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05/20/2005:

"Earth 'still ringing' from tsunami quake"

The Indian Ocean earthquake that triggered the great Boxing Day tsunami literally shook the world and triggered a swarm of minor earthquakes 11,000 kilometres away in Alaska.
It set new records - the longest fault rupture ever seen; the longest duration and the most energetic swarm of aftershocks ever observed.

The calamity began with a sudden shift on average of more than 16.5ft (5 metres) along an 800 mile fault line deep below the ocean. Just off Banda Aceh in northern Sumatra, the ocean floor suddenly moved north-eastward, pushing as much as 20 metres under the Burma tectonic plate.


It raised the tip of the Burma plate several metres, and it lifted the ocean itself, setting up a tsunami that slammed into the coasts of Sumatra, Malaysia, India and Sri Lanka, killing 300,000 people.
The earthquake was so catastrophic that its effects could be measured from space, according to scientists reporting today in the US journal Science. It rearranged the Earth's surface and caused measurable deformation almost 2,800 miles away.

"The Earth is still ringing like a bell today," said Roland Bürgmann of the University of California, Berkeley. "We have never been able to study earthquakes of this magnitude before, where a sizable portion of the Earth was distorted. Normally, we see deformation of the surface a few hundred kms away. But here we see deformation 4,500 kms away, and five or six times the deformation we have seen in previous quakes."
Full: guardian.co.uk

Seismologists now believe that the 9.15 magnitude earthquake was probably twice as powerful as previously estimated. The violence was also was more enduring: much of the movement along the fault line happened half an hour after the initial shock and continued for up to three hours.

Readings from 41 GPS stations were used to reconstruct the biggest shock in 40 years. At one site, 45,000 kms from the epicentre, the surface shifted by just a millimetre. It shifted two cms in southern India.

The shock waves caused the ground to rise and fall 9 cms in Sri Lanka. It moved massive slabs of rock 20 metres, along a 1,300km section of the fault. And it set the Earth ringing.

"Just like thumping a watermelon to hear if it is ripe, after a big earthquake thumps our planet we measure the natural tones from seismograms to detect properties of the Earth's deep mantle and core," said Jeffrey Park of Yale University. "The Sumatran-Andaman earthquake produced the best documentation of the Earth's free oscillations ever recorded."

Previous comparable earthquakes all occurred at least 40 years ago: in Kamchatka in Russia in 1952; the Aleutian islands in 1957; southern Chile in 1960 and Prince William Sound, Alaska, in 1964.

"This really is a watershed event. We've never had such comprehensive data for a great earthquake, because we didn't have the instrumentation to gather it 40 years ago. And then the sheer size of the event is so awesome.

"It is nature at its most formidable, and it has been humbling to all of us who have studied it, " said Thorne Lay of the University of California, Santa Cruz. "Even among seismologists, we call this a monster earthquake."