October 18, 1988
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Knoxville News-Sentinel October 18, 1988 |
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by Morgan Simmons, News-Sentinel staff writer In the early morning of Dec. 16, 1811, the residents of southeastern Missouri were awakened by a loud rumbling as the earth began to quiver, then break into visible waves that survivors said reminded them of the ocean under storm. Sulfurous gas and sand spewed from fissures in the ground, dark clouds obscured the sun, and as large areas of land rose and fell, the Mississippi River reversed its flow. By the time the earthquakes ended five months later, hundreds of frontier families had lost their homes, and what had been a swamp in northwest Tennessee was now Reelfoot Lake. The great New Madrid Earthquake was actually a series of 2,000 shocks in five months. Seismographs, which measure the intensity of earthquakes, were not invented until 1870, but scientists estimate the 1811 quake to equal 8.6 on the Richter scale, with two subsequent quakes estimated to be 8.4 and 8.7 in magnitude. No other earthquake in the history of North America -- not even the San Francisco earthquake (7.7 to 7.9 on the Richter scale) of 1906, or the Mexico City quake (8.1) of 1985 -- has packed more seismic energy. Today the New Madrid Fault, which encompasses West Tennessee as well as parts of Illinois, Missouri and Arkansas, ranks as the most active and famous earthquake zone in the United State east of the Rocky Mountains. What many people don't realize is that the second most active earthquake zone in the eastern United States extends from northwestern Georgia to southeastern Kentucky, right through the heart of East Tennessee. And while the East Tennessee Seismic Zone differs markedly from the New Madrid Zone in many ways, it poses the same fundamental questions: What is the likelihood of a severe earthquake occurring here, and when would it happen? Area earthquakes are monitored by the University of Tennessee's Department of Geology, which operates a network of 17 seismographic stations scattered throughout the Tennessee Valley. The system -- originally installed by the Tennessee Valley Authority as part of its nuclear power program -- transmits earthquake data continuously by radio and telephone telemetry to a central location, at which point the data is analyzed and posted on the University's Earthquake Information Web site, http://tanasi.gg.utk.edu/quakes.html. Dr. Rick Williams, a geophysicist and associate professor in the University of Tennessee's Department of Geological Services, said many of the region's earthquakes are level 2.0 magnitude or below, too small to be felt. In terms of distribution, Williams identified three persistent clusters of earthquake activity in East Tennessee -- one in the Vonore-Greenback area, one just northeast of Knoxville, and another just west of Dayton. "We were getting a lot of earthquake activity just west of Dalton, Ga.," Williams said. "That area was lit up, and now it's quiet. That's typical earthquake activity. An area can go weeks without any earthquakes of any size, and then light up again." The strongest earthquake on record to hit East Tennessee occurred on Nov. 30, 1973, in Maryville. The quake registered 4.6 on the Richter Scale, large enough to be widely felt and do minor damage. On average, the New Madrid fault of the central Mississippi Valley produces about six earthquakes of magnitude 3.0 (the minimum size that's generally "felt") or larger per year, while the East Tennessee Seismic Zone averages just one or two 3.0 or larger earthquakes per year. The most recent earthquake of perceptible strength was a June 17 quake of 3.7 magnitude that originated in Oak Ridge, just west of the East Tennessee Technological Park, the former site of the K-25 Plant. The quake was felt in six counties -- Roane, Anderson, Knox, Blount, Loudon and Morgan -- but caused no significant property damage. On July 30, 1997, a 3.8-magnitude earthquake occurred just north of Tazewell, Tenn., and could be felt a short distance from Knoxville. Williams said earthquake activity in the area follows a clear pattern and does not appear to be growing in magnitude or frequency. He said the frequency of East Tennessee earthquakes in the magnitude 4.0 range -- no structural damage but widely felt --is approximately one every 20 years, and that the area is "pretty much overdue for a quake in the magnitude 4 range." "The central question in East Tennessee is what is the largest earthquake we need to worry about?" Williams said. "We have record in the recent past of a 5.6 quake in Maryville, but could we have, say a 6.5, a 7.5, or even an 8?" Scientists say that despite the similarities between the New Madrid Seismic Zone and East Tennessee Seismic Zone, there are a number of important differences, most notably that East Tennessee lacks any known earthquakes with magnitude of 6.0 or larger. "We believe earthquakes reoccur at intervals of 500 to 1,000, maybe 2,000 years," Williams said. "We've never had a catastrophic earthquake like the ones that occurred in Charleston (S.C.) in 1886, or New Madrid in 1811, but our records only go back 200 years. "The alarmist possibility right now is that we're ready for the next big one. I don't subscribe to that. On the other hand, could the East Tennessee Seismic Zone be incapable of generating over a 6? That's the conservative theory." Unlike most earthquake areas, which occur along the shifting boundaries of large crustal plates that make up the outermost shell of the earth, the East Tennessee Seismic Zone lies in the interior of a plate, making it an intraplate seismic zone. Geologists believe earthquakes in the East Tennessee Seismic Zone originate along ancient faults located deep in the bedrock -- so deep, in fact, that they're extremely difficult to study and predict. "Earthquakes in this area present a lot of challenges because they're not directly related to surface geology," Williams said. "In most earthquake zones you can see the consequences. In California you can stand on the San Andreas Fault at Point Rey where the 1906 quake actually originated. And in the Mississippi Valley you can see surface deformities such as sand blows and sand volcanoes. "We're still looking for surface features associated with earthquakes (in East Tennessee). In the meantime, we try to figure out clever ways to figure what's 5 to 25 kilometers beneath the surface. "If we had $50 million, we might drill a hole to half that depth, and even that wouldn't give us the answers. |
 
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