Tsunami-Causing Earthquake Trimmed Bulge off Earth's Middle
The Dec. 26 earthquake off the coast of Indonesia was the fourth largest in one hundred years.? Scientists have determined that this major shift in the Earth's plates changed the planet's shape - enough to shorten the day by fractions of a second and to shift the North Pole by an inch.
The general shape of the Earth is slightly oblate - that is, it is not a perfect sphere but is slightly squished down, making it about 26 miles wider at the equator than between the poles.? This shape, however, is not rigid, with climate being a major distorting force.??
But the magnitude nine earthquake last month almost certainly altered the shape as well.? Recent calculations have estimated that this catastrophic land displacement caused a small reduction in the bulge, making the planet more round.
"The waistline was reduced by not quite a millimeter because of the earthquake," said Benjamin Fong Chao from NASA's Goddard Space Flight Center.
This slimming down sped up the rotation of the Earth, much like when a spinning ice skater pulls in her arms to increase her speed.? The length of the day correspondingly decreased by 2.68 millionths of a second.?
No watches need to be changed because of this.? In fact, Chao told LiveScience in a telephone interview that this change is too small for current detection methods.? But, he said, the change in the Earth's shape and pole location might be observable, once all the relevant data is reviewed.
Chao and his colleague, Richard Gross of NASA's Jet Propulsion Laboratory, have analyzed seismological data from 20,000 earthquakes with magnitudes greater than five.? They have modeled how each has affected the shape of the Earth, and subsequently the rotation.?
In two thirds of the cases, the planet has become less oblate, or skinnier, following a temblor.? The other one third of the time, the planet has become more oblate.??
None of these changes, however, were big enough to directly measure - until maybe now.
"The Earth got hit pretty hard on December 26," Chao said.? "For the first time, we hope to see the effect of an earthquake, but it will take a couple of months to sort through the data."?
The Earth's profile can be measured with satellite laser ranging (SLR).? By precisely tracking the orbit of a satellite, scientists can infer the gravitational pull from that part of Earth below the satellite.?
"We measure the gravity change, and from that, we infer the shape change," said Minkang Cheng from the University of Texas at Austin.
Seeing the effect of the earthquake, though, will be difficult because there are other processes that cause greater distortions.?
Cheng and his co-worker, Byron D. Tapley, have analyzed 28 years of SLR data, identifying several cyclical patterns in the variations of the Earth's oblateness, which they have correlated with weather and climate changes.
The amplitudes of these cycles are 10 times bigger than the expected change from the earthquake.
"There's no question that a signal [from the earthquake] is there in the satellites, but it is very tough to separate this from the climate-induced signal," Tapley said.
The Weight of Water
One of the largest variations in the SLR data is a seasonal "breathing" in and out.? On average, the Earth's shape fluctuates by 2.38 centimeters over the course of a year, Cheng said.?
The cause of this annual shift is the redistribution of water.? Evaporation over the ocean leads to precipitation over land, which eventually makes its way back to the ocean.? Major climate changes can alter this water cycle.
"The magnitude of that mass transport of water varies from year to year."? Tapley said.? "The change is very dramatic in El Nino years."??
Cheng and Tapley found that every 4-6 years the Earth becomes slightly more oblate - by about 0.7 centimeters - due to the redistribution of precipitation caused by an El Nino event.?
"Essentially, El Nino puts more moisture in the lower latitudes," Tapley said.
Tapley and Cheng plan to look at how other climate changes - specifically global warming - may affect the Earth's shape in their next paper.
The Weight of Ice
There is one other interesting variation in the SLR data due to the effects of the last ice age, called the postglacial rebound (PGR).?
Ten thousand years ago, when parts of the continents were covered in ice, that weight deformed the earth by squeezing down on the poles.? When the ice melted, the land did not immediately pop back into place but is even now still recovering.?
That recovery has the Earth becoming less and less oblate - at a rate of about a tenth of a centimeter per year, according to Cheng.?
Chao said that the PGR also shifts the direction of the North Pole by about four inches a year.? Since this is only four times bigger than the expected shift from the earthquake, Chao believes this is the best hope for measuring the earthquake's effect.
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