A lot of very wealthy folks around the world are buying up private islands and tracts of land in New Zealand and the Caribbean and the South Pacific and building fortresses mostly underground with provisions of all kinds to last many years. There are also outfits building underground luxury bunkers. So some folks think they can weather anything.
It's more likely a nuke is gonna come from a small player like N Korea where he takes out a few cities on the West Coast. I mean what are you gonna do to destroy N Korea. Pretty much the whole country is backwards with no real infrastructure. There ain't much to destroy.