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The earthquake which devastated the city of Padang in Sumatra, Indonesia, this week, killing more than 1100 people, may have been only a hint of worse to come. Since 2004, geologists have been predicting a far nastier earthquake in the region – a shallow tremor that will rip the sea floor apart, trigger a devastating tsunami and kill far more people.

"Another earthquake is on its way, and all it will take to trigger it is the pressure of a handshake," says John McCloskey, a seismologist at the Environmental Sciences Research Institute at the University of Ulster in Coleraine, Northern Ireland.

Padang experienced a magnitude-7.6 earthquake on 30 September, just after 5 pm local time. Images of terrified relatives waiting to identify dead bodies, their T-shirts clutched over their noses to mask the stench, military officials stalking between bright yellow, zipped-up body bags and centuries-old Dutch colonial mansions obliterated in an instant have flooded around the world.

At first, geologists assumed this was the earthquake they had predicting for many years. "Padang has bad geology," explains McCloskey. "It sits 40 kilometres above the most earthquake-prone stretch of the interface between the Indo-Australian and Eurasian plates."

This interface has not experienced the stress relief of an earthquake for over 200 years, according to McCloskey's analysis of historical coral growth rings, which show no sign of seafloor uplift. GPS measurements of the rate of plate motion suggest that there has been around 13 metres of movement in this area over the same period. "A shallow earthquake at the plate interface off Padang is long, long overdue," says McCloskey.
Freak event

Yet the earthquake which struck this week off Padang did not occur at the plate interface, which lies 500 kilometres offshore. The epicentre was just 45 kilometres from Padang, far away from the plate interface. What's more, it originated 80 kilometres underground, far deeper than the place at which the Indo-Australian and Eurasian plates crunch together.

Further evidence comes from the orientation of the rupture caused by this week's quake. "The rupture spread in a north-south orientation, rather than east-to-west, as we would expect along the plate interface," says McCloskey.

All the clues add up to the earthquake being a freak rupture of an ancient stressed fracture zone embedded deep within the Indo-Australian plate rather than slippage at the plate interface. "What we're looking at is probably a vestigial crack left over from some distant spreading centre," says McCloskey.

So, what kind of damage will the tsunami-triggering earthquake that the geologists have been predicting near Padang inflict? McCloskey has built computer models of over 125 scenarios in which shallow, powerful earthquakes at the interface off Padang jolt the sea floor, triggering tsunamis. In most, devastating tsunamis are generated. They will reach the city about 30 minutes after the earthquake hits.

His simulations suggest that 25 per cent of tsunamis would be over 5 metres tall as they reached the coast; the highest waves would be 12 metres tall. "In reality, of course, waves will gather height and become more turbulent as they power inland, which means they could be far higher over the city," says McCloskey.
Escape routes

If the people of Padang are well prepared, then most should survive, says McCloskey. Within 30 minutes, the young and the fit should be able to reach the 10-metre elevation contour that rises 2 kilometres back from the coast, he says, which would at least protect them from waves lower than 10 metres.

However, over 100,000 people – a seventh of the city's population – are blocked from running directly to higher ground by the barbed wire-laced, 10-metre-high walls of a huge military airport.

"Padang needs to build a tunnel under that airport, because if they don't these poor people will have to run parallel to the coast for several hundred metres while the tsunami is coming at them," says McCloskey. So far, no steps have been taken to build such an exit route. "Sometimes you despair," he says.

Source : Journal references: Earth and Planetary Science Letters, DOI: 10.1016/j.epsl.2007.09.034; Nature, DOI: 10.1038/nature07572

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