Running out of time?

NASA scientists say Chilean earthquake shortened day by fraction of a second.

March 4, 2010 04:19
3 minute read.
Firemen work on a destroyed building in Concepcion

Chile earthquake cool night buildings falling over 311 ap. (photo credit: ASSOCIATED PRESS)


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If during the last five days you have been unable to get things done in time, it may have been due to Saturday’s earthquake in Chile shifting the Earth’s axis and shortening the day by 1.26 millionths of a second.

Maybe you didn’t notice. But Dr. Richard Gross, a geophysicist at the US National Aeronautics and Space Administration’s Jet Propulsion Laboratory in Pasadena, California, fiddled with computer models and said the subterranean tectonic shifts caused the axis to move by about 7 centimeters, thus shortening the day by one-and-a-quarter microseconds.

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Quakes can move hundreds of kilometers of rock by a few meters, thus altering the distribution of mass on Earth. The change is too minute to detect physically and is based only on models, however.

The shortening of the day would not affect the behavior of animals or humans, assured Prof. Nir Shaviv of the Hebrew University’s Rekah Institute of Physics. He told The Jerusalem Post on Wednesday that in fact, there are various influences on the Earth’s axis that have changed the length of the day over eons.

About 500 million years ago, he said, the day was only 22 hours in length; since then, it has grown by about two hours to 86,400 seconds. This fact, he said, was determined by sediment analysis.

Some influences on the axis and length of the day are seasonal, said Shaviv, while others are long-term. For example, during the winter, when more ice forms at the poles, the day becomes slightly shorter as the weight slows the Earth’s rotation on its axis.

“But the changes in our lifetimes are very small,” he added.

In 2004, the earthquake in the Indonesian island of Sumatra – 9.1 on the Richter scale, compared to 8.8 in Chile on Saturday – caused India to push into Tibet and raise the Himalayan mountains a bit, thus causing the Earth to rotate a bit slower, he said.

But the strongest effect of the speed of rotation is the moon’s effect on the Earth over many millions of years. This causes the ocean tides to rise and fall, causing a loss of energy. The moon is moving away from the Earth by a few centimeters every few years, making the tides less strong and the rotation slower, thus making the day longer.

The effect of earthquakes depends on how far they occur from the equator, said Shaviv, but in general, these have little affect on the length of the day. Nuclear tests have an even more negligible effect, he added.

Gross calculated that the quake in Chile should have moved Earth’s figure axis (the axis about which Earth’s mass is balanced) by about 8 centimeters. By comparison, he calculated, using the same model, that the Sumatran earthquake should have shortened the day by 6.8 microseconds and shifted Earth’s axis by 7 centimeters.

The NASA scientist said that even though the Chilean earthquake was much smaller than the Sumatran quake, it is predicted to have changed the position of the figure axis by a bit more for two reasons. First, unlike the 2004 Sumatran earthquake, which was located near the equator, last week’s Chilean earthquake was located in Earth’s mid-latitudes, which makes it more effective in shifting Earth’s figure axis.

Second, the fault responsible for the 2010 Chilean earthquake dips into Earth at a slightly steeper angle than does the fault responsible for the 2004 earthquake. This makes the Chile fault more effective in moving Earth’s mass vertically and thus more effective in shifting Earth’s figure axis, Gross concluded.

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