The earthquake that shook southern Asia yesterday was one of the most powerful since the start of the 20th century, the US Geological Survey said. ?We?ve just updated it to 8.9 magnitude. That makes it the fifth largest earthquake since 1900,? said Julie Martinez, geophysicist for the U.S. Geological Survey?s Earthquake Hazards Programme in Golden, Colorado.

It was the largest quake in the world since 1964, she said. That year, a major earthquake hit Alaska?s Prince William Sound.

Yesterday?s quake struck at 7.59 a.m. off the coast of Aceh province on the northern Indonesian island of Sumatra and appeared to swing north into the Andaman islands in the Indian Ocean.

It triggered a tsunami that killed hundreds in Sri Lanka, Thailand, Indonesia and India. ?These big earthquakes, when they occur in shallow water, ... basically slosh the ocean floor ... and it?s as if you?re rocking water in the bathtub and that wave can travel basically throughout the ocean,? USGS geophysicist Bruce Presgrave told the BBC.

He said there had been no signs of the impending quake. ?Unfortunately, we are not able to predict earthquakes at this time and one of the big reasons is typically these big earthquakes occur with no warning, no foreshock activity or anything like that,? he added.

As the Earth moves and its plates hit each other, it breaks in one place and pressure builds up in a different area, Martinez said. When that pressure increases, another earthquake occurs.

?About 1 000 km of the Andaman thrust (or faultline) broke, which is a huge area,? Martinez said. ?This doesn?t occur that often. To have a break along that long of a faultline ... is more unusual.?

The magnitude of the quake increased the likelihood of aftershocks ? the tremors that follow as minor readjustments occur along the fault after the main shock.

?I would say it makes aftershocks more likely and it makes them more likely to be large,? Presgrave said. ?We have already seen one aftershock of magnitude 7.3 and there have been a large number of aftershocks in the magnitude 5-6 range.?

Martinez said aftershocks are usually in more or less the same area. ?Because of the size of this quake, you will see more quakes in a larger area because the break or the faultline is larger.?

<B>Strongest earthquakes in the last century</B>

The world?s fifth strongest quake in a century hit southern Asia yesterday, triggering a speeding tsunami that crashed into Sri Lanka and India and swamped tourist islands in Thailand and the Maldives.

Here is a factbox showing the 10 strongest earthquakes in the last century by magnitude.

?<B> May 22, 1960</B> - Chile - An earthquake measuring 9.5 struck Santiago and Concepcion, triggering tidal waves and volcanic eruptions. Some 5 000 people were killed and 2 million made homeless.

?<B>March 28, 1964 </B>- Alaska - An earthquake and ensuing tsunami claimed 125 lives and caused about $311 million in property loss. The quake, measuring 9.2, was felt over a large area of Alaska and in parts of western Yukon Territory and British Columbia, Canada.

?<B>March 9, 1957</B> - Alaska - An earthquake measuring 9.1 hit the Andreanof Islands. On Umnak Island, Mount Vsevidof erupted after being dormant for 200 years, generating a 15-metre high tsunami that continued to Hawaii. Nov 4, 1952 - Russia - An earthquake with a magnitude of 9.0 generated a tsunami which struck the Hawaiian islands. No lives were lost.

?<B> Dec 26, 2004 </B> - Indonesia - A quake measuring 8.9 struck the coast of Aceh province on the northern Indonesian island of Sumatra and triggered a tsunami that killed thousands in Sri Lanka, Thailand, Indonesia and India.

?<B> Jan 31, 1906 </B> - Ecuador - An earthquake with a magnitude of 8.8 struck near the coast of Ecuador and Colombia, generating a strong tsunami that killed up to 1 000. It was felt all along the coast of Central America and as far north as San Francisco and west to Japan.

?<B> Feb 4, 1965 </B> - Alaska - Measuring 8.7, the quake generated a tsunami reported to be about 10.7 metres high on Shemya Island.

?<B> Aug 15, 1950</B> - Tibet/India - Two thousand homes, temples and mosques were destroyed in a quake measuring 8.6. Hardest hit was the Brahmaputra Basin in northeast India. At least 1 500 people were killed.

?<B> Feb 3, 1923</B> - Russia - Kamchatka was struck by a quake with a magnitude of 8.5.

?<B> Feb 1, 1938</B> - Indonesia - An 8.5 magnitude earthquake in the Banda Sea generated tsunamis causing great damage on Banda and Kai, volcanic islands in eastern Indonesia.

<I>The magnitude of the quake increased the likelihood of aftershocks ? the tremors that follow as minor readjustments occur along the fault after the main shock. </I>