Scientists study earth in New Madrid zone

~ They are using vibrations to map the fault in Arkansas.

LEPANTO, Ark. -- A 12,000-pound truck is sending soft booms through the earth above the New Madrid fault zone in eastern Arkansas in an effort to learn more about the potential danger of another major earthquake in the United States.

Researchers from the U.S. Geological Survey, the University of Memphis, and the University of Texas were plodding -- and plotting -- along Highway 140 on Tuesday as part of a study to understand what causes earthquakes in the region.

Through the study, "we can see images of the fault from thousands to even millions of years ago," said geophysicist Robert A. Williams from the USGS in Denver.

The group uses a fat-tired vibroseis truck to vibrate the ground below and create small waves that travel deep into the earth. The vibrations, weaker than those of an 18-wheeler, are reflected by the layered sediment, and those waves are then recorded by seismometers along the route. Called a seismic reflection study, the research produces an image of the fault zone.

The study should provide a better reading of the potential danger of an earthquake for the region and provide information important to building in an earthquake zone.

The activity has piqued the curiosity of residents in the small community of 2,133.

"The people in town have questioned us," Williams said. "Some of them have thought that we were from the cable company."

Williams said the work, which began last week, will take several more days. When finished, the group should have a six-mile long sonogram image that will become part of the data collected in the multiyear study.

The New Madrid fault zone runs from Marked Tree, Ark., to near Cairo, Ill.

In 1811-1812, the faults produced some of the strongest temblors ever known to have struck the continental United States. Scientists say quakes like those in the 1800s could kill and injure thousands from St. Louis to Memphis, Tenn., but they're uncertain about how much strain and movement is occurring along the fault zone.

The faults remain active, though most of the quakes are too small to be felt at the surface.