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Japan's 9.0 Magnitude Quake Truly Moved the World March 18, 2011
Quake Map of Japan
Japan’s 9.0-magnitude quake on March 11 moved the country’s coast about 8 feet and shifted the Earth on its figure axis by about 4 inches toward 133 degrees east longitude.

While this did not change the planet’s north-south axis — only external forces like the gravitational forces of other heavenly bodies can do that — it will cause the planet to wobble a big differently as it rotates.

The massive seismic thrust, the fifth-largest since 1900, also affected Earth’s rotation.

NASA says the distribution of Earth’s mass was changed, causing the planet to rotate a bit faster and thus shortening the length of day by about 1.8 microseconds. That’s about 2 millionths of a second.

In comparison, last year's magnitude 8.8 earthquake in Chile is believed to have shortened the length of day by about 1.26 microseconds and shifted Earth's figure axis by about 3 inches.

A calculation performed after the 2004 magnitude 9.1 Sumatran earthquake revealed it should have shortened the length of day by 6.8 microseconds and shifted Earth's figure axis by about 2.76 inches.

How an individual earthquake affects Earth's rotation depends on its magnitude, location and various aspects of how the surface slipped.