Via Wired: 5 Most Dangerous U.S. Earthquake Hot Spots Beyond California and one anomaly.
Most of the major earthquakes in the world occur at tectonic plate boundaries where land masses are colliding or pushing past one another. But in the middle of the country lurks a geological enigma near New Madrid, Missouri, that has produced some of the largest quakes on record for the United States but has yet to be fully explained by scientists.
"It's a big mystery," said geologist Eugene Schweig of the U.S. Geological Survey, who has studied the area for 23 years. "New Madrid is about as far from a plate boundary as you can get."
In 1811 and 1812, a swarm of at least three massive earthquakes struck near New Madrid, the largest of which exceeded a magnitude 8 and caused violent, damaging shaking in an area 10 times larger than did the 1906 earthquake. The quake was felt over an area of two million square miles — nearly two-thirds of the country.
During the quakes, the ground rose and fell, trees were bent, deep cracks opened up in the ground, large landslides swept down hills, huge waves washed boats out of the Mississippi River and river banks, islands and sand bars gave way.
Damage was widespread, but only a few people died thanks to the sparse population in what was then the Louisiana Territory but today is near the junction of Missouri, Illinois, Kentucky, Tennessee and Arkansas. Were another magnitude 8 earthquake to strike the region today, the toll would be much, much higher.
By analyzing deposits of sand that squirted out of the ground during past major quakes, Schweig and others estimate an average time between earthquakes of 500 years. But earthquakes are impossible to predict, and because scientists aren't even sure why major quakes occur away from plate boundaries, it is even more difficult to estimate a probability for New Madrid.