News in Science

Quake start of Indo-Australia plate split

Breaking up An 8.7 earthquake that struck west of Indonesia in April was the biggest of its kind ever recorded and confirms suspicions that a giant tectonic plate is breaking up.

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, according to a study in the journal Nature.

"We've never seen an earthquake like this," says Keith Koper, a geophysicist at the University of Utah.

"Nobody was anticipating an earthquake of this size and type, and the complexity of the faulting surprised everybody I've spoken to about this," says 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 per cent greater than thought, according to investigations.

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 26 December 2004 quake, which measured 9.1 and resulted in a tsunami that killed a quarter of a million people, is one such example.

But the 11 April 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.

Plate breaking apart

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 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," says Koper.

Distant effect

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," says 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."