Finding fault with Midwest seismic maps.

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. At the St. Louis meeting, geophysicists described an enigmatic linear structure running underground for more than 400 kilometers from near Little Rock, Ark., to Vincennes, Ind.

Victoria E. Langenheim and Thomas G. Hildenbrand of the USGS in Menlo Park, Calif., call the feature the Commerce geophysical lineament because it appears most prominently in magnetic and gravity readings taken near Commerce, Mo.

They say the lineament could be a major fault zone, an injury left over from when North America almost split apart 600 million years ago.

The discovery of recent quakes at English Hill, above one section of the lineament, has scientists wondering whether other sections may also trigger tremors. "The lineament is an exceptionally long feature," says Hildenbrand, "and anything that is long in earthquake country is scary, because the longer a fault is, the greater magnitude earthquake that could occur on it."

Hildenbrand has a history of finding buried structures. In the 1970s, he identified a geological scar called the Reelfoot rift underlying the Mississippi River valley near New Madrid. Geologists surmise that faults within the rift caused the trio of magnitude 8 shocks that rocked the young United States.

The Commerce lineament runs parallel to the Reelfoot rift, so the newfound structure may have a favorable orientation for producing quakes, suggests Langenheim. At this point, though, scientists know too little about the feature to say whether it represents a threat.

"When geologists and seismologists don't know what a feature is, they usually call it a lineament," says seismologist Arch C. Johnston of the University of Memphis. "Anytime you see that word, it indicates that we've got something there, but we're just not sure what it is."