My experience dovetails with yours Hikermor. There are always some people who will take advantage of a situation for personal benefit, but the vast majority will help each other out.

I will say, however, that a prolonged situation that has very strong and negative impacts can wear down societal structures and values and reduce many people to survival mode. Even then, there are some who will remain pro-social. I think it depends on leadership and group identities. These instances are more in the nature of wars and famines and not hurricanes, fires or earthquakes.