PASADENA, Calif. — The earthquake that struck beneath the Indian Ocean off Indonesia's Sumatra island on Dec. 26 slightly sped Earth's rotation, shortening the length of a day by about 2.6 millionths of a second, according to a computer model developed at the Jet Propulsion Laboratory.

Earth's rotational speed changes depending on how mass moves on the planet's surface. Mass can be moved by shifting winds, ocean currents — and earthquakes. When the Indian Ocean's tectonic plate lurched beneath Indonesia's plate, mass shifted toward the planet's center.

"This is analogous to a spinning ice skater bringing her arms closer to her body. She gets more compact and spins faster," said JPL geophysicist Richard Gross, who created the model.

Because Global Positioning System readings that measure the planet's rotation are accurate only to 20 millionths of a second, the shift is not likely to show up in data, Gross said.

Any changes caused by the quake are overshadowed by changes in winds and currents that can slow or speed Earth up to a thousandth of a second — hundreds of times more than the quake.

Gross' model also shows that the quake caused the planet to tilt by an extra inch, a fraction of the 33 feet that the Earth wobbles on its axis during each year of rotation.