(1) the Atomic war which will either destroy outright our present civilization, or cause social changes so sudden and violent that such destruction results.
(2) The incident (13 November 1949) when Eugene Seger set off a bomb, made by Fred Reich, on Art Rapp's lawn in Saginaw after an MSFS meeting. (He was cheered on by the other members, but made the goat for the ensuing events.) The blast blew in a couple of windows and brought police, firemen, and unwelcome notoriety. Rapp announced his resignation from the club in MICHIFAN for 14 November 1949, and what with one thing and another Michigan fandom, like civilization in (1), was never the same again.
(3) The civil war in Shangri-LA described under LASFS was also given the name of Blowup, because of its shattering effect.
from Fancyclopedia 2 Supplement ca. 1960:
The cited one was merely the Blowup. There was more than one bomb incident -- several months before, after a meeting at Norm Kossuth's house, Reich set off a noisy but harmless firecracker on the lawn. [Norm declined to permit any more meetings at his house because of this. "In Michifandom", deprecates Art Rapp, "that was hardly an unusual enough incident to be noticed."