A fault line located in what one would think is an unlikely place: central Missouri. Quakes from the New Madrid fault (which is, surprisingly, near New Madrid, MO) are rare, but strong. The last few bad ones include an 1895 quake (comparable to the Los Angeles quake of 1994), and an entire series of 8+ magnitude between 1811 and 1812, which were strong enough to make the Mississippi River flow backwards for days.. (Thanks Uberfetus for dates)