Richard A. Lovett
for National Geographic News

The magnitude 9.0 earthquake that struck Japan on March 11 was powerful enough to shorten Earth's day by 1.8 microseconds and throw an extra 6.7 inches (17 centimeters) into the planet's wobble, scientists say.
That doesn't mean shockwaves from the event somehow knocked Earth off its north-south axis, around which the planet revolves.
Instead the quake shifted what's called Earth's figure axis, an imaginary line around which the world's mass is balanced, about 33 feet (10 meters) from the north-south axis.
Earth naturally wobbles slightly as it spins, because shifting surface mass such as melting glaciers and moving ocean currents can throw the planet off balance.
Data from high-precision GPS instruments show that parts of Japan shifted by as much as 13 feet (4 meters) as the fault plates lurched due to the earthquake. This allowed scientists to calculate how much Earth's overall mass distribution had shifted and thus how much the wobble was affected.

http://news.nationalgeographic.com/news/2011/03/110316-japan-earthquake-shortened-days-earth-axis-spin-nasa-science/