How earthquake jolted the planet

29 December 2004 | 11:53 Code : 4594 Geoscience events
Sumatra is thought to have moved by as much as 120ft and the Earth to have shifted on its axis
Sumatra is thought to have moved by as much as 120ft and the Earth to have shifted on its axis
THE earthquake responsible for the Asian tsunami was so powerful that it changed the local geography, shifting islands and the mainland of Sumatra by as much as 120ft, American seismologists said yesterday. The energy released by the quake was so huge that it may even have caused the Earth to wobble on its axis. Geophysicists are calling it a megathrust, the term used for the most powerful of changes in the Earth’s surface. Pressure built up over nearly two centuries was released in a single snap.
“That earthquake has changed the map,” Ken Hudnut, of the US Geological Survey (USGS), said. “Based on seismic modelling, some of the smaller islands off the southwest coast of Sumatra may have moved to the southwest by about 20 metres. That is a lot of slip.”
The northwestern tip of Sumatra may also have shifted to the southwest by about 36 metres (120ft), he said. In addition, the energy released as the two sides of the undersea fault slipped against each other probably made the Earth wobble on its axis. “We can detect very slight motions of the Earth and I would expect that the Earth wobbled when the earthquake occurred due to the massive amount of energy exerted and the sudden shift in mass,” he said.
The energy released was prodigious. Normally, seismologists reckon that an earthquake of magnitude 8 releases energy equivalent to six million tons of TNT. For every step up in the magnitude scale, the energy released is increased by a factor of 32, so a magnitude 9 quake such as Sunday’s would be equivalent to more than 190 million tons of TNT. The tectonic plates that carry the Earth’s continents around move at about 6cm a year, roughly as fast as fingernails grow. Although the region around Indonesia is highly active, there had been no big earthquakes along this section for more than a century. There had been huge quakes in 1797 and 1833, but little since. That explains why so much pressure had built up, to be released so quickly early on Sunday morning. Kerry Sieh, of the California Institute of Technology (Caltech), an expert in the area, said: “Sunday was
one of the biggest earthquakes in the region in the past 200 years.”
Geophysicists are not yet sure just how far the land moved. Stuart Sipkin, of the USGS National Earthquake Information Centre in Golden, Colorado, believes that it is more likely that the islands off Sumatra had risen higher out of the sea, rather than moving laterally. “In this case the Indian plate dived below the Burma plate causing uplift, so most of the motion to the islands would have been vertical, not horizontal,” he said. Extensive damage and flooding was preventing investigators from reaching the scene. When they do, global positioning systems will be used to detect exactly how much movement has taken place, and where. Beneath the ocean the edges of the plates might have shifted vertically by as much as 18.3m relative to each other. But even that kind of displacement would lift or lower the Sumatran coast by only a few feet or less, the experts said, and sea levels would not change dramatically.
“Basically, the run-up of high tide will be just a little further up or further back,” Paul Earle, a USGS geophysicist, said.
Inland, ground levels in northern Sumatra might have changed noticeably in places, Dr Sieh said. “As the block of land on top of the subduction zone lurches out west towards the Indian Ocean, you expect that area behind it to sink,” he said. “The western coast of Sumatra has probably sunk by a metre or two.” Banda Aceh, the regional capital, may now be below sea level. Television pictures had shown people in the capital walking around in knee-deep water after the tsunami had receded, indicating that the land was lower than before. As for causing the Earth to wobble, most seismologists agree that this is likely but it has yet to be tested. “It causes the planet to wobble a little bit, but it is not going to turn Earth upside down,” Dr Sieh said. The seismologist Hiroo Kanamori, of Caltech, said that the quake would probably have affected the Earth’s rotation and the regular wobble of its axis. “The question is how much and can it be detected,” he told the Los Angeles Times. The tsunami generated by the earthquake was so huge that some of its energy spilt into the Pacific. At Manzanillo, Mexico, waves rose more than 8ft and fluctuations were reported in New Zealand and Chile.
After the initial quake there were several aftershocks, some as substantial as magnitude 7.3. Seismologists do not now expect further major earthquakes, at least not in the immediate vicinity. The pressure has been released, and will now start to build up again for fresh quakes decades or centuries from now.

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