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Quake threat rises after tsunami slip

March 16, 2005 By Michael Hopkin This article courtesy of Nature News.

Nearby faults are under increased strain after December's catastrophe.

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The huge earthquake that caused the Indian Ocean tsunami has increased the stress on neighbouring fault lines, experts have calculated. The knock-on effect could be another large earthquake in Sumatra or even another tsunami, they warn.

Seismologists have studied the fault lines around the Burma microplate, whose border with the Indian plate was the scene of the huge jolt in December 2004. Using a mathematical model, they conclude that the increased strain on the neighbouring Sumatra fault, which runs through the island itself, makes Sumatra the most likely place for a subsequent earthquake.

The December earthquake shunted almost 250,000 square kilometres of the Indian plate under the Burma microplate's western edge, explain John McCloskey and his colleagues at the University of Ulster in Coleraine, UK. This has increased the pressure on the Sumatra fault on the plate's eastern edge, which runs close to the already ravaged city of Banda Aceh.

It is urgent. Earthquakes cluster in time and in space.
John McCloskey
Earthquake hazard researcher, University of Ulster, UK
The most immediate threat is probably an earthquake of magnitude 7-7.5 on the island, McCloskey's team predicts. This region has produced such earthquakes in the past, although not for more than a century, meaning that the fault may now be close to breaking point.

Also under increased strain is the Sunda trench, to the southeast of the region where December's earthquake struck. Like its neighbour, it is also an undersea 'subduction zone' with a history of destruction: in 1833 and 1861, earthquakes on the Sunda fault triggered fatal tsunamis.

Under pressure

The model used by McCloskey's team shows how the stresses have moved through the region in the wake of December's slip. They report in this week's Nature1 that pressure on a 50-kilometre stretch of the Sunda trench has increased by up to 5 bars, and a 300-kilometre segment of the Sumatra fault is now under an extra 9 bars of strain.

It's hard to tell whether this will trigger an earthquake at either location, McCloskey says. "The problem is that we don't know the failure stress for any of these structures."

And the calculations are based on a crude estimate of the amount of slip that occurred on 26 December, which does not take account of any subsequent shifts in the region, points out Kerry Sieh, a geologist at the California Institute of Technology in Pasadena. "This is just a first crack at the problem," he says.

But seismologists have used similar information to 'telegraph' earthquakes before, McCloskey says. He points out that the earthquake that devastated Izmit, Turkey, in 1999 was preceded by a 2-bar increase in pressure that seismologists spotted some 18 months before the quake.

A tsunami triggered by the Sunda trench would mostly dissipate in the Southern Ocean, McCloskey told news@nature.com. But Indonesia, particularly Sumatra, would almost certainly be struck again, and the wave would probably wash up on southern African shores too.

By showing how one earthquake can trigger another nearby, the researchers have underlined yet again the case for installing a tsunami warning system in the Indian Ocean, McCloskey argues.

"It is urgent," he says. "People think 'well you've had your bad luck', but earthquakes cluster in time and in space."

A warning system will be most useful to people on distant shores, rather than the Sumatran population, Sieh adds. "Sumatrans will most likely have ample warning of an impending tsunami given by the severe shaking of the earthquake," he reasons.

References

  1. McCloskey J., Nalbant S. S. & Steacy S. Nature, 434. 291 (2005 ).

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