Chile earthquake: Shock effect on Earth's axis

The earthquake that struck Chile on Saturday may have shifted the Earth's axis and created shorter days, according to scientists at Nasa. Richard Gross, a geophysicist at Nasa's Jet Propulsion Laboratory in Pasadena, California, said the 8.8 magnitude quake could have moved the Earth's axis by 2.7 milliarcseconds (about 8cm) – enough to shorten a day by about 1.26 microseconds.

A large quake can shift huge amounts of rock and alter the distribution of mass on the planet. When that distribution changes, it changes the rate at which the planet rotates, which determines the length of a day.

"The length of the day should have got shorter by 1.26 microseconds," Gross told the Bloomberg news agency. "The axis about which the Earth's mass is balanced should have moved by 2.7 milliarcseconds."

Gross previously used the technique to estimate the shift caused by the 2004 Sumatran quake that caused the Indian Ocean tsunami. That 9.1 magnitude quake shifted the Earth's axis by 2.3 milliarcseconds and shortened a day by 6.8 microseconds.

David Kerridge, a seismologist with the British Geological Survey, said the Chile and Sumatra earthquakes were based on subduction, in which one tectonic plate slides under another, redistributing the Earth's overall mass. The effect was similar to that for an ice dancer who moved their arms in and out to accelerate and slow their spin.

"As the ice skater puts when she's going around in a circle, and she pulls her arms in, she gets faster and faster. It's the same idea with the Earth going around if you change the distribution of mass, the rotation rate changes."

Earthquakes caused by plates sliding past each other, such as the recent event in Haiti, do not have the same impact on the Earth's rotation.

Gross said the Chilean earthquake shifted the Earth's axis a greater distance than the larger Sumatran event because it was further from the equator. The fault that caused the Chilean quake also dips into the Earth at a steeper angle, which meant it moved more mass.