Big One for Big Mo?

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 in the middle of a plate, a place where earthquakes are generally not seen.

Unfortunately, scientists believe, the town has the bad luck to sit atop a weak spot, and as the plate moves, it stretches -- and sometimes cracks. Now a report in Science bears out that belief: a new survey reveals that stresses in the earth have been rising sharply in the area since the 1950s. The amount of change points to a big quake in about 1,000 years, which is roughly the historical average indicated by the geological evidence. But that is just an ^ average. It could also happen tomorrow.