the plate like a motorcycle with a sidecar, says Matthias Delescluse, a marine geophysicist at the École Normale Supérieure in Paris. The motorcycle — the part of the plate carrying Australia — is driving quickly northeast beneath Indonesia. But the sidecar — the part carrying India — is slamming into a geological wall of the Himalayas. The motorbike and sidecar are thus shearing apart. Millions of years from now, the Indo-Australian plate will split into an Indian and an Australian plate.

April’s quakes reminded scientists that this is happening, maybe even faster than once thought. The 2004 quake, to the east, sped up the rate of earthquakes across the region and probably hastened the April quakes, Delescluse and his colleagues report in Nature. They calculated how the monster 2004 and 2005 quakes changed stress patterns in the Earth’s crust, and found that releasing stress on the faults diving under Sumatra to the northeast actually raised stress in the strike-slip faults to the southwest, in the Indian Ocean.

In a second Nature paper, seismic records illuminate the complex way the seafloor ruptured in April.

The first April 11 quake unzipped four perpendicular faults one after another in less than two minutes, the scientists found. Each fault ruptured with the equivalent energy of at least a magnitude 8.0 quake in that event. Two hours later, the magnitude 8.2 aftershock struck just south of the main rupture. “This was a gee-whiz event for us,” says team member Thorne Lay, a seismologist at the University of California, Santa Cruz.

But the story wasn’t over once the two quakes were done. They continued to resonate around the globe, triggering big aftershocks as far away as Mexico, a third study finds.

Fred Pollitz, a seismologist at the U.S. Geological Survey in Menlo Park, Calif., became interested in the quakes when his colleagues’ pagers kept going off for days afterward with alerts of other big quakes. “That struck me as rather suspicious,” Pollitz says. So he and his colleagues went through catalogs of global earthquakes, looking for changes in patterns of seismicity.

They found that the number of quakes of magnitude 5.5 or greater, located more than 1,500 kilometers from the April 11 quakes, went up nearly fivefold for six days afterward. The biggest such quake was a magnitude 7 in Baja California, about 22 hours afterward.

Most giant quakes don’t trigger temblors so far away — or if they do, the triggered quakes are well below magnitude 5. The difference, Pollitz says, lay in the strike-slip nature of the April 11 quakes. This type of fault geometry allows the stress of a crustal movement to propagate much farther across the planet’s surface, compared with deep-diving plates that transmit their energy into the bowels of the Earth. The quick rupture also allowed seismic wave energy to travel out in pulses that “we believe shook up the faults more efficiently,” Pollitz says.

Though the strike-slip geometry may have triggered other quakes, it also meant that only a few people died in the April 11 events. Horizontal ground movements don’t push the ocean water around in ways that generate a deadly tsunami, like the one that killed a quarter of a million people in 2004.

Still, the seismic risk around Sumatra remains high, because other parts of the plate diving beneath Indonesia have not broken for some time. And the fact that big earthquakes can pop off where they’re not expected, like along strike-slip faults in the east Indian Ocean, suggests to researchers that other surprises lie in store.

Back Story | How the plates break up

Geoatlas/Graphi-Ogre, adapted by E. Feliciano

As geologists’ defining theory of the processes that shape the face of the planet, plate tectonics is rock solid. Yet there’s no clear definition on exactly what constitutes a tectonic plate or how many plates there are. At its most basic, a tectonic plate is a chunk of Earth’s outer surface (its crust and a portion of the upper mantle) that moves as a single entity. Where plates meet, they grind against or dive beneath one another.

Seven huge plates dominate the plate tectonic scene. At least eight smaller plates show up on most maps, mostly on the margins of oceans where Earth’s crust is thinner and more prone to fracture. But some of these plates, big and small, are in the process of breaking up (see main story), and in a few cases may have already done so. Several scientists argue that the Indo-Australian plate, which is the third largest plate and is in the midst of a drawn-out breakup, has already shed one fragment, a smaller plate dubbed Capricorn, on its southwest edge. Global-positioning satellite data show that the southwest part of the larger plate is not moving in exactly the same direction as the northwest and east. That, some geologists argue, suggests that Capricorn has already broken away.