When people think of earthquakes in the United States, they tend to think of the west coast. But earthquakes also happen in the eastern and central U.S. Until 2014, when the dramatic increase in earthquake rates gave Oklahoma the number one ranking in the conterminous U.S., the most seismically active area east of the Rocky Mountains was in the Mississippi Valley area known as the New Madrid seismic zone. Since 1974, seismometers, instruments that measure ground shaking, have recorded thousands of small to moderate earthquakes. The faults that produce earthquakes are not easy to see at the surface in the New Madrid region because they are eroded by river processes and deeply buried by river sediment. A map of earthquakes epicenters, however, reflects faulting at depth and shows that the earthquakes define several branches of the New Madrid seismic zone in northeastern Arkansas, southwestern Kentucky, southeastern Missouri, and northwestern Tennessee. Other relatively young faults, which are not necessarily associated with recent earthquakes, or the main seismicity trend in the New Madrid region, are shown in this map. It shows 20 localities where geologists have found and published their findings on faults or evidence of large earthquakes (from sand blows; see image to the right).
In the winter of 1811 and 1812, the New Madrid seismic zone generated a sequence of earthquakes that lasted for several months and included three very large earthquakes estimated to be between magnitude 7 and 8. The three largest 1811-1812 earthquakes destroyed several settlements along the Mississippi River, caused minor structural damage as far away as Cincinnati, Ohio, and St. Louis, Missouri, and were felt as far away as Hartford, Connecticut, Charleston, South Carolina, and New Orleans, Louisiana. In the New Madrid region, the earthquakes dramatically affected the landscape. They caused bank failures along the Mississippi River, landslides along Chickasaw Bluffs in Kentucky and Tennessee, and uplift and subsidence of large tracts of land in the Mississippi River floodplain. One such uplift related to faulting near New Madrid, Missouri, temporarily forced the Mississippi River to flow backwards. In addition, the earthquakes liquefied subsurface sediment over a large area and at great distances resulting in ground fissuring and violent venting of water and sediment. One account of this phenomena stated that the Pemiscot Bayou "blew up for a distance of nearly fifty miles."
After the earthquake [of 1811-1812] moderated in violence, the country exhibited a melancholy aspect of chasms, of sand covering the earth, of trees thrown down, or lying at an angle of forty-five degrees, or split in the middle. The Little Prarie settlement was broken up. The Great Prarie settlement, one of the most flourishing before on the west bank of the Mississippi, was much diminished. New Madrid dwindled to insignificance and decay; the people trembling in their miserable hovels at the distant and melancholy rumbling of the approaching shocks.
The New Madrid seismic zone is located in the northern part of what has been called the Mississippi embayment. The Mississippi embayment is a broad trough filled with marine sedimentary rocks about 50-100 millions years old and river sediments less than 5 millions years old. The upper 30 meters of sediment within the embayment includes sand, silt, and clay deposited by the Mississippi, Ohio, St. Francis, and White Rivers and their tributaries over the past 60,000 years. Wisconsin valley train deposits formed during the glacial period from 10,000-60,000 years ago, and the Holocene meander belt deposits were laid down during the past 10,000 years.
The Mississippi embayment is underlain by Paleozoic sedimentary rocks up to 570 millions years old. The Paleozoic rocks are underlain by even older rocks that appear to have been deformed about 600 million years ago when the North American continent almost broke apart. During the process of continental rifting, a deep valley formed that is bounded by faults and known as the Reelfoot rift. The Reelfoot rift is identified today as a subsurface system of fractures and faults in the earth's crust. New Madrid seismicity is spatially associated with the Reelfoot rift and may be produced by movement on old faults in response to compressive stress related to plate motions.
