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Last Updated: Friday, 31 December, 2004, 16:10 GMT
Earthquake 'redraws the map'
Satellite imagery reveals what happened to places like Aceh

The devastating earthquake that struck the Indian Ocean probably caused some islands to move by several metres.

The massive thrust of the tectonic plates may have heaved the Indian Ocean floor towards Indonesia by about 15m (50ft), seismologists think.

The movement is likely to have altered the geography of islands like Sumatra.

The force of the earthquake was probably also so great that it made the Earth wobble on its axis and cut our day length by fractions of a second.

Building pressure

The earthquake follows more than a century and a half of growing pressure between the Indian tectonic plate and the Burma microplate, upon which Sumatra, Nicobar and the Andaman Islands sit.

"In terms of the specific position of Sumatra, it will have moved," Bill McGuire, a geophysicist at University College London, UK, told the News@Nature website. "Things have shifted literally within minutes."

It is not only likely that these islands have shifted geographically, they may also be higher or lower than before, Professor McGuire believes.

Since the Burma microplate shot upwards during the quake, the Andaman and Nicobar Islands are likely to have been elevated. Slightly further from the fault itself, water levels indicate that the Indonesian city of Banda Aceh has sunk.

A team from the Southern California Earthquake Hazards Assessment Project (SCEHA) plans to visit the area to examine the changes with the help of the global positioning survey.

"The work will take weeks or months to complete," said Dr Kenneth Hudnut of SCEHA.

Changing wobble

According to US scientists, the deadly earthquake was forceful enough to accelerate the Earth's rotation, and may have made the planet wobble on its axis.

Richard Gross, a geophysicist with the US space agency's Jet Propulsion Laboratory in California, believes a shift of mass toward the Earth's centre during the quake caused the planet to spin three microseconds faster and tilt about 2.5cm (one inch) on its axis.

The Earth's poles travel a circular path that normally varies by about 10m (33ft), so an added wobble this scale is unlikely to cause long-term effects.

"That continual motion is just used to changing," Dr Gross said. "The rotation is not actually that precise. The Earth does slow down and change its rate of rotation."

Scientists have long theorised that changes on the Earth's surface such as tide and groundwater shifts and weather could affect its spin, but they have not had the measurements to prove it.

"Even for a very large event, the effect is very small," said seismologist Hiroo Kanamori, from the California Institute of Technology. "It is very difficult to change the rotation rate substantially."

Beach disappearance

But images from space have already begun to reveal the exact nature of the changes wrought on the region.

In Indonesia and Thailand, whole sections of the coastline have been drastically altered, according to German satellite imaging specialists.

"In many areas where the tsunami had a lot of energy or was moving laterally to the shoreline, it has washed away beaches and rock, leaving underground material exposed to the sea and air," said Dr Stefan Voigt of the German Aerospace Center (DLR).

"We saw some islands and coral reefs washed away or drastically changed," he told the BBC News website.

In Indonesia, the tsunami created temporary lakes and wetlands, which have probably made some areas impassable to survivors and aid workers.

"In some places, where the topography was not very steep, the water travelled several kilometres inland," said Dr Voigt.

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