Midwesterners who worry about earthquakes got some good news last week: their risk of catastrophe may have been vastly overstated.

    New measurements taken around New Madrid, MO - the epicenter of devastating earthquakes in 1811 and 1812 - show that the ground there is scarcely moving.  According to many scientists, this means that it will take 2,500 to 10,000 years before another very large earthquake could occur in the region, although smaller, less damaging earthquakes are possible.

    "The motions are small to zero," said Dr. Seth Stein, a professor of geological sciences at Northwestern University in Evanston, Ill., who made the new measurements.  Earlier evidence showing rapid regional ground motion, a geologic sign that large quakes are probable, "was based on honest scientific errors," Dr. Stein said.

    The lack of ground motion means that regional building codes should be relaxed rather than strengthened, Dr. Stein said.  It is even possible, he said, that Missouri will never experience another large earthquake.

    But other scientists challenged Dr. Stein's findings, reported in the current issue of the journal Science.  "We see evidence for very strong ground shaking over the past 2,000 years," with great earthquakes recurring as often as every 500 years, said Dr. Eugene Schweig, the Central United States coordinator for the United States Geological Survey's Earthquake Hazards Program at the University of Memphis in Tennessee.  Dr. Schweig said he believed Dr. Stein had gone too far in calling the hazard overstated, and cautioned, "We need to exercise great caution when presenting interpretations of our results to the general public."

    Dr. Stein responded:  "This is a classic example of a geological argument.  We have to wait hundreds of thousands of years for real answers."

    Geologists do agree, at least, on what is happening now.  Small earthquakes, most too small to be felt, are occurring regularly in the region.

    Also, evidence for past earthquakes continues to accumulate.

    But neither side can say for sure what causes earthquakes in New Madrid.  This part of the country is in the middle of the North American plate, one of a dozen floating on hot, molten rock deep inside the planet.  When these plates collide, scrape or dive past one another, earthquakes occur.  According to every current model of earthquake dynamics, the new Madrid region should be unceasingly immobile, stuck as it is in the middle of a large plate.

    The debate began in the 1980's when geologists found signs that there has once been massive upheaval in the New Madrid landscape.  Whole forest tracts sank.  Soils liquefied.  The Mississippi River temporarily changed course.

    Using carbon dating and other techniques, scientists concluded that catastrophic earthquakes occurred there in A.D. 500, 900 and 1530.  Two or perhaps three earthquakes were centered there in 1811 and 1812, causing widespread damage in the Midwest and parts of the East Coast.  All these events, the scientists said, were probably magnitude 8, among the most powerful in the world.

    Geophysicists found that the ground beneath New Madrid was moving almost as fast as the ground in California, where earthquakes are relatively common.  The scientists calculated that a magnitude 8 earthquake could occur in New Madrid every 500 to 800 years.

    As a result, the Geological Survey identified the New Madrid area as one of most dangerous places to live in the nation.  "Memphis is said to be more dangerous than L.A. or San Francisco," Dr. Stein said.  "I think this is unlikely."

    Saying the numbers "just didn't make sense," Dr Stein and colleagues from four institutions re-did the ground motion measurements using far more accurate satellite techniques.  "It hardly budged," Dr. Stein said.

    It might be that the ground is not moving very fast, he said.  Or it might be that the previous earthquakes were much smaller than thought.

    Dr. Paul Segall, a geophysicist at Stanford University, is redoing his previous measurements of New Madrid using the advanced satellite technique.  "Although we are still comfortable in revising down the rates of strain of what we observed."

    In other words, Dr. Segall and his colleagues also see little ground motion in New Madrid.  But he said the lack of motion today did not rule out the possibility of a large earthquake.  After a large quake, strain can build rapidly for several decades and then slow way down, he said.

    Dr. Schweig said standard models of strain accumulation based on plate motions should not be used in New Madrid.  "This is an intraplate region," he said.  'We don't know or understand why these quakes happen, let alone when or why they might occur again."

    But Dr. Stein said the answer was simpler.  "We can concoct a variety of complicated explanations for why we see no motion," he said, "but the simplest explanation for nothing moving is that nothing is moving."

    Dr. Stein agreed that the New Madrid earthquakes were a mystery.  Since there is little evidence of earthquakes in the region before 2,000 years ago - and no mountains built up in the wake of seismic forces - it is possible that the seismic activity is temporary, he said.  Perhaps there is an ancient fracture in the North American plate and somehow geologic forces are causing it to rupture, Dr. Stein said. "It could be dying out or already dead," he said.  "The risk is not zero, but it's much lower than the maps say."

    Others, though, are not ready to turn off the alarm.  "I would hate for people t o think they don't have to take any precautions, to not worry about their foundations or water heaters," Dr. Segall said.  "In my mind, there are too many uncertainties."
 

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This page last modified on November 4, 1999  /st