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.
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 yesterday.
"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 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.