The Indian Ocean earthquake that triggered the great Boxing Day tsunami literally shook the world and triggered a swarm of minor earthquakes 11 000km away in Alaska.
It set new records — the longest fault rupture seen to date, the longest duration and the most energetic swarm of aftershocks observed to date.
The calamity began with a sudden shift on average of more than 5m along an 1 280km fault line deep below the ocean. Just off Banda Aceh in northern Sumatra, the ocean floor suddenly moved north-eastward, pushing as much as 20m 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 on Friday in the United States journal Science. It rearranged the Earth’s surface and caused measurable deformation almost 4 480km 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 kilometres away.
”But here we see deformation 4 500km away, and five or six times the deformation we have seen in previous quakes.”
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 000km from the epicentre, the surface shifted by just a millimetre. It shifted 2cm in southern India.
The shock waves caused the ground to rise and fall 9cm in Sri Lanka. It moved massive slabs of rock 20m, 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.” — Guardian Unlimited Â