Twist of plate. The northwestern tip of the Indo-Australian tectonic plate is slamming into southern Asia and slowing down, while
the central and eastern portions are sliding full speed beneath their tectonic neighbors, producing stresse

Keith Koper, University of Utah Seismograph Stations

Indian Ocean Quakes Part of Slo-Mo Seafloor Breakup

The magnitude-8.7 quake that wrenched the seafloor west of Sumatra on 11 April wasn't merely the largest quake of its type ever recorded. That quake—and the magnitude-8.2 aftershock that occurred just 2 hours later—were part of a much bigger story: the slow ripping-apart of the Indo-Australian tectonic plate, a process that has been unfolding for millions of years and will undoubtedly continue long into the future. The end result, scientists say, will be a new tectonic boundary stretching across the seafloor of the eastern Indian Ocean.

The crescent-shaped Indo-Australian tectonic plate is named, aptly enough, after the two largest landmasses it hosts—the Indian subcontinent at its northwestern tip, and Australia in its eastern reaches. In between, there's a broad arc of seafloor, with a large part of the plate's edge running offshore of the Indonesian archipelago. That arc includes the portion of the plate shoving its way under the western edge of Sumatra, which in December 2004 was rocked by a magnitude-9.1, tsunami-spawning quake, as well as a magnitude-8.6 quake that occurred a few hundred kilometers to the southeast just 3 months later.

The April 2012 quakes occurred far from a major boundary between tectonic plates, however. The magnitude-8.7 quake is the largest intraplate quake ever measured by instruments, says Matthias Delescluse, a marine geophysicist at France's National Center for Scientific Research in Paris. It also holds the record for strongest "strike/slip" earthquake -- one in which the steep sides of a fault push and slip past each other, as happens along much of California's San Andreas fault, rather than one caused by the bulldozing of a thrust fault, like that which produced the 2004 earthquakes.

Now, analyses by Delescluse and his colleagues suggest that the Wharton Basin, the patch of seafloor in the northeast Indian Ocean where the April 2012 quakes occurred, was seismically awakened by the Sumatran quakes of 2004 and 2005. For one thing, release of seismic energy in the Wharton Basin in 2005 (in the months after the Sumatran quakes) was more than five times the amount released each year, on average, during the previous century. The quakes of April 2012 alone released about 1000 times the seismic energy released each year in the previous century, the researchers report online today in Nature. In the 7 years between the 2005 Sumatran quake and the April 2012 temblors, the Wharton Basin experienced 47 quakes with a magnitude of 5.0 or higher. By comparison, in the 8 years before the Sumatran quake of December 2004 the region experienced only 14 such quakes, the researchers note.

The magnitude-8.7 temblor that occurred on 11 April "was a remarkably complicated quake that took place along a complex network of faults," says Keith Koper, a seismologist at the University of Utah in Salt Lake City. "There's no precedent for this in recorded seismic history," he notes. Analyses of the seismic waves that radiated from the 2-minute, 40-second-long quake suggest that the opposing sides of at least four major seafloor faults slipped between 6 meters and 37 meters past each other during the event.

That quake first sprang to life about 330 kilometers west-southwest of the epicenter of the December 2004 Sumatran temblor. The first fault to rupture, oriented west-northwest to east-southeast, slipped as much as 37 meters; the slippage lasted for 50 seconds. The rupture extended almost through the full thickness of the tectonic plate, reaching depths of more than 50 kilometers, Koper says. Even before the first fault stopped slipping, another fault ruptured. The second fault, which runs for 100 kilometers to 200 kilometers perpendicular to the first and intercepts it, slipped as much as 8 meters. Then a third fault let loose; that one is about 150 kilometers southwest of the first and parallel to it, and slipped as much as 21 meters along a rupture zone about 150 kilometers long. Finally, a fourth fault slipped, the sides of the fault moving as much as 6 meters past each other along a 50-to-100-kilometer segment of fault in just 15 seconds. Koper and his colleagues describe this multistage quake online today in a second study in Nature.

Long-term seismic stress in the Wharton Basin region is still accumulating as a result of differing stressors to different parts of the Indo-Australian tectonic plate. These different forces are shearing the tectonic plate apart: While the northwestern tip of the plate is slamming into southern Asia and slowing down—and has been doing so for 8 million years or so—the central and eastern portions of the plate are still free to slide beneath neighboring plates, Delescluse says. Thousands more quakes like the one that occurred on 11 April will be needed for the Indo-Australian plate to fully rip in two. In the meantime, he notes, no one has a really good idea about where the new tectonic boundary will form or how the edges of the new plates will move relative to each other.

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