continue recolonizing america

i think you mean a large bomb with a blast radius of 3/4 the size of north america, if it itself was 3/4 the size of north america it would instantly wipe out all life on earth

I mean how much tsar bombs to take out 3/4 of north aemrica

take namibia and botswana

boat to south argentina

Since there are no zombies in the eastern continents I'll continue expanding southward