Many disasters that would be sufficient to wreck civilization will probably leave at least some survivors. The inhabitants of Easter Island ended up pretty screwed, but they didn't go extinct. Similarly, the collapse of the Mayan civilization left plenty of people left alive, many of which ended up settling in a different area than the former center of civilization. If a major disaster occurs that doesn't manage to kill off basically all animal life on Earth, I suspect that there will still be at least a few people carrying on one hundred years later, even if they have to live as subsistence farmers or hunter-gatherers.