Explainer

# Earthquakes vs. the Earth’s Rotation

## How major tremors alter the planet’s wobble.

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.