ST. LOUIS — Glancing out the window as he drove along a narrow county road in Missouri's Bootheel region, Dave Hoffman noticed a cut-away hillside in an auto salvage yard.
He was visiting the area near Bloomfield, about 140 miles south of St. Louis, and found himself on the road by happenstance. But for Hoffman, Missouri's state geologist, stumbling upon a slice of bare earth was like a kid finding an unknown playground.
Hoffman pulled over and asked the salvage yard owner if he could take a look. What he discovered digging through the dirt and clay could prompt a round of earthquake jitters in a region that has long been waiting for the notorious New Madrid Fault to rumble again.
Hoffman found a layer of silt that had shifted vertically about 20 feet. He said the shift suggested that one or several massive earthquakes may have struck the region sometime in the last 10,000 years.
Of more immediate concern, the finding suggests that the New Madrid Fault isn't the region's only earthquake threat, Hoffman said.
"We know of hundreds of faults [in Missouri], but basically all of them are considered inactive--not assumed to be a current threat," Hoffman said Dec. 29. "We thought the only thing we had to be concerned about was New Madrid.
"The question becomes: How many more of these things are out there that we aren't aware of?"
The New Madrid Fault is named after another Bootheel town that is at the center of the fault line, which runs 125 miles from southern Illinois to northeast Arkansas.
It produced the largest earthquakes ever in the continental United States. A series of New Madrid quakes in 1811 and 1812 had estimated magnitudes exceeding 8.0. Those quakes could be felt as far away as New England and caused the Mississippi River to briefly flow backward.
The fault has been relatively quiet since. It produced a magnitude 6.4 temblor in 1843, centered near Marked Tree, Tenn., and a magnitude 6.8 quake near Charleston in southeast Missouri in 1895. Recent quakes have been in the magnitude 4.0 range.
Scientists with the U.S. Geological Survey and area universities give a probability of up to 60% for a damaging New Madrid quake, magnitude 6.0 or higher, within 15 years. Over 50 years, that figure climbs to more than 95%.
Hoffman believes his discovery--dubbed the Holly Ridge Fault for its proximity to a nearby conservation area--may be part of a vast multi-state fault that could rival New Madrid in size and potential for destruction.
The Commerce Geophysical Lineament, identified by the U.S. Geological Survey, extends from near Little Rock, Ark., up through southeast Missouri and southern Illinois to Vincennes, Ind. Scientists believe it may be a fault, but haven't yet proved it.
Robert McDowell, a geologist with the U.S. Geological Survey, said it is too early to know whether Hoffman's discovery is an earthquake fault or some other geological abnormality.
McDowell downplayed the potential for an increased earthquake threat, citing the lack of frequency between quakes in the region.
"There is an earthquake potential in the area, but it's not like it's something to stay awake and worry about," he said.
The finding has since been studied by Hoffman and other scientists from the Missouri Division of Geology and Land Survey. Hoffman said he hopes to secure state and federal funds to continue the studies.
There may be many undiscovered faults in the United States, Hoffman said. He noted that the magnitude 6.7 earthquake in Los Angeles in 1994 was the result of a previously unknown fault. That quake killed 57 people-- and caused $20 billion in damage.
Hoffman isn't making any predictions on when the Holly Ridge Fault might rumble.
"But there are significant implications for quake-hazard analysis and what we should be planning for," he said.