by Staff Writers
Paris (AFP) Sept 26, 2012
An 8.7 earthquake that struck west of Indonesia on April 11 was the biggest of its kind ever recorded and confirms suspicions that a giant tectonic plate is breaking up, scientists said on Wednesday.
The quake, caused by an unprecedented quadruple-fault rupture, gave Earth's crustal mosaic such a shock that it unleashed quakes around the world nearly a week later, they said.
"We've never seen an earthquake like this," said Keith Koper, a geophysicist at the University of Utah in the western United States.
"Nobody was anticipating an earthquake of this size and type, and the complexity of the faulting surprised everybody I've spoken to about this," said Thorne Lay, a planetary sciences professor at the University of California, Santa Cruz.
The quake occurred around 500 kilometres (300 miles) west of Sumatra in the middle of the Indo-Australian plate, a piece of Earth's crust that spans Australia, the eastern Indian Ocean and the Indian sub-continent.
It was initially reported as measuring 8.6 on the "Moment magnitude" scale.
But a new calculation places it at 8.7, which under this logarithmic scale means the energy release is 40 percent greater than thought, according to investigations published in Nature.
It was the biggest "strike-slip" earthquake ever recorded, meaning a fault which opens laterally rather than up or down, and the 10th biggest quake of any kind in the last century.
It was followed two hours later by an 8.2 event on another fault a little farther to the south, and both were felt from India to Australia.
Earthquakes of such intensity are typically "subduction" quakes, where one tectonic plate slides beneath another at a plate boundary, causing vertical movement that can displace the sea and unleash a tsunami.
The December 26 2004 9.1 quake off Sumatra, whose waves killed a quarter of a million people around the Indian Ocean, is one such example.
But the April 11 event caused no tsunamis because the movement was sideways. Fatalities, too, were few -- 10, according to the Indonesian authorities -- because it occurred under the Indian Ocean.
Taking a scalpel to what happened that day, the seismologists believe there was a near-simultaneous rupturing of at least four faults, stacked up and lying at right angles to one another.
They ripped open one by one, all within 160 seconds, in a process known by the French term "en echelon."
Even more remarkable, though, was where the event took place.
It occurred nowhere near a boundary between the plates which like a jigsaw puzzle comprise Earth's crust.
Instead, it occurred in the heart of the Indo-Australian plate, tearing a gash up to 40 metres (yards) wide and confirming long-held suspicions that the plate is fragmenting.
According to this theory, the process began roughly millions of years ago, and is caused by a pulling-apart of the plate: the western part is colliding with Asia, which stops its movement, while the eastern part is gliding beneath Sumatra.
"It will take millions of years to form a new plate boundary and, most likely, it will take thousands of similar large quakes for that to happen," Koper said.
Another study in Nature found that quakes occurred around the world for at least six days afterwards.
They included a 7.0 quake in Baja California, Mexico, and in Indonesia and Japan.
Mercifully, the big shakes occurred in rural areas, not in urban areas where the outcome "could potentially have been disastrous," said Roland Burgmann of the University of California at Berkeley.
"Until now, we seismologists have always said, 'Don't worry about distant earthquakes triggering local quakes.' This study now says that, while it is very rare -- it may only happen ever few decades -- it is a real possibility if the right kind of earthquake happens."
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