UNITED STATES, LOWER MISSISSIPPI DELTA REGION
Fault Line: New Madrid
Last big quake: 1812
Reasons to worry: A string of earthquakes in the early 19th century along the New Madrid fault -- covering parts of Illinois, Missouri, Arkansas, Kentucky, Tennessee, and Mississippi -- caused the Mississippi River to flow backwards, rang church bells in Boston, and affected an area more than three times as large as the famous San Francisco quake of 1906.
Two hundred years ago, the at-risk population was minimal. Today, the major cities of Saint Louis and Memphis lie within the danger zone of arguably the United State's most threatening fault line. FEMA warned in 2008 that a major New Madrid fault earthquake could cause "the highest economic losses due to a natural disaster in the United States," largely due to a relative lack of earthquake preparation compared with California and the Pacific Northwest.