Those rings show the area of near total destruction and death.
The area of severe damage and less instant fatality is maybe twice as wide.
Outside of that there's another ring of significant damage, but the probability of survival increases (as long as you're not near a window when the shockwave hits.)
The map also doesn't show fall out or the effects of a possible firestorms.
It's an option you can select - you can also set wind speed and direction (drag the windsock icon about). You also need to make sure it is a ground burst as an air burst won't produce much fallout.
The area of severe damage and less instant fatality is maybe twice as wide.
Outside of that there's another ring of significant damage, but the probability of survival increases (as long as you're not near a window when the shockwave hits.)
The map also doesn't show fall out or the effects of a possible firestorms.