BioEd Online

As fears spread about damaging aftershocks from Sunday's killer earthquake, experts are keen to reassure people that such tremors are unlikely to worsen the situation seriously.

The official death toll from the 26 December quake in the Indian Ocean now tops 120,000. Five million people in the area are thought to be in urgent need of aid, but efforts to tackle the devastation and to help survivors are being hampered in India and Sri Lanka as people flee the shore again, after warnings of impending aftershocks and further monstrous waves.

Strong aftershocks are likely to continue for weeks, months or even a year, confirms Waverly Person, a geophysicist at the United States Geological Survey in Golden, Colorado. "But they're not large enough to generate additional tsunamis," he says.

Strong shocks

The largest aftershock measured so far occurred about three hours after the initial quake, had a magnitude of 7.1 and was classified as a major quake in its own right. And as of 30 December, around 85 aftershocks had been recorded and categorized as being either strong or moderate, Person says.

But although violent, quakes such as these are unlikely to cause more large tsunamis, which were responsible for the vast majority of the destruction after Sunday's initial 9.0 magnitude quake.

The scale used to measure quakes is logarithmic, so the original tremor was orders of magnitude more powerful than any subsequent ones (see "How is the severity of earthquakes measured?"). Smaller aftershocks, of magnitude 8 or less, lack the power to trigger killer waves and are likely just to cause land tremors on Indonesian islands close to their epicentre.

Aftershocks are occurring because the stress built up between the neighbouring India and Burma tectonic plates, off the Sumatran coast, was not all released in the original earthquake. There are subsequent smaller adjustments of the plates as they settle into a new stable position.

Before the current disaster, the last quake to top 9 in magnitude was in Alaska in 1964. Aftershocks there continued for more than a year.

The Asian region could yet experience an aftershock as large as 8 in magnitude, predicts geophysicist Rob McCaffrey, who studies seismic activity at Rensselaer Polytechnic Institute in Troy, New York. But "the likelihood of another 9 is infinitesimally small", he says.