<p>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.<br /><br /></p>.<p>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.<br /><br />"We've never seen an earthquake like this," said Keith Koper, a geophysicist at the University of Utah in the western United States.<br /><br />"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.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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.</p>
<p>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.<br /><br /></p>.<p>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.<br /><br />"We've never seen an earthquake like this," said Keith Koper, a geophysicist at the University of Utah in the western United States.<br /><br />"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.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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.<br /><br />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.</p>