1,000 Earthquakes in Arkansas Since Last September
April 14, 2011 • 9:32AM

Since last September nearly 1000 earthquakes have occurred in the state of Arkansas which is part of the New Madrid Seismic Zone. Two of these earthquakes registered above a magnitude of 4. This includes a magnitude 4.7 quake on Feb. 27, the largest quake to hit the state in 35 years, and an earlier magnitude 4 on Feb. 15.

The New Madrid Seismic Zone, which covers four states along the Mississippi River, has repeatedly produced major earthquakes, including several of magnitude 7 to 8, over the past 4,500 years, most recently in 1811-12. The key evidence for large earthquakes that occurred in the past are sand blows that formed when underground sand and water erupted to the surface as a result of violent shaking. In addition to the quake in 1811-12, large, widespread sand blows were produced from previous large earthquakes around A.D. 1450, A.D. 900, and 2350 B.C.

The New Madrid Seismic Zone is the most seismically active area of the United States east of the Rockies. The New Madrid region is located in the middle of the vast North American tectonic plate.

According to the U.S. Geological Survey: "The 1811-12 earthquakes caused many types of ground failures, including landslides along the Mississippi River bluffs from Mississippi to Kentucky. Ground failures also included lateral spreading and ground subsidence by soil liquefaction across the Mississippi River flood plain and along tributaries to the Mississippi River over at least 15,000 square kilometers. Today, a repeat event could be expected to produce similar effects in northeastern Arkansas, southeastern Missouri, western Tennessee and Kentucky, and southern Illinois. Roadways in the Mississippi Valley of Arkansas and Missouri (such as Interstate 55) could become impassable because of bridge failures and fissuring of road surfaces. Venting of large quantities of water, sand, and mud as a result of liquefaction could flood fields and roads and disrupt agriculture for weeks to months. Flooding of farmland, where agricultural chemicals are stored onsite, could contaminate rivers and streams. Failure of levees, especially during high water, would contribute to flooding, and failures of riverbanks could make the Mississippi River and its tributaries difficult to navigate for many weeks.

"The City of Memphis and the surrounding metropolitan area of more than one million people would be severely impacted. Memphis has an aging infrastructure, and many of its large buildings, including unreinforced schools and fire and police stations, would be particularly vulnerable when subjected to severe ground shaking. Relatively few buildings were built using building codes that have provisions for seismic-resistant design. Soil liquefaction and related ground failures are likely to occur in downtown Memphis along the Mississippi River and along the Wolf River that passes through Memphis. The older highways and railroad bridges that cross the Mississippi River, as well as older overpasses, would likely be damaged or collapse in the event of a major New Madrid earthquake. Some of the bridges and pipelines crossing the Wolf River might be damaged or destroyed. Although Memphis is likely to be the focus of major damage in the region, St. Louis, Mo., Little Rock, Ark., and many small and medium-sized cities would also sustain damage."

Tonight the Seismological Society of America will be holding a town meeting on the subject in Memphis, Tennessee. Also, two earthquake drills are scheduled to take place in the area. The Great Central U.S. Shakeout Earthquake Drill is scheduled for April 28, 2011, and the National Level Exercise 2011 (NLE 2011), scheduled for May 16-20.