Kirk with a little more recent information about whether or not you need to evacuate:
https://www.bousai.go.jp/oukyu/hinanjouhou/r3_hinanjouhou_guideline/pdf/EN_ura.pdf
I think the content is basically the same as what I posted earlier but this is easier to follow.
I found the link to it on this page. There are links to many other languages: https://www.bousai.go.jp/oukyu/hinanjouhou/r3_hinanjouhou_guideline/