By Richard A. Lovett
A series of earthquakes that hit the North American heartland nearly 200 years ago were considerably smaller than reported in the history books, according to research presented at a meeting this week.
The quakes struck the New Madrid fault zone 200 kilometers south of St Louis, Mo., in 1811 and 1812, long before modern seismometers allowed accurate measurements of their intensity. In the 1980s, however, some scientists estimated that the magnitudes of these quakes were over 8.0, says Susan Hough, a seismologist at the U.S. Geological Survey's Pasadena office in California.
"You'll still find claims that these were the largest earthquakes ever in the contiguous United States," says Hough, who presented her findings April 23 at a meeting of the Seismological Society of America, in Portland, Ore.
Previously, Hough had stated that the earthquake magnitudes were only about 7.5. Now, she has reduced her estimates by another half point, to "right around magnitude 7. Possibly a bit below, possibly a bit above, but not as big as 7.5."
In making this stepwise reduction, Hough questioned the accuracy of the report that the largest of the earthquakes rang church bells in Boston, Mass., more than 1,700 kilometers away. Boston newspapers carried no mention of such an occurrence, says Hough, who speculates that the legend arose because church bells in Charleston, S.C.--500 kilometers closer to the epicenter--were confused with those in the Charlestown section of Boston.
To determine the most likely magnitude of the earthquake, Hough assembled historic accounts of the shaking, and asked experts in Canada, Italy, the U.S. and India to estimate the magnitude of the earthquake that produced them. "There were 300, maybe 400 accounts that had to be gone through carefully," she says.
Helping with the interpretation were findings from her agency's "Did You Feel It?" web page, in which people are invited to submit reports of earthquake experiences. Hough says that the reports have proved to be good indications of the severity of shaking measured by on-site instruments.
Still, common sense is needed. "The older the account and the more fragmentary, the easier it is to exaggerate," she says. "You have an account that says people were frightened and ran outside and chimneys came down. It's all breathless, but the bottom line may be that it was just a couple of chimneys."
Her experts fairly consistently estimated the magnitude of the New Madrid temblors at about 7.0.
"That's impressively small," says Andrew Newman, a geophysicist at the Georgia Institute of Technology in Atlanta. "I've always felt the early estimates were dramatically overestimated."
"I think this is something that grew wildly out of proportion and is now coming down to some level of reality," says Seth Stein, a geophysicist and New Madrid expert at Northwestern University in Evanston, Ill. "This was a quite respectable earthquake, but not the biblical cataclysm some people have been coming up with."
Newman says that the reduced estimates fit well with the fact that section of the New Madrid fault zone that ruptured two hundred years ago is relatively small by fault-line standards--only about 100-150 kilometers long. "There is no way you really can fit a magnitude-8 earthquake in such a small seismic zone," he says. "Even a 7.5 could have a bit of trouble."
Less understood are the forces causing earthquakes in the middle of the continent. Hough says that it is due to stress occurring as North America slowly rises after being depressed into Earth's mantle by the weight of ice-age glaciers.
Another possibility, Newman says, is that the continent is being squeezed from both sides, producing earthquakes at widely disparate locations, only one of which is New Madrid.
Seismic hazard maps of the U.S. need to take the new findings into account, says Zhenming Wang, a seismologist and geotechnical engineer with the Kentucky Geological Survey. At present, he says, the New Madrid area ranks higher than California on U.S. ratings of seismic risk. "Clearly, this doesn't make sense," he says.
Still, a magnitude-7 earthquake isn't to be sneered at. "Haiti was a magnitude 7, and it's clear what that did in a region that wasn't prepared," says Hough.