While large-scale disasters such as Hurricane Sally and the March 2011 earthquake and tsunami in Japan caused many Americans sadness and sympathy for the people affected, the events of September 11, 2001, caused a serious nationwide stress reaction. Why do you think there is a difference?
The difference is that that didn't need to happen. An Earthquake, a storm is out of everyone's control of its nature. A terrorist attack requires that some dark-hearted scumbag plans it out then sends people to execute the plan. Then afterward one of the most powerful nations in the world is in inner turmoil. How do we prevent another one? We need to retaliate! We can't do that violates our rights. With all this confusion and stress impactful decisions are made based on emotion, people take advantage of fear and stress for their own gain, and worst of all the idea of "the other" is created. "The other" is the worst idea in human history. It dehumanizes a group of people a produces fear and anger that affects innocent people that happen to share uncontrollable characteristics with the heinous human being. That is why the stress of an event like 9/11 is different than the stress of a natural disaster.