Are nurses obligated to help in a public emergency?

No, they are not. If you are not on the clock, you are not required to do anything. You are a good Samaritan with a little more knowledge than the average Joe. Buyer be ware though, you touch you buy....
Becareful not to turn it into a neglectful situation.

Morally yes. But legally no.

Agree with Sheryl Shantz. Whether peeps agree or not, when I am off the closer I am an ordinary person.