Using a complex model to perform a theoretical calculation based on a U.S. Geological Survey, Richard Gross of NASA's Jet Propulsion Laboratory (JPL) has determined that by changing the distribution of the Earth's mass, the earthquake that devastated Japan last Friday should have sped up the Earth's rotation, resulting in a day that is about 1.8 microseconds (1.8 millionths of a second) shorter.

The calculations, which will likely change as the data on the Japan quake is further refined, have also been used to examine the effects of other recent quakes. Gross estimated that last year's 8.8 earthquake in Chile shortened the length of a day by about 1.26 microseconds, while similar calculations revealed the 9.1 magnitude Sumatran quake of 2004 shortened the day by 6.8 microseconds. Just how much an earthquake affects the Earth's rotation depends on the magnitude of the quake, its locations and details of how the fault slipped.

Gross's calculations also indicate the Japan quake should have shifted the position of the Earth's figure axis by about 17 cm (6.69 in), towards 133 degrees east longitude. Not to be confused with the Earth's north-south axis, the figure axis is that about which the Earth's mass is balanced. While the slight shift will cause the Earth to wobble a bit differently as it rotates, it won't cause a shift of Earth's axis in space, which can only be affected by external forces such as the gravitational pull of the sun, moon or planets.

Gross points out that the changes to the Earth's rotation and shift of its axis aren't anything to be worried about. "Earth's rotation changes all the time as a result of not only earthquakes, but also the much larger effects of changes in atmospheric winds and oceanic currents," he said. "Over the course of a year, the length of the day increases and decreases by about a millisecond, or about 550 times larger than the change caused by the Japanese earthquake. The position of Earth's figure axis also changes all the time, by about one meter (3.3 feet) over the course of a year, or about six times more than the change that should have been caused by the Japan quake."

Although scientists are able to measure the larger effects of the atmosphere and ocean on the Earth's rotation, the changes due to earthquakes have been too small to measure as the computed change in the length of a day caused by earthquakes is much smaller than the accuracy with which scientists can currently measure changes in the length of a day.

However, the effects from the 9.0 magnitude Japan quake, which is the fifth largest since 1900, may actually be large enough for scientists to observe. This is because the position of the figure axis can be measured to an accuracy of about five cm (two inches), so the 17 cm shift from the Japan quake may be observable if the scientists can adequately remove the larger effects of the atmosphere and ocean from the equation.