Look at the experience of Hurricane Katrina. It was a week before the government got around to doing anything. People died. One family dragged their grandmother around for days and they could not help her. She died in the street. People went without food for a week and [because they lacked water filters] they were forced to drink the muddy polluted water.
Eventually the troops came pouring in to save the day (and to clean up the mess). They were shot at by gang members who had been robbing, raping, and terrorizing the locals. Some of the police had given up and fled with their families. It wasn’t a complete breakdown of civilization, but it was close.
Most people's response is that "It can't happen here." Let me point out that the most severe earthquake in American history happened in the mid-west. It was on the New Madrid fault. Luckily it hit in the 19th century were there were few settlers there. The Mississippi ran backwards and changed it's course of flow. It also created a large lake in the area.
The American population is growing and makes it more likely that a small event could kill lots of people and make many homeless. Simple and small steps to prepare can make a difference in whether or not you survive. It doesn’t hurt to have a pantry of extra food on hand for special occasions.