From this image here, you have five circles that represent an effect, the green circle is heavy radiation, and you can read what it does on the image or on the website, that encircles most of Hoenn's major island, all of Hoenn, Johto, and most of Kanto gets hit by the actual fireball, the strong airblast that follows hits the rest of Kanto that wasn't hit. A weaker, but still deadly airblast moves up into Sinnoh, and the last circle, which is thermal radiation, is so large, it actually comes back around to create a small circle near South America which is the only area NOT affected by this at all, Unova and Kalos would experience thermal radiation.
Fun Fact: Guyana, the supposed birth place of Mew, was just outside of the unaffected circle by maybe 1,000 miles.
So yeah, it's a good thing Rayquaza complied and destroyed it.
To put it simply, it doesn't matter if it's an asteroid or a nuke, something that powerful would essentially destroy the planet.
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