Quake Moves Japan Closer to U.S. and Alters Earth’s Spin

The magnitude-8.9 earthquake that struck northern Japan on March 11 not only violently shook the ground and generated a devastating tsunami, it also moved the coastline and changed the balance of the planet.

Global positioning stations closest to the epicenter jumped eastward by up to 13 feet. Not all of Japan jumped 13 feet closer to the United States, said Kenneth W. Hudnut, a geophysicist with the United States Geological Survey. The shifts occurred mostly in the area closest to the epicenter, and stations farther away reported much less movement. On a larger scale, the unbuckling and shifting moved the planet’s mass, on average, closer to its center, and just as a figure skater who spins faster when drawing the arms closer, the Earth’s rotation speeds up. Richard S. Gross, a scientist at NASA’s Jet Propulsion Laboratory, calculated that the length of the day was shortened by 1.8 millionths of a second. The earthquake also shifted the so-called figure axis of the Earth, which is the axis that the Earth’s mass is balanced around. Dr. Gross said his calculations indicated a shift of 6.5 inches in where the figure axis intersects the surface of the planet. That figure axis is near, but does not quite align with, the rotational axis that the Earth spins around. Earlier great earthquakes also changed the axis and shortened the day. The magnitude-8.8 earthquake in Chile last year shortened the day by 1.26 millionths of a second and moved the axis by about three inches, while the Sumatra earthquake in 2004 shortened the day by 6.8 millionths of a second, Dr. Gross said.