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Quake analysis rewrites history books

New Madrid quakes were smaller than originally thought.

Richard A. Lovett

The New Madrid earthquakes may have been considerably smaller than scientists had estimated.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 kilometres south of St Louis, Missouri, 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 US 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 on 23 April at a meeting of the Seismological Society of America, in Portland, Oregon.

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, Massachusetts, more than 1,700 kilometres away. Boston newspapers carried no mention of such an occurrence, says Hough, who speculates that the legend arose because church bells in Charleston, South Carolina — 500 kilometres closer to the epicentre — were confused with those in the Charlestown section of Boston.

Minor catastrophe

To determine the most likely magnitude of the earthquake, Hough assembled historic accounts of the shaking, and asked experts in Canada, Italy, the United States 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?' webpage, 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.

“This was a quite respectable earthquake, but not the biblical cataclysm some people have been coming up with.”

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, Illinois. "This was a quite respectable earthquake, but not the biblical cataclysm some people have been coming up with."

Tumbling temblors

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 kilometres 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 United States 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 US 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.

References
1.Hough, S. E., Armbruster, J. G., Seeber, L. & Hough, J. F. J. Geophys. Res. 105, 29839–23864 (2000).

New Madrid: New take on a very old earthquake

By TOM CHARLIER
Scripps Howard News Service
Seattle Times May 19, 2010

MEMPHIS, Tenn. -- Sure, the Mississippi River flowed backward and church bells rang on the East Coast, but the 1811-12 New Madrid earthquakes weren't nearly as powerful as generations have been told.

That's the conclusion of a U.S. Geological Survey scientist who has spent years studying the legendary series of quakes that largely defined the perceived risks associated with the New Madrid seismic zone, which stretches from southern Illinois to the Memphis area.

USGS seismologist Susan Hough contends that the three main quakes occurring between Dec. 16, 1811, and Feb. 7, 1812, probably were no more than 7.0 in magnitude. While that's still a large event -- the Jan. 12 quake that killed more than 200,000 people in Haiti was a 7.0 -- it's only about one-twentieth as powerful as the 7.7-plus magnitudes previously estimated.

Hough's findings, presented last month at a meeting of the Seismological Society of America in Portland, Ore., carry significance because the 1811-12 quakes -- generally considered to be the strongest that the New Madrid can generate -- serve as a basis for seismic building-code standards in the region.

Hough bases her conclusions on a new method of interpreting historic accounts of the temblors, which occurred long before the advent of modern instruments capable of measuring their power.

"The magnitudes hinge on how you interpret the accounts of the earthquakes,"she said. "The accounts are limited."

Hough compiled the accounts of the shaking -- everything from landslides along the river to falling chimneys -- and submitted them to a team of international experts, none of whom had previous familiarity with the New Madrid zone.

She asked the experts to assign the accounts "intensity values."The consensus values then were plugged into a computer program estimating the magnitudes of the quakes.

"When you do that, across the board the magnitudes are lower,"said Hough, who is based in Pasadena, Calif.

However, some other scientists say Hough's conclusions are tenuous at best.

Chris Cramer, a research associate professor with the Center for Earthquake Research and Information at the University of Memphis, said that without instrument-based measurements to calibrate the types of accounts reported in 1811-12 effects, it makes little sense to assign magnitudes based solely on them.

"There is a little bit of a disconnect,"Cramer said. "Scientifically, you cannot say what she is stating is true."

Scientists over the years have claimed the 1811-12 temblors were perhaps comparable in power to the 1906 San Francisco earthquake, which is believed to be nearly 8.0 in magnitude, because of the dramatic effects they produced and the strong shaking that was felt across more than 1 million square miles. Waterfalls and a backward current were reported on the Mississippi, for instance, and church bells rang as far away as Charleston, S.C.

But Hough's analysis indicates the 1811-12 events are more similar, strength-wise, to the magnitude-6.6 San Fernando Valley earthquake of 1971 and 6.7-magnitude Northridge temblor of 1994.

Some of the more cataclysmic events reported by witnesses two centuries ago, she said, reflect "site amplification."That's the process in which areas underlain by loose soil shake more violently that those on bedrock. Land along the Mississippi would be especially prone to amplification.

Based on archeological evidence, scientists know that the New Madrid zone has been producing major earthquakes every 500 years or so.

But Hough's report is among the latest in a series of studies suggesting that the seismic hazards of the zone might have been overstated in recent decades.

Seth Stein, a Northwestern University researcher, has published papers concluding that the fault zone is shutting down, although scientists at the U of M and elsewhere strongly dispute that.

While he and Hough differ on whether the zone is shutting down, Stein agrees with her assessment that the power of 1811-12 quakes has been exaggerated based on sometimes-sketchy accounts.

"There's a tendency for these things to grow," Stein said.