The move Monday comes after Japan's Meteorological Agency did the same. It's not unusual for scientists to tweak the magnitude of a giant quake after some number-crunching.
U.S. government scientists originally put the Japan quake at 8.9. The change to 9.0 means that the quake was about 1.5 times stronger than initially thought.
The Japan quake is now the fourth largest in the world since 1900 behind the 2004 magnitude-9.1 Sumatra quake.