Richard Gross, a geophysicist at NASA's Jet Propulsion Laboratory who studies the earth's rotation, explains.
Scientific American: Did the undersea earthquake affect the earth's rotation?
Models predict that the earthquake should have affected rotation of the earth by shortening the length of a day by about three microseconds, or three millionths of a second. This happens because during the earthquake one of the tectonic plates [the India plate] subducted down beneath another plate [the Burma plate]. The downward mass movement of the plate changed the earth's rotation just like a spinning ice skater bringing her arms closer to her body increases her rotation. When the earth spins faster, the days are shorter.
SA: Has this shift been measured?
This rotation change is a prediction from a model, and the data [collected by ground- and space-based position sensors] is being analyzed to see if the predicted change actually occurred. The data comes in every day, but it will take a few weeks for the most accurate data to be received and analyzed.
SA: Is this change permanent, or will it shift again?
The length of the day changes all the time in response to many different processes such as changes in the atmospheric winds or ocean currents. Changes in winds have by far the greatest effect on the length of the day: their effect is actually about 300 times larger than that predicted to be changed by this earthquake.
SA: Did the tilt of the earth's axis change as well?
The earth wobbles as it rotates because its mass is not balanced about its rotation axis, just like a tire on a car will wobble as it rotates if the tire is not perfectly balanced. The size of the planet's wobble is usually about 33 feet. As the India plate subducted beneath the Burma plate, the mass of Earth was rearranged, not only causing the speed of rotation to change, which causes the length of the day to change, but also causing the wobbling motion of the planet to change by about an inch. The wobble is also affected by other influences, such as changes in atmospheric pressure.