With a little luck, the bicentennial of three of the strongest earthquakes in the history of the continental United States will pass quietly this winter. Two hundred years ago, the center of the United States shook violently as the first of three strong earthquakes — estimated to be of magnitude 7 to 8 — rocked parts of a five-state area in the Mississippi River Valley south of St. Louis.
The New Madrid Earthquakes, named for the southeast Missouri town at the epicenter of the last quake, are considered to be three among the top seven ever recorded in the lower 48 states, according to the U.S. Geological Survey. Some sources consider them the strongest earthquakes ever to strike the lower 48 states, even though there were no seismic instruments back then. Still, evidence left by the quakes and accounts by those who experienced them give scientists some idea of their strength. “On the 16th of December 1811, about two o’clock a.m., we were visited by a violent shock of an earthquake, accompanied by a very awful noise resembling loud but distant thunder,” wrote eyewitness Eliza Bryan. In Bryan’s letter, he painted a picture of fear and chaos in the shock’s aftermath. “The screams of the affrighted inhabitants running to and fro, not knowing where to go or what to do — the cries of the fowls and beasts of every species — the cracking of trees falling, and the roaring of the Mississippi — the current of which was retrograde for a few minutes, owing as one is supposed, to an interruption in its bed — formed a scene truly horrible.” Dozens of aftershocks followed, including one on Dec. 16 that was nearly as strong as the first quake. The ground rumbled for the next two months, including two additional quakes that struck Jan. 23 and Feb. 7, 1812.
And that might not be the end of the story. Evidence also points to serious earthquakes in 950 and 1400 A.D. Geologists say the New Madrid Seismic Zone, stretching for 150 miles and covering portions of Missouri, Arkansas, Tennessee, Kentucky and southern Illinois, still has the potential to produce a major earthquake. Not your coastal earthquake. The New Madrid quakes defy the understanding most people have about earthquakes because they occurred far from any evident fault lines or continental plate boundaries. New Madrid, Mo., is about 165 miles south of St. Louis. The New Madrid Seismic Zone is a failed rift, a place where the North American Plate tried to tear itself apart when the continents drifted apart hundreds of millions of years ago. It’s not one fault, but three — corresponding to three earthquakes in 1811-12. The tears in the Earth’s crust are buried deep underground, covered by millions of years of sediments. “In 1811-12, there were three very large main shocks on each one of those faults,” said Robert Bauer, principal engineering geologist with the Illinois State Geological Survey. “And there were thousands of aftershocks. “When seismic activity is monitored,” he said, “it all lines up, giving you three lines that people believe are the faults that produce 100-200 small earthquakes a year — most not felt by people, but (they) can be picked up by seismographs.” Bauer said tectonic plates continue to move, squeezing the New Madrid faults and allowing them to build up enough energy for future earthquakes. “The plates are moving around, dragged by convection cells underneath,” he said. “Through most of the central United States, we are being squeezed in an east-west direction.”
What could happen. Because the potential for earthquakes remains, scientists have developed sophisticated monitoring networks that will get information in the hands of first responders and government officials as quickly as possible in the event of a serious earthquake. “There have been a number of estimates that have been done throughout the years to determine the impact of a sizable New Madrid event,” Bauer said. “Potential casualties, fatalities, estimates of the number of structures damaged in different categories — and many of these have been run through a computer loss estimation program that FEMA has developed,” he said. Most emergency plans are based on higher-magnitude earthquakes. A FEMA report issued in 1990 estimated a 7.6 magnitude quake would cause $2.8 billion in damage, 260 deaths and 1,060 serious injuries in St. Louis — almost a three-hour drive to the north of New Madrid. In lightly populated areas, deaths are expected to be fewer at night due to children being at home and not in schools. A 1985 study called for Poplar Bluff, in the Missouri Bootheel region, to suffer $693 million in damage and one death if the quake occurs at night or 17 if it happens during the day. The 1811-12 quakes destroyed most of the buildings in New Madrid and Little Prairie, according to the Missouri Department of Natural Resources.
Earthquakes can occur outside the New Madrid Seismic Zone. “We use the New Madrid Seismic Zone as a forum to prepare for earthquakes,” Bauer said. “We have earthquakes throughout the state. A large event or events in the Wabash Valley Seismic Zone or smaller events near your city could do more damage than a large one far away.” Bauer said earthquake scenarios are useful preparation for all kinds of natural disasters. “There are a couple of professors saying the New Madrid Seismic Zone is dead, and that nobody should be preparing for earthquake damage,” he said. “But these preparations could deal with multiple hazards. “There was a great case in Kentucky where their director of emergency management had gone through an exercise where a New Madrid event knocked out communications,” Bauer said. “In January 2009, an ice storm totally knocked out communication for the western part of the state, so he said, ‘Take down the earthquake plan,’ and they used it to help deploy National Guardsmen to re-establish communications. He called it an ‘icequake.’”
Saint Louis University and the University of Memphis form the two main earthquake-monitoring stations. Robert Hermann, professor of geophysics at SLU’s Earthquake Center, said his work there is two-fold. Hermann’s research interest is to work with engineers to develop a better understanding of what happens to a building when shaking occurs. He is interested in the safe design of buildings, those engineered to the proper level of strength, and re-evaluating existing structures and their safety. First responders are concerned with life safety after an earthquake, Hermann said. “But life safety also is in the preservation of property before you have an earthquake,” he said. “We take this all very seriously, as we must.” The second part is maintaining a network of monitoring equipment that will feed information quickly and accurately to earthquake centers, including SLU and Memphis. “What we do is we ensure the instruments work properly and that the data streams are well-defined,” Hermann said. Thirty-five monitoring sites gather information, helping scientists pinpoint the location, depth and size of an earthquake. Hermann said the whole thing exists almost virtually. Modern instruments contain a sensor connected to a computer in the field, which is connected to a satellite or the Internet. Information flows through to Saint Louis University, and then it is forwarded electronically to the University of Memphis and the U.S. Geological Survey Earthquake Information Center in Golden, Colo. Information then is distributed to the U.S. Departments of Defense, Energy, FEMA, state emergency management agencies and others. “We want to provide information to the responder, letting them know where the earthquake occurred and an initial estimate of what the problems are,” Hermann said. “Within the first hour you don’t know how bad it was, especially if you are not there,” he said. “You have to gear up the response, get the playbook out and get emergency services there to save lives and help injured people.” - SJ-R.