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Quake changed earth's shape, rotation

January 11, 2005 09:01 IST

The earthquake off the coast of Indonesia, which spawned tsunamis in the Indian Ocean region killing over 150,000 people, has slightly changed the earth's shape, shaved microseconds from the length of the day, and shifted the North Pole, according to NASA scientists.

"Although all earthquakes leave an impact, the devastating mega-thrust earthquake registered nine on the new 'moment' scale (modified Richter scale), making it the fourth largest quake in one hundred years," Dr Benjamin Fong Chao of NASA's Goddard Space Flight Centre and Dr Richard Gross of NASA's Jet Propulsion Laboratory said.

Chao and Gross routinely calculate the effects of earthquakes on earth's shape and rotation. They also study changes in polar motion, which shows the shifting of the North Pole.

According to their latest calculations, the December 26 earthquake shifted the earth's "mean North Pole" by about 2.5 centimetres in the direction of 145° East Longitude, more or less toward Guam in the Pacific Ocean.

This shift is continuing a long-term seismic trend identified in previous studies.

The quake also affected the earth's shape. Chao and Gross calculated that Earth's oblateness (flattening on the top and bulging at the equator) decreased by a small amount -- about one part in 10 billion.

This continues the trend of earthquakes making the earth less oblate.

They also found that the earthquake decreased the length of the day by 2.68 microseconds. (A microsecond is one millionth of a second.) The earth spins a little faster than it did before.

This change in spin is related to the change in oblateness. It is like a spinning skater drawing arms closer to the body resulting in faster spin, they said.

None of these changes have yet been measured -- only calculated. But Chao and Gross hope to detect the changes when the earth rotation data from ground-based and space-borne sensors are reviewed.

Seema Hakhu Kachru in Houston