Samoa, then Indonesia: are the earthquakes linked?

Vision in memorial di 8:12 PM

Two earthquakes within 16 hours of each other. Cause and effect, or coincidence?

American Samoa and Indonesia, the victims of two earthquakes in 24 hours, both lie on the so-called Ring of Fire. This volcano- and quake-plagued semi-circle runs for 40,000km around the edge of the Pacific Ocean and accounts for approximately 90 per cent of the World's earthquakes and 75 per cent of its volcanoes.

At 17:48GMT on Tuesday September 29, a magnitude 8.3 earthquake struck south of American Samoa. Just 16 hours later, at 10:16GMT yesterday, the second quake, of magnitude 7.6, hit 30 miles off the east coast of the Indonesian island of Sumatra. A third, smaller, earthquake struck Sumatra in the early hours of this morning. (The earthquake which caused the devastating Asian tsunami on Boxing Day 2004 was magnitude 9.1.)

So why might the Samoan and Indonesian quakes be linked? The epicentres of these earthquakes both lie near to the edge of the Australian plate, about 4,000 miles apart (see map). It seems plausible that one movement of the Australian plate caused both earthquakes. But David Booth, senior seismologist at the British Geological Survey, is not so sure.

"There are sound physical reasons for expecting one earthquake to cause another, but that is very unlikely to have happened in this case," he says. "The quakes were in different fault zones, and the chance of one earthquake triggering another by seismic waves moving up the fault line are very slight over such a great distance.

"Indonesia is one of the most seismic zones in the world - perhaps the most seismic - so it's no real surprise that a large earthquake has happened there, but the fact that it happened within hours of the Samoan one is almost certainly a coincidence."

One thing Booth is certain about is that there has been an upsurge in seismic activity in the Indonesian faultline since the 2004 Asian tsunami - and recent earthquakes in that region have only added to the likelihood of further disasters.

"As one earthquake happens, the stress in the fault changes," he explains. "The stress on unfractured segments increases, making an earthquake in that segment more likely."

This is indeed what has happened in Indonesia this morning. Sumatra was hit by a second earthquake of magnitude 6.8 at 01:52GMT. It won't be the last.