BioEd Online

On 17 March, a team of seismologists published a paper in Nature that attempted to evaluate the knock-on effects of the 26 December earthquake. Eleven days later, another horrific earthquake struck, almost exactly where they said it would.

The magnitude-8.7 tremor, which has left some 2,000 feared dead, occurred in the Sunda trench. This runs south from the site of the previous Sumatra rupture and was one of two fault lines identified by the researchers as being under increased stress¹. When the researchers predicted that the Sunda fault was likely to give way as a result (see " Quake threat rises after tsunami slip"), they didn't know how quickly they would be proved right.

"We got the location and the size bang on, but not the timing," says team leader John McCloskey of the University of Ulster, UK. He notes that seismologists are notoriously wary of the term 'prediction': "It conjures up all that's worst about the science."

Experts prefer to talk about 'earthquake risk'. Although modelling can tell us where an earthquake might strike next, it is still almost impossible to say when, because we don't know enough about the breaking strains of individual fault lines.

However, given that McCloskey's team were so accurate this time around, where does that leave the region's other at-risk fault line: the Sumatra fault that runs directly underneath Banda Aceh. Has the latest earthquake increased the danger still further?

Wider stress

"It doesn't look as if the size of the stress has increased, but the extent has," McCloskey says. The first earthquake affected some 300 kilometres of the Sumatra fault; the latest event could have elongated the stressed section to some 600 kilometres. "The extent is important," he adds. "It increases the chance of there being a weak patch."

This is based on crude estimates of how much slip might have occurred in the latest tremor, McCloskey admits. Researchers at the California Institute of Technology in Pasadena are "burning the midnight oil to calculate these things", he says, adding that a detailed profile of the event should be available within days.

What is clear is that this was another fearsomely big earthquake. "I wouldn't be surprised if it's the biggest in the world this year," McCloskey says. Anything with a magnitude above 7.5 is generally classed as a giant earthquake.

Log scales

So why was there no tsunami this time? The answer may be that this earthquake, although huge by everyday standards, is still small compared with that of 26 December. Researchers in this week's Nature confirm² that the earlier shake had a magnitude of 9.3.

"Superficially, 8.7 and 9.3 seem similar. But because the scale is logarithmic, Monday's event released a fraction of the energy," McCloskey explains. "We're stunned with superlatives for the Boxing Day event: it was a true giant. It seems that damaging tsunamis are thankfully very rare."

Nonetheless, he was impressed by the speed at which Indonesian villagers fled their homes for higher ground. "The response was exactly right. I would recommend that people err on the safe side: children in Japan are taught to just get away from the sea if the ground is shaking. Those closest to the epicentre are not helped by high-tech warning systems."