Reelfoot lies in wait over earthquake zone

Posted: Tuesday, December 23, 2008 9:27 pm
By: Bud Grimes

 By BUD GRIMES Special to The Messenger The view from Fish Gap Hill in Obion County offers a postcard-like perspective of the upper Reelfoot Lake basin. The lake rests quietly near the Mississippi River among acres of farmland that spread across western Kentucky and northwest Tennessee. The bluff overlooks the eastern-most portion of the Mississippi River floodplain. Hidden beneath the surface of this tranquil setting is the power to alter life in this region in a way not witnessed in modern times. A series of earthquakes in 1811-12 formed Reelfoot Lake and changed the region’s landscape. Left for current and future generations to consider is, “Could it happen again?” Experts agree that it’s not so much a question if such an event will happen, but when. And when a major earthquake occurs in the eastern United States, likely in the New Madrid Seismic Zone, preparation will determine how the region fares in the quake’s aftermath. That’s the message of Dr. Stan Dunagan, assistant professor of geology at the University of Tennessee at Martin. Dunagan sometimes takes students to Fish Gap Hill, where he not only teaches but also shows them about earthquakes. On a clear day earlier this year, he explained where portions of the bluff gave way under the rolling force of the quakes. Back in his geology classroom, he uses simple illustrations to explain a complex set of actions that lead to earthquakes. In summary, rocks are under pressure, and if enough pressure or force is applied, they will eventually crack, much like bending a pencil, he said. He demonstrates to his classes by showing that you can apply force and bend the pencil — apply enough force, and the pencil will crack. “Similarly, rocks act that way when enough force is placed on them. Just like that pencil will break, the rocks will eventually break,” he said. “And when you have rocks break, they release energy, and that break, that release of energy — that’s your earthquake.” The good news is that earthquakes come in all different sizes. A recent example is a magnitude 5.2 earthquake that occurred in the early morning hours of April 18 in Southern Illinois. The quake was felt in several states, including West Tennessee, and was considered moderate, he said. The epicenter was actually located in the Wabash Valley Seismic Zone and caused relatively little damage. The quakes that formed Reelfoot Lake were probably in the 7.7 range, so fortunately, the “big ones” don’t happen as often. But, earthquakes are occurring, whether people feel them or not. In fact, Dunagan said, the New Madrid Seismic Zone, which includes West Tennessee, has the greatest amount of seismic activity and the greatest potential for large quakes in the eastern United States. “Interestingly, East Tennessee is second (the East Tennessee Seismic Zone), he said. “Now, they don’t have big earthquakes, but they have lots of very small earthquakes, magnitude typically less than 2.0, but there are lots of faults in East Tennessee. …” To support his point, he referred to the U.S. Geological Survey Web site (http://www.usgs.gov/) that recorded 25 earthquakes from March to July in East Tennessee, a fact probably surprising to many. Based on history and available geologic information, Middle Tennessee residents can rest the easiest with the smallest probability of a large-magnitude quake. Of greater concern is that West Tennessee, more than any area of the state, faces significant consequences from a large-magnitude quake. “Geologists view the New Madrid Seismic Zone as one that’s low probability but high consequences,” Dunagan said. “In other words, if there is an earthquake, there is a chance that it could be a big one.” The U.S. Geological Survey and the University of Memphis maintain the Center for Earthquake Research and Information at the Memphis campus. The center has released numbers for the probability of an earthquake measuring 7.7, which is comparable to the 1811-12 quakes that formed Reelfoot Lake. “If you have a magnitude 7.7, the probability of an earthquake of that magnitude occurring over the next 50 years is 7 to 10 percent,” Dunagan said. “Now, the good news is that that probability is a little lower today than what we might have been talking about 10 to 15 years ago, largely because there’s been a little bit more research.” As for the location of greatest damage if such a quake occurred, Dunagan said that it depends. He drew the shape of the New Madrid Seismic Zone as a crooked “S” shape that encompasses an area from the Missouri Bootheel and western Kentucky winding south to Memphis. The scope of damage will be determined by where the earthquake occurs along the fault, he said, adding, “And it’s certainly possible that it occurs along faults that we already have identified, but there may be additional faults that we’re not aware of that it could occur along. “Memphis is certainly a major concern among various municipal entities, because it’s the largest city in this area and for the important transportation routes that run through that area.” Highlighting the concern, he noted that Memphis, Light, Gas and Water — in cooperation with the National Science Foundation, the Army Corps of Engineers and University of Memphis researchers — recently traveled along the Mississippi River performing seismic surveys to locate faults and to assess the seismic potential. But, residents in major seismic zones aren’t the only ones who should be concerned about major earthquakes. “Most of West Tennessee, if there’s a large earthquake, will have significant damage,” he said. “There’s lots of soft, sandy sediment beneath us. That blesses us with great high-quality water resources but, in terms of earthquakes, that’s not where you want to be.” The important work of monitoring seismic activity is largely coordinated through the U.S. Geological Survey and through several consortiums. UT Martin, which has a seismic station, is part of a larger seismic network that includes St. Louis University and the University of Memphis center. Another project called EarthScope aims to gradually move GPS and seismic stations across the country to better understand the earth’s structure in a way that might help better identify the potential for earthquake hazards. To date, almost 700 seismic and more than 900 GPS instruments have been deployed starting in the western United States and Alaska, Dunagan said. He added that portable instruments should be moved into Tennessee beginning in 2011 for West Tennessee and 2012 for Middle and East Tennessee. While history might help to identify a pattern that could predict a major quake, Dunagan suggests “not to get bogged down” with these details. “There will be an earthquake one day, and the question is, ‘Are you prepared when it happens?’ And, there are lots of things that you can do to get prepared and to really mitigate any potential losses that there might be, whether it’s injuries, loss of life or property losses,” he added. While Dunagan said that many Tennesseans would cite a tornado or flood as the most likely natural event or hazard to occur, preparations for these calamities work equally well for earthquake preparedness. Such steps are keys to short and long-term survival following a catastrophic earthquake. His core recommendations include: have a family emergency plan, have supplies available (water, food, medication), know how to turn off utilities and have a communication plan. Specifically for earthquakes, he’s a big proponent of earthquake insurance. He also recommended that people secure appliances, bookshelves and large furniture. “Most injuries are not due to shaking. Most injuries are due to things hitting people and falling on people in an earthquake,” he said. “Everyone can take an l-bracket and screw it to a piece of furniture and back into the wall. If someone has a gas appliance, it needs to be secured.” For people building new homes, they might do well to consider some steps that his family took in constructing their new home near Paris. He had corner bracing added when the house was being framed, as well as some additional horizontal blocking and bracing. He also had the top wooden structure bolted to the basement in multiple places. “The last thing you want, if you have some serious shaking, is one part of your house go one way and the other go the other direction. And if it’s not firmly tied on, that will certainly occur,” he said. Such preparations seem far removed as visitors take in the scenery from atop Fish Gap Hill. However, this panoramic view of Reelfoot Lake reminds visitors what happened when the earth shook in the early 1800s. Stan Dunagan’s advice offers a sobering reminder to be prepared when the earth shakes again. Editor’s note: Bud Grimes is director of University Relations at the University of Tennessee at Martin. Published in The Messenger 12.23.08