It should be noted that the Richter Scale is logarithmic. Each point increase represents a 10x stronger quake. So, the Japanese quake at 8.9 is nearly 100x stronger than the 7.0 that the California plant is designed for. I hope they picked the right number!
The relationship between wave energy and peak destructive force is not linear. That is why the volume control on your sound system is logarithmic too.