Not to be a fiddly bitch, but...

Kansai airport is nowhere near Tokyo. Kansai is named so because it is the major airport for Kansai prefecture - the prefecture Osaka is located in, several hours train ride to the southwest of Tokyo. When I went to Japan in '01, I flew into Narita and flew out of Kansai. It's possible that a massive quake in Kanto (like high 8's or low 9's minimum) could damage Kansai international, but there are plenty airports closer to Tokyo than Kansai that it would kind of be a non-issue.

Is the idea you pose that a third airport was constructed somewhere in the Tokyo area (maybe down towards Yokohama, a fair distance away but still able to serve Tokyo) in the same way that Kansai was constructed; ie. on a manmade island? Call it Kanto or something like that and you'd still be good, I would think.

Kansai IS absolutely gorgeous, btw.