Magazine article Science News

Finding Fault with Midwest Seismic Maps

Magazine article Science News

Finding Fault with Midwest Seismic Maps

Article excerpt

In the U.S. heartland, faults seem a thing of the past. Almost all known surface faults in the midcontinent formed hundreds of millions of years ago and have remained quakeless since before the time of the dinosaurs. Now, the discovery of active faults in Missouri may illuminate a seismic threat stretching from Arkansas to Indiana.

The Missouri faults zigzag through the southeastern part of the state at a site called English Hill, say David Hoffman of the Missouri Department of Natural Resources in Rolla and his colleagues. The faults lie north of New Madrid, Mo., which in the winter of 1811-1812 saw the largest series of earthquakes in U.S. history. The scientists reported their findings this week at the annual meeting of the Seismological Society of America in St. Louis.

Although geologists in the 1930s noted the existence of some faults in this area, the age and extent of them remained unclear. Hoffman and his coworkers spent several years looking for the signs of faults-fractured sediment layers-which they identified by drilling in the ground and digging trenches.

They found evidence of several quakes within the last 10,000 years. Although they could not determine the timing of these jolts, the scientists estimate that they were fairly large, magnitude 6 or greater.

"The generally accepted standard is that anything active in the last 10,000 years is considered to be an active fault and will be active again," says Hoffman. The English Hill faults are becoming a tourist attraction for geologists around the central United States, who rarely get a chance to run their hands over active faults. East of the Rocky Mountains, most quake-producing faults are hidden beneath thick sedimentary deposits. "It's quite spectacular," comments Eugene S. Schweig of the U.S. Geological Survey (USGS) in Memphis.

The English Hill faults present potential threats on their own, but an even larger seismic hazard may lie beneath them. …

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