Worst U.S. Natural #Disaster?

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So, the weirdest natural disaster may be the Boston Molasses Disaster but is the worst natural disaster the 1906 San Francisco earthquake?  I suppose it depends on the definition of “worst”.

http://en.wikipedia.org/wiki/1906_San_Francisco_earthquake

One of the issues I see with this one is the unknown loss of life.  At the time, of course, they had a paper record and many time no record of people living certain areas.  My understanding is that there were many, many people killed in the Chinatown area but they really do not know the exact number.

Check out the wikipedia article on it.  What do you think?  What defines “worst” disaster?  Loss of life?  Confusion?  Recovery time?  Lack of preparation?  Cost to repair infrastructure?