The Disaster as Pressure Test
Every disaster your characters survive should reveal something about them that normal life conceals. The character who seems dependable in ordinary times may freeze when it counts. The one who seemed selfish may be the first to go back for someone left behind. The disaster's job is to remove the social scaffolding that allows people to perform who they would like to be, leaving only who they actually are. Design your disaster specifically to test your protagonist's central flaw or fear. The most resonant disaster fiction feels inevitable in retrospect: of course this is the disaster that would unmake and remake this particular person.