Sunday, January 09, 2005

The effect of Eartquakes on the Earth Rotation

There was discussion at some point about the effect of the latest earthquake that hit South East Asia on the Earth's Axis of rotation. The fact is that large earthquakes of magnitude of 9 and up induce a displacement of the mass important enough to affect the speed at which the earth rotates around itself. A displacement from the center to the surface will induce a slower rotation, while a displacement from the surface to the center will induce a faster rotation. The latest Earthquake that hit South East Asia induced a displacement from the surface to the center, which in theory would have induced a slightly faster rotation. According to one study this would have made our days 2 microseconds shorter. Nothing to worry about.

Another effect of this huge displacement of matter is the change in the position of the axis of rotation of the Earth. According to the same study this would be of the order of an inch, or two and a half centimeters.

For more on this, here is an interesting article.

Earthquakes vs. The Earth's Rotation

How major tremors alter the planet's wobble.
By Sam Schechner
Posted Monday, Dec. 27, 2004, at 5:05 PM PT

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...


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