Science: Middle America's Fault

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It may some day cause another major earthquake

It was as frightening a natural cataclysm as had befallen the young nation. Buildings tumbled and forests were destroyed. Giant fissures opened in the ground, accompanied by a thunderous roar and a spreading sulfurous odor. Wrote one eyewitness: "The whole land was moved and waved like waves of the sea." The usually placid Mississippi became an angry torrent of whirlpools and rapids, overflowing its banks and possibly even briefly reversing course.

Three times, between Dec, 16, 1811, and Feb. 7, 1812, such scenes were repeated as major earthquakes jolted the region around the small Mississippi River town of New Madrid, Mo. Because the region was sparsely inhabited, few lives were lost. Still, the shocks were so powerful that they caused church bells to ring as far away as Charleston, S.C., stopped pendulum clocks in Washington, D.C., and shook buildings in New York City. No seismographs existed at the time, but detailed descriptions by survivors indicate that the intensities of the three quakes would have ranged between 7.3 and 7.5 on the Richter scale. By comparison, the big quake that destroyed San Francisco in 1906 was 8.3, and the 1964 quake in Alaska registered 8.5.

The trio of quakes has another distinction. Most major quakes occur around the boundaries of the great moving plates that form the earth's outer layers. One such region lies along California's quake-prone San Andreas fault, where the North American plate and the adjacent Pacific plate are grinding horizontally against each other as they move in opposite directions. When friction causes these plates to stick, stresses build up that are eventually released in a quake when the rock suddenly fractures and the plates lurch ahead. Yet the New Madrid area lies in the very heart of the North American plate, far from its boundaries. Why should it have shaken so violently in the early 1800s and, in fact, continued to quiver occasionally ever since?

Seismologists now believe they have part of the answer. Using oil-exploration equipment, they bounced sound waves off the subterranean rock where the tremors were centered. The echoes yielded a surprising profile of this hidden structure.

Along a large crack, corresponding layers of rock were offset vertically—in some places by as much as 1,000 meters (3,300 ft.). In other words, a break had occurred and some of the rock sank. Apparently, at some time in the remote past, the rock had been uplifted—perhaps by volcanism.

Then, as the volcanic flows cooled, part of the rock collapsed, creating sharp breaks. Subsequently the breaks were obscured by the soft sediments of the Mississippi River Valley.

The U.S. Geological Survey scientists mapped only a small section of this fault zone—in Arkansas' Mississippi and Craighead counties—but they suspect it continues north for some 100 km (62 miles), through Arkansas and northwestern Tennessee. There the fault system veers off past New Madrid and probably continues into southern Illinois. In all, the scientists count about half a dozen associated faults, although their data are still sketchy. Says St. Louis University Geophysicist Sean-Thomas Morrissey: "You can't go out and stick your finger in the fault like in California."