Melt down isn't likely to happen, it is possible but they build in fail safes for stuff like this. But a 9.8 is REALLY hard to simulate in full scale. The fact that they made it this far is a sign of our species ability to design and build something.
As long as they can keep the core cool, and most of the time the extra fuel rods are stored in containers like this:
http://www.youtube.com/watch?v=N_JhruRobRI
For reasons such as this earthquake. They are used for transportation and from what I read a few years back they are often moved into similar housings on the facility and or the actual transport containers. When was the last time you saw something mess up the front of a train?
If they do melt down our impact in the states here will probably be the same as going tanning in a tanning both once more per year. But it's something to be said about the amount of radioactivity that will be in that country for years, decades or even a century or two to come if they have one or more melt downs. Say goodbye to nuclear power ever being built in the US now as well because of this scare.