9.0

When the news first broke of the Japanese earthquake the preliminary magnitude was given at 7.9.  I remember thinking "wow, that was a big one".  Then within a few minutes the magnitude jumped to 8.8.  Then 8.9.  And now 9.0:

The USGS has updated the magnitude of the March 11, 2011, Tohoku earthquake in northern Honshu, Japan, to 9.0 from the previous estimate of 8.9. Independently, Japanese seismologists have also updated their estimate of the earthquake’s magnitude to 9.0. This magnitude places the earthquake as the fourth largest in the world since 1900 and the largest in Japan since modern instrumental recordings began 130 years ago.

That means that within a few days an 8.0 aftershock is not out of the question. And many aftershocks in the 7+ range could occur.

And dozens and dozens of 5's and 6's. Japan will be on the move for some time.

These kind of corrections are not unusual.  As more and more data becomes available from around the world the scientists often update their figures.  And by the way, that move from 8.9 to 9.0 means the quake was about 1.5 times stronger than originally thought.