Quakes Signal Indo-Australian Plate Breakup: April Temblors Released Energy West of Sumatra

Article excerpt

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 (SN Online: 4/11/12), startling seismologists with their size (magnitudes 8.6 and 8.2) and location (hundreds of kilometers from the active seismic 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 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.

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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 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 Ecole Normale Superieure in Paris. The motorcycle--the part of the plate carrying Australia--is driving quickly northeast. But the sidecar--the part carrying India--is slamming into 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 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 just over two minutes, the scientists found. Each fault ruptured with the equivalent energy of at least a magnitude 8.0 quake in that magnitude 8.6 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. …