Chilean Quake Has Effects on Movement of Earth
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The Feb. 27 earthquake in Chile has not only caused the Earth to spin faster but caused it to wobble differently as well, a US scientist said Saturday.
Richard Gross of NASA's Jet Propulsion Laboratory said he had computed the changes the massive 8.8-magnitude quake had caused to the movement of the Earth.
The California-based scientist had posted his calculation on the NASA (National Aeronautics and Space Administration) website.
"The Earth spins faster just like an ice skater spins faster as she moves her arms closer to her body," Gross told Xinhua on Saturday.
"The same thing happened as a result of the earthquake. The earthquake caused the Earth's mass to be rearranged in such a way that, on average, the Earth's mass is a bit closer to its rotation axis, so the Earth is rotating a bit faster."
The earthquake has also caused the Earth to wobble a bit differently, he said.
"The Earth rotates about its rotation axis, but the mass of the Earth is balanced about a different axis, its figure axis," explained the scientist.
He said because the Earth is not rotating around its figure axis, it wobbles as it rotates, just as the tire on the car will vibrate or wobble if the tire is not perfectly balanced.
"By balancing the tire on your car, you are making the rotation axis be the same as the figure axis of the tire. In this way, the tire rotates smoothly. But for the Earth, the rotation and figure axes are different so the Earth does not rotate smoothly but wobbles as it rotates," Gross said.
The scientist explained that now the Earth's wobble is a bit different from what it used to be in that the Chile temblor had rearranged the Earth's mass only to change its figure axis which in turn has affected the way the planet wobbles.
"I used seismic estimates for the slip on the fault caused by the earthquake and computed the resulting change in the arrangement of the Earth's mass and hence the change in the Earth's rotation," said Gross.
But Gross noted since the change in the Earth's rotation caused by the Chilean earthquake is so small that it will have no noticeable effects.
The Earth's rotation is constantly changing, mainly in response to changes in the strength and direction of atmospheric winds and oceanic currents.
"In fact, the changes caused by the winds and currents are about 800 times larger than the changes caused by the earthquakes. So the earthquake-induced changes are really quite small and will have no noticeable effects," said Gross.
NASA quoted Gross as saying on March 1 that the Chile quake should have shortened the length of an Earth day by about 1.26 microseconds. A microsecond is one millionth of a second.
According to Gross' calculation, the Chile earthquake should have moved Earth's figure axis by 2.7 milliarcseconds (about 8 centimeters, or 3 inches).
The Earth's figure axis is offset by about 10 meters with its north-south axis.
"The Chilean quake shifted enough material to change the mass balance of our entire planet," Gross said.
Researches explained that a shifting of the figure axis is nothing new.
On its own, the figure axis moves about 10 centimeters per year as a result of the "Ice Age rebound."
After the last great glacial period some 11,000 years ago, many heavy ice sheets disappeared and this unloaded the crust and mantle of the Earth, allowing the planet to relax or "rebound" back into a more spherical shape. The rebounding process is still under way and so the figure axis naturally moves all the time.
But the Chilean quake may have moved the figure axis as much in a matter of minutes as it normally moves in a whole year.
The changes, however, were still the result of calculation and speculation.
"We haven't actually measured the shift," said Gross. "But I intend to give it a try."
He said the key is GPS.
"Using a global network of GPS receivers, we can monitor the rotation of the Earth with high precision."
"Changes in Earth's spin and the orientation of Earth's axes affect (the phase and timing of) signals we get from the satellites in Earth orbit," he added.
"I have to take the GPS Earth rotation measurements and subtract the effects of tides, winds and ocean currents," he explained. "Then the (change of) earthquake should stand out."
(Xinhua News Agency March 14, 2010)