Post-apocalyptic science fiction is set in a world or civilization after such a disaster. The time frame may be immediately after the catastrophe, focusing on the travails or psychology of survivors, or considerably later, often including the theme that the existence of pre-catastrophe civilization has been forgotten or mythologized. The fall of civilization may also be the fall of a space based civilization. This plot device allows writers to write Soft science fiction while accounting for the lack of technological advancement and thus remain relevant to the present day no matter how far in the future the events are set.
There is a considerable degree of blurring between this form of science fiction and that which deals with false utopias or dystopic societies.
After London by Richard Jefferies; the nature of the catastrophe is never stated, except that apparently most of the human race quickly dies out, leaving England to revert to nature.
The Purple Cloud by M.P. Shiel (A volcanic eruption floods the earth with cyanide gas, leaving only two survivors)