The most obvious effects of the 1811-1812 earthquakes are the large sandy deposits, known as sand blows, resulting from eruption of water and sand to the ground surface. This phenomenon called earthquake-induced liquefaction is the process by which water-saturated, sandy sediment temporarily loses its strength due to the buildup of water pressure in the pores between sand grains as seismic waves pass through the sediment. If the pore-water pressure increases to the point that it equals the weight of the overlying soil, the sediment liquefies and behaves as a fluid. The resulting slurry of water and sediment tends to flow towards the ground surface along cracks and other weaknesses. Overlying soil "floating" on liquefied sediments moves down even gentle slopes, causing fissuring and lateral and vertical displacements. This type of landslide known as lateral spreading is commonly responsible for damage to infrastructure (bridges, roads, buildings) during major earthquakes.
During the 1811 and 1812 earthquakes, liquefaction and resulting lateral spreading was severe and widespread. Sand blows formed over an extremely large area about 10,400 square kilometers. Effects of liquefaction extended about 200 km northeast of the New Madrid seismic zone in White County, Illinois, 240 km to the north-northwest near St. Louis, Missouri, and 250 km to the south near the mouth of the Arkansas River. In the New Madrid region, sand blows can still be seen on the surface today. In the past, the sand blows were attributed to the 1811-1812 earthquakes. We now know that some of the sand blows pre-date 1811 and formed as the result of prehistoric New Madrid earthquakes.
In the New Madrid seismic zone, many sand blows appear as light-colored sandy patches in plowed fields. Flood deposits bury other sand blows. Viewed from above, sand blow have circular, elliptical, and linear shapes and can range up to tens of meters in width and hundreds of meters in length. Viewed in cross-section or in excavations and riverbanks, sand blows commonly take the form of large lenses 1 to 2 m in thickness. Sand blows composed of several layers that fine upward from coarse sand to silt and capped by clay probably formed as a result of multiple earthquakes. Sand blows usually contain clasts, pieces of underlying deposits and soil horizons ripped from the dike walls as the liquefied sand erupted to the surface.
The lower Mississippi River Valley was a fertile homeland to Native Americans from about 9500 B.C. to 1670 A.D. The presence of Native Americans is still evident today in the occasional mound not yet destroyed by modern agricultural practices and the abundant potsherds, lithic tools and points, and bone fragments found in plowed fields and river and ditch cutbanks. Most artifacts encountered during studies of New Madrid sand blows are from the Woodland and Mississippian cultures, which thrived from about 200 B.C. to 1000 A.D. and 800 to 1670 A.D., respectively. Both cultural periods are subdivided into early, middle, and late intervals. Woodland ceramics are characterized by grog (ground up potsherds or fired clay) and sand tempering; whereas, Mississippian ceramics are characterized by shell tempering.
Although there are uncertainties regarding their age ranges, certain pottery and point types, as well as plant remains, are considered diagnostic of various cultural periods. For example, Bell Plain, Campbell Appliqué, and Parkin Punctate pottery and Nodena points are diagnostic of the Late Mississippian period; Old Town Red pottery and Madison points are diagnostic of the Middle Mississippian period; Varney Red Filmed pottery is diagnostic of the Early Mississippian period; and Barnes pottery and Table Rock stemmed points are diagnostic of the Late Woodland period. Zea maize, or corn, became dominant in the Native American diet about 1000 to 1050 A.D. and is as an important temporal marker in the region.
Archeology has played an important role in recognizing and dating prehistoric earthquake-induced liquefaction features in the New Madrid region. Sand blows found below Native American mounds and occupation horizons no doubt formed prior to 1811 because few Native Americans lived in the area after the 17th Century. Diagnostic artifacts found in association with sand blows provide a preliminary estimate of the age of the causative earthquake. Detailed investigations can further constrain the age of the event. For example, artifacts in an occupation horizon buried by a sand blow can provide an estimate of the maximum age of the liquefaction feature; whereas, artifacts in an horizon developed in the top of a sand blow can provide an estimate of its minimum age. Similarly, plant remains and other organics found in cultural horizons