Did the Chilean Quake Shift Earth's Axis?

Mar 11, 2010 by Dr. Tony Phillips
A USGS map of the Chilean quake.

Pictures of widespread devastation leave no doubt: Last month's 8.8 magnitude earthquake in coastal Chile was extremely strong. Indeed, say NASA scientists, it might have shifted the axis of Earth itself.

"According to our calculations, the quake moved Earth's figure axis by about 3 inches (8 cm)," says geophysicist Richard Gross of the Jet Propulsion Laboratory in Pasadena, California.

If the Earth tilted over 3 inches, you probably think you would have noticed. But that's not how the "figure axis" works. "The figure axis defines not how Earth is tilted, but rather how it is balanced," says Gross.

Consider the following:

Earth is not a perfect sphere. Continents and oceans are distributed unevenly around the planet. There's more land in the north, more water in the south, a great ocean in the west, and so on. As a result of these asymmetries, Earth slowly wobbles as it spins. The figure axis is Earth's axis of mass balance, and the spin axis wobbles around it.

"The Chilean quake shifted enough material to change the mass balance of our entire planet," Gross says.

A shifting figure axis is nothing new. On its own, the figure axis moves about 10 centimeters per year as a result of "Ice Age rebound." After the last great some 11,000 years ago, many heavy ice sheets disappeared. 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 underway and so the figure axis naturally moves.
On Feb. 27, 2010, the Chilean quake may have moved the figure axis as much in a matter of minutes as it normally moves in a whole year. It was a truly seismic shift—no pun intended.

So far, however, it's all calculation and speculation. "We haven't actually measured the shift," says Gross. "But I intend to give it a try."

The key is GPS1. "Using a global network of GPS receivers, we can monitor the rotation of Earth with high precision," he says. "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."

GPS is already used to monitor seasonal changes in Earth's spin. It turns out that tides, winds, ocean currents, and circulation patterns in Earth's molten core modulate Earth's rotation on a regular basis. For instance, a typical day in January is about 1 millisecond longer than a typical day in June. The roughly six-month variation is driven mainly by seasonal winds; there are also changes on time scales of weeks, years, decades and centuries.

Observed changes in Earth's length of day caused by tides, winds, ocean currents and other factors. From Treatise on Geophysics, 2007, section 3.09, "Earth Rotation Variations--Long Period" by Richard Gross.

Earthquakes throw a "spike" into GPS signals, which Gross believes he can find.

"I have to take the GPS Earth rotation measurements and subtract the effects of tides, winds and ocean currents," he explains. "Then the earthquake should stand out."

Recent news reports have focused on Earth's length of day, noting that the Chilean earthquake might have shortened days by as much as 1.26 microseconds out of 24 hours. That's true. But it's also negligible compared to the normal effect of wind and tides, which can lengthen or shorten days a thousand times more than earthquakes can.

The real news, as Gross sees it, is the possible shift in Earth's figure axis. He has a very "JPL perspective" on the issue: "The antennas we use to track spacecraft en route to Mars and elsewhere are located on . If our tracking platform shifts, we need to know about it."

The normal wobble of Earth's axis since Jan. 2009 as reported by the International Earth Rotation Service. The grid is scaled in milliarcseconds (mas); 1 mas = 1/3,600,000 deg.

No one has ever measured a shift in Earth's axis due to an earthquake before. Back in 2004, Gross looked for a shift from the magnitude 9.1 in Sumatra, but failed to find a signal. The Sumatra quake was less effective in altering Earth's figure axis because of its location near the equator and the orientation of the underlying fault. The Chilean quake, albeit weaker, may have produced a bigger shift.

The stage is set for discovery. "Computing power is at an all-time high. Our models of tides, winds and ocean currents have never been better. And the orientation of the Chilean fault favors a stronger signal."

In a few months Gross hopes to have the answer. Stay tuned.

Explore further: Magnitude-7.2 earthquake shakes Mexican capital

Source: Science@NASA, by Dr. Tony Phillips

4.6 /5 (16 votes)

Related Stories

Probing Question: Why does the Earth rotate?

Aug 06, 2007

We spend our lives on a spinning globe -- it takes only 24 hours to notice that, as night follows day and the cycle repeats. But what causes Earth to rotate on its axis?

Earth's orbit creates more than a leap year

Feb 08, 2008

The Earth's orbital behaviors are responsible for more than just presenting us with a leap year every four years. According to Michael E. Wysession, Ph.D., associate professor of earth and planetary sciences ...

Recommended for you

Magnitude-7.2 earthquake shakes Mexican capital

Apr 18, 2014

A powerful magnitude-7.2 earthquake shook central and southern Mexico on Friday, sending panicked people into the streets. Some walls cracked and fell, but there were no reports of major damage or casualties.

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

nuge
not rated yet Mar 12, 2010
What's the big deal? If some ant walks 5cm east then the figure axis of the Earth will have changed from that too. This particular Earthquake just happened to change it by a lot. So what?
gwrede
1 / 5 (1) Mar 12, 2010
Exactly. The title is truly misleading. It should be something like "Can we already measure axis and day length shifts as small as those from earthquakes?"

Which reminds me, as a child I once was at an airport in Lapland, and there was this brass line across the lounge floor, with the text Arctic Circle. I wondered whether that was just BS, since I'd expect the circle to move every year.

The latter graph, above, proves the case.

More news stories

China says massive area of its soil polluted

A huge area of China's soil covering more than twice the size of Spain is estimated to be polluted, the government said Thursday, announcing findings of a survey previously kept secret.

UN weather agency warns of 'El Nino' this year

The UN weather agency Tuesday warned there was a good chance of an "El Nino" climate phenomenon in the Pacific Ocean this year, bringing droughts and heavy rainfall to the rest of the world.

Poll: Big Bang a big question for most Americans

Few Americans question that smoking causes cancer. But they have more skepticism than confidence in global warming, the age of the Earth and evolution and have the most trouble believing a Big Bang created the universe 13.8 ...

Making graphene in your kitchen

Graphene has been touted as a wonder material—the world's thinnest substance, but super-strong. Now scientists say it is so easy to make you could produce some in your kitchen.