THREE OF THE MOST POWERFUL EARTHQUAKES EVER to hit the U.S. -- each topping 8.0 on the Richter scale -- struck near the town of New Madrid, Missouri, in the early 1800s. It is hardly an obvious location. The theory of plate tectonics says quakes should happen most often along the edges of crustal plates, pancakes of rock a few score miles thick and thousands of miles across, which carry the continents on their backs as they slide across the semimolten mantle below. The plates ride over each other or grind together, and the earth shakes. But New Madrid is right...
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