Quake Knocked Earth For a Loop

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michael_ital
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Quake Knocked Earth For a Loop

Post by michael_ital »

2004 catastrophe felt even in space

Drop in gravity affected satellites
Aug. 12, 2006. 01:00 AM


The giant earthquake that set off a devastating tsunami across the Indian Ocean in December 2004 disrupted the Earth enough to change gravity and to deflect satellites passing hundreds of kilometres above.

Two identical satellites, collectively known as the Gravity Recovery and Climate Experiment, or GRACE, travel one behind the other in a polar orbit separated by about 210 kilometres.

By recording small changes in the distance between them when their orbits are deflected, the satellites provide data used to calculate variations in the Earth's gravitational field.

In a report in the current issue of the journal Science, scientists at Ohio State University and the University of California, Santa Barbara, report that in the aftermath of the magnitude 9.1 earthquake, the largest in four decades, GRACE recorded a sudden drop in gravity near the quake's epicentre off Sumatra.

The rupture raised thousands of square miles of the seafloor, reducing the density of rocks in the Earth's crust and diluting their gravitational pull.

The data, combined with models of the Earth's interior, indicate that the density changes extend hundreds of kilometres.

"It really gives an insight of the Earth's interior down to the mantle area," said Shin-Chan Han, an Ohio State research scientist and an author of the Science paper.

It was the first time that the gravitational effect of an earthquake had been observed.

The gravity at the Earth's surface decreased by as much as about 0.0000015 per cent, meaning that a 150-pound person would experience a weight loss of about one-25,000th of an ounce.

In other places, where the force of the earthquake compressed rocks, gravity increased by a similar amount.

The force of gravity is changing in other areas of the Earth, too. In Hudson Bay, which was crushed downward by the weight of ice during the last ice age, the ground is still rebounding upward. That change adds about one-400,000th of an ounce to the weight of a 150-pound person every year.



New York Times
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