Really large events are termed by the actusl event. Ex: the Challenger Disaster, the Kennedy Assasination, etc.
Fictitious events: Since The Day, Warday, ww III, etc.
I like the idea of naming it if it's really freakin' huge.
One thing is for sure, if you're discussing the proper terminology to call a really freakin' huge disaster, then you're definitely not in one.