Richard Gross, a geophysicist with NASA (news - web sites)'s Jet Propulsion Laboratory in California, theorized that a shift of mass toward the Earth's center during the quake on Sunday caused the planet to spin 3 microseconds, or one millionth of a second, faster and to tilt about an inch (2.5 cm) on its axis.

When one huge tectonic plate beneath the Indian Ocean was forced below the edge of another "it had the effect of making the Earth more compact and spinning faster," Gross said.

Gross said changes predicted by his model probably are too minuscule to be detected by a global positioning satellite network that routinely measures changes in Earth's spin, but said the data may reveal a slight wobble.

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The Earth's poles travel a circular path that normally varies by about 33 feet, so an added wobble of an inch (2.5 cm) is unlikely to cause long-term effects, he said.

"That continual motion is just used to changing," Gross said. "The rotation is not actually that precise. The Earth does slow down and change its rate of rotation."

When those tiny variations accumulate, planetary scientists must add a "leap second" to the end of a year, something that has not been done in many years, Gross said.

Scientists have long theorized that changes on the Earth's surface such as tide and groundwater shifts and weather could affect its spin but they have not had precise measurements to prove it, Caltech seismologist Hiroo Kanamori said.

"Even for a very large event, the effect is very small," Kanamori said. "It's very difficult to change the rotation rate substantially."