just to add my two cents, when i was thinking of the "disaster type film" i immediately thought of The Road.

if you're unfamiliar with it:
http://en.wikipedia.org/wiki/The_Road_(film)

it has been made after the book by Joe Penhall. it was very enjoyable to me, suspenseful, and serves as a personal inspiration for me.

x

So i has an idea to this...

Ever seen the movie (Ground Hog Day) where everyday is a repition of the previous and only 1 person knows it and knows what events will unfold thru out the day...

If zero budget is what you are after, then it should be easy to get volunteer actors to repeat the same thing over and over again and it can be in everyday life...

Outline of story: Unknowingly to all except a few on planet earth, the world falls into an alternate reality where one day is repeated continously. All earths inhabitant's memories are wiped clean and they are stuck in repeating the events of their life for this paticular day everyday. Only a handfull of people begin to realize the same thing is happening over and over again. They don't know what is going on, but are determined to find out. Ironically enough the repeating day is Lotto day where the numbers are shown and repeated daily, but to add insult to injury the people who remember the previous day can never cash in the winning...

just some thoughts...