The March 11, magnitude 9.0 earthquake in Japan may have shortened the length of each Earth day and shifted its axis. Using a United States Geological Survey estimate for how the fault responsible for the earthquake slipped, research scientist Richard Gross of NASA's Jet Propulsion Laboratory, Pasadena, Calif., applied a complex model to perform a preliminary theoretical calculation of how the Japan earthquake-the fifth largest since 1900-affected Earth's rotation. His calculations indicate that by changing the distribution of Earth's mass, the Japanese earthquake should have caused Earth to rotate a bit faster, shortening the length of the day by about 1.8 microseconds (a microsecond is one millionth of a second). There are about 86,400 seconds (86 billion microseconds) in a day, so the impact of the earthquake is quite small. The calculations also show the Japan quake should have shifted the position of Earth's figure axis (the axis about which Earth's mass is balanced) by about 6.5 inches, towards 133 degrees east longitude. The Earth's figure axis is not the same as its north-south axis in space, which it spins around once every day at a speed of about 1,000 mph. The figure axis is the axis around which the Earth's mass is balanced.In comparison, following last year's magnitude 8.8 earthquake in Chile, Gross estimated the Chile quake should have shortened the length of day by about 1.26 microseconds and shifted Earth's figure axis by about 3 inches. A similar 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 7 centimeters, or 2.76 inches. How an individual earthquake affects Earth's rotation depends on its size (magnitude), location and the details of how the fault slipped. There are many other factors that affect the length of the day. For example tidal friction (caused by the Moon)slowly increases the length of the day over millenia. Earth's rotation period relative to the Sun (true noon to true noon) is its true solar day or apparent solar day. It depends on the Earth's orbital motion and is thus affected by changes in the eccentricity and inclination of Earth's orbit. Both vary over thousands of years so the annual variation of the true solar day also varies. Generally, it is longer than the mean solar day during two periods of the year and shorter during another two periods. Gross said that, in theory, anything that redistributes Earth's mass will change Earth's rotation. "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 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." In some ways the Japan earthquake is part of the more general phenomena known as continental drift which is typically measured in centimeters per year. However, the drift rate varies and the earthquake in Japan is part of the same tectonic process that moves the continents. Some scientists have determined that the temblor's force may have moved parts of eastern Japan as much as 12 feet closer to North America and that Japan may have shifted downward about two feet.