BioEd Online

A huge earthquake struck the coast of Indonesia on Monday, just three months after last year's devastating tsunami. But researchers are scratching their heads over why this one did not trigger giant waves.

The magnitude-8.7 earthquake struck just after 11:00 pm local time on 28 March, around 200 kilometres west of northern Sumatra. It is classified as a 'great earthquake' and occurred south of the earthquake that hit the region on 26 December 2004.

There are reports of significant damage and around 1,000 deaths on the nearby Indonesian island of Nias, although the final figures are not yet clear. "I'm sure the death toll will go up," says Waverly Person, a geophysicist at the United States Geological Survey in Golden, Colorado.

Initial reports from the region suggest however that this earthquake did not generate vast waves. By contrast, last year's earthquake, now thought to be magnitude 9.3, triggered tsunamis that spread as far as Africa and killed an estimated 300,000 people.

Deep tremor

Seismologists are not yet clear why the ocean remained calmer this time. The earthquake released only a quarter of the energy of its predecessor (the scale on which magnitude is measured is logarithmic). Nonetheless, it is one of the eight most powerful earthquakes measured since 1900.

Lesser earthquakes in 1861 and 1833 in the same region did trigger tsunamis. "It's very puzzling," says geophysicist Rob McCaffrey, who studies seismic activity at Rensselaer Polytechnic Institute in Troy, New York. "It's probably one of the biggest earthquakes in history."

One possible explanation for the absence of tsunamis could be that the latest tremor occurred much deeper in the fault line that slices through the Indian Ocean, McCaffrey says. This might have avoided shifts in the sea bed that can displace water and prompt a tsunami. Seismologists will be studying the position and depth of the earthquake to try and answer these questions.

Yesterday's tremor is not classified as an aftershock of the December event, because it did not occur in exactly the same area. But Waverly says that it was probably triggered by last year's earthquake.

Researchers had predicted that the earlier judder, which increased stress on neighbouring fault lines, would prompt more earthquakes (see ' Quake threat rises after tsunami slip').

Monday's event seems to have occurred in the Sunda trench, one of the places pegged as likely candidates for a quake as a result of stress changes caused by December's monster. "This quake may be a vindication [of our understanding] of the stress transfer process," says Bill McGuire, an expert on earthquake hazards at University College London.

Last year's disaster prompted governments to commit resources to a tsunami warning system in the Indian Ocean, but it is not yet up and running. Even so, some coastal areas did issue alerts and evacuated coastal areas more promptly because of lessons learned from the earlier catastrophe.

Additional reporting by Michael Hopkin.