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Earthquakes vs. the Earth's RotationHow major tremors alter the planet's wobble.

By Sam Schechner
Posted Monday, Dec. 27, 2004, at 8:05 PM ET

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In covering the massive, tsunami-generating earthquake off the northwest coast of Sumatra this weekend, many news outlets picked up a statement from Enzo Boschi, head of Italy's National Institute of Geophysics, saying the temblor was strong enough to disturb the Earth's rotation. Can an earthquake really affect the way the planet spins on its axis?

Yep. As you'll recall from science class, the rotating Earth resembles a spinning top: The planet's axis does not always point in exactly the same direction but wobbles very slightly, describing small but measurable circles at the poles. A very large earthquake—one of a magnitude of 9.0 or greater—can shift enough mass relative to that of the entire Earth to alter, very minutely, the course of that wobble. But the planet's speed of rotation (which, of course, determines the lengths of our days) remains unchanged, so we don't need to worry about adjusting our watches.

In this case, the 9.0-magnitude shock was a "megathrust" quake, which occurs where one tectonic plate is forced beneath another. Initial U.S. Geological Survey data from the quake and its dozens of powerful aftershocks indicate that some 740 miles of the boundary between the India plate and the Burma plate slipped an average of 15 meters and that the sea floor thrust up several meters. It is difficult to determine the total mass of the crust that shifted because the movement was irregular, but when so much of the Earth moves so far, the wobble of its axis will jog slightly, too.

Geophysicists have still not calculated the exact effect of this earthquake on the wobble, but they will as data from the quake and aftershocks are plugged into complex mathematical models developed for that purpose in the 1950s and early '60s. Wobble was a hot topic back then because a series of powerful megathrust quakes—the 9.0 Kamchatka quake in 1952, the 9.1 Andreanof Islands quake in 1957, the 9.5 Chile quake in 1960, and the 9.2 Prince William Sound quake in 1964—provided a lot of data to work with.

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Update, Dec. 29, 2004: Explainer spoke too quickly; you may still need to adjust your watch early next millennium. According to Richard Gross of NASA's Jet Propulsion Laboratory, it's possible that the Earth's rotation did indeed speed up slightly as a large chunk of the crust fell toward the planet's core, just as a spinning figure skater speeds up when she pulls in her arms.

"I used a model of the elastic properties of the Earth along with the seismically determined source properties of the earthquake to compute the change in the distribution of the Earth's mass caused by the earthquake, and hence its effect on the Earth's rotation, including the change in the length of the day and in the Earth's wobble," Gross told Explainer in an e-mail. "This calculation predicts that the earthquake should have shortened the length of the day by about 2.7 microseconds, and caused the Earth to wobble by about another 1 inch."

Stuart Sipkin, a research geophysicist who has been studying the quake at the USGS's National Earthquake Information Center, doesn't dispute the calculation but urges caution until the model's projection can be confirmed with observed data. Unfortunately, that's not possible; according to Gross, current length-of-day measurement techniques are accurate to only 20 microseconds.

Explainer thanks Jim Devine at the U.S. Geological Survey.

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Sam Schechner is a writer in New York.
Photograph of the damaged house on Phi Phi Island by Surapan Boonthanom/Reuters.
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