Richard A. Lovett
for National Geographic News
Published March 15, 2011
The magnitude 9.0 earthquake that struck Japan last Friday was powerful enough to shorten Earth’s day by 1.8 microseconds and throw an extra 6.7 inches (17 centimeters) into the planet’s wobble, scientists say.
That doesn’t mean shockwaves from the event somehow knocked Earth off its north-south axis, around which the planet revolves.
Instead the quake shifted what’s called Earth’s figure axis, an imaginary line around which the world’s mass is balanced, about 33 feet (10 meters) from the north-south axis.
Earth naturally wobbles slightly as it spins, because shifting surface mass such as melting glaciers and moving ocean currents can throw the planet off balance.
Data from high-precision GPS instruments show that parts of Japan shifted by as much as 13 feet (4 meters) as the fault plates lurched due to the earthquake. This allowed scientists to calculate how much Earth’s overall mass distribution had shifted and thus how much the wobble was affected.
The shifting mass also affected the planet’s spin rate, according to geophysicist Richard Gross, of NASA’s Jet Propulsion Laboratory in Pasadena, California. He compares what happened to a figure skater pulling her arms closer to her body, causing her to spin faster.
Because Earth is big, the effect is tiny—a microsecond is only a millionth of a second. For most of us, Gross said, “it has no real practical consequence.”
Researchers are more intrigued by the change in Earth’s wobble, which could inform future space missions, and the data collected on small earthquakes leading up to the main event, which may help with earthquake prediction.
Japan Earthquake Data “Unbelievably Good”
Similar changes to Earth’s mass distribution were calculated from GPS data obtained during the 2004 Sumatra earthquake and the 2010 Chile earthquake. In the case of Sumatra, the change in the length of the day was larger: 6.8 microseconds.
But for the Japan earthquake, the change in Earth’s wobble was more than twice as large as those calculated for the 2004 and 2010 events.
That’s exciting, Gross said, because the wobble is large enough that scientists might actually be able to measure it, not just calculate it, by looking for small changes to Earth’s tilt.
Still, since other factors also redistribute mass in the form of air and water, random changes to Earth’s wobble might mask the effect of the earthquake.
Other geophysicists say that there are even more exciting things that can be done with the GPS data from the Japan earthquake.
For example, Japanese GPS instruments, strain meters, and seismometers recorded dozens of smaller quakes leading up to the main event, said Ken Hudnut, a geophysicist at the U.S. Geological Survey’s Pasadena office.
Scientists poring over the data will be trying to figure out if there was anything unusual in the prior earthquakes that might have indicated they were foreshocks to a bigger event, rather than more ordinary tectonic rumblings.
“The question is, did the GPS or strain meters show a precursor,” Hudnut said. “Because if they did, it will revolutionize earthquake research forever.”
In Sumatra, Hudnut added, the GPS data was “pretty good.” In Chile it was “much better,” and for Japan, the positioning and nature of the instruments made the data “unbelievably good.”
“We may not get another data set like this until I don’t know when. Here, we have a monster earthquake not too far offshore, and GPS instruments along the coast,” he said.
“So much of what scientists do is about getting the right instruments in the right position to record some natural phenomenon, so we can understand it better. If there was anything precursory associated with those foreshocks, it should have been seen on that array of instruments.”
Earth’s wobble calculations aren’t relevant to this particular quest, NASA’s Gross said, but they are more than just a curiosity. Understanding Earth’s spin movement is critical in space launches, for instance.
“When we navigate spacecraft to land a rover on Mars, we have to be able to account for changes in the Earth’s rotation in order to [launch so that we] precisely land the rover where we want to,” he said. “If we didn’t, we might miss Mars altogether.”