Wednesday, March 3, 2010

Chile temblor tilts Earth

Martin Bernetti/AFP/Getty Images - Firefighters searched for victims in the debris of a house in Curanipe.


Temblor Tilts Earth by Inches


The earthquake that struck Chile was so powerful it shifted the planet's axis enough to make it spin slightly faster, meaning our days will be shorter by 1.26 millionths of a second, according to preliminary calculations by scientists at the National Aeronautics and Space Administration.

"This is an esoteric effect that physics says has to happen," notes David Kerridge, the British Geological Survey's head of natural hazards, who studies earthquakes. "It's interesting, but it has no particular consequence on anything."

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Scientists have long noted that just about any event that shifts a large amount of mass from one part of the planet to another will have a tiny—and sometimes measurable—effect on the Earth's rotation. Such events include changes to ocean currents, big shifts in the atmosphere, earthquakes, and possibly even the creation of more and more reservoirs from the damming of rivers.

The 8.8-magnitude temblor that struck Chile on Saturday is one of the largest quakes in a century. It was the result of an ocean tectonic plate—a shard of the earth's crust—sliding under the South American plate; over time, the two became locked together. When the pent-up energy overcame the forces of friction and the South American plate sprang upward, it unleashed a huge amount of energy in the form of the quake.

The planet was jolted to its roots. A chunk of the Earth's mass was redistributed vertically, which caused the planet's figure axis—on which the Earth's mass is balanced—to move by about three inches, according to calculations by NASA scientist Richard Gross. The net effect of that mass redistribution made the earth spin slightly faster, just as a figure skater speeds up when she pulls in her arms.

"It's important for us to know how the earth's rotation changes," said Dr. Gross, of NASA's Jet Propulsion Laboratory. "It helps us figure out where a spacecraft is and to navigate it for a precise pinpoint landing" on Mars, the moon or another planet. Dr. Gross and his colleagues had conducted a similar analysis following the even bigger 9.1-magnitude quake in Indonesia. They found that the 2004 temblor decreased the length of a day by 6.8 microseconds and shifted the North Pole by a few centimeters.

For a long time, earthquakes have been gradually changing the shape of the earth as well. Calculations suggested that the earth's oblateness—the way it is flat on top and bulges at the equator—had decreased by a tiny amount following the Indonesian earthquake. In other words, the world is getting slightly rounder.

Dr. Gross said that though the Chilean quake was less powerful than the 2004 Indonesia temblor, it likely changed the position of the figure axis a bit more. (The planet's figure axis is separate from the north-south axis; they are about 33 feet apart.)

The vertical redistribution of mass caused by the Chilean quake had a slightly greater impact in shifting the figure axis because the quake happened near the mid-latitudes, NASA concludes. Plus, the fault in Chile dips into Earth at a slightly steeper angle, which again has a greater effect on shifting the figure axis.

The Haiti quake, by contrast, didn't make the Earth wobble in quite the same way, according to Dr. Kerridge. The Haiti quake was caused by one tectonic plate sliding past another, not above or under it. "There was almost no vertical movement there when the quake happened, so you wouldn't expect the same effect," says Dr. Kerridge.

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