I wrote about this issue the other day. My message was about how a major eruption of Mt Aso might spread volcanic ash over a very wide area:

https://www.facebook.com/Kumamotoi/posts/719982528075182

This article presents an even scarier perspective on the same report from experts a Kobe University.

http://blogs.wsj.com/japanrealtime/2014/10/23/major-volcanic-eruption-could-make-japan-extinct-study-warns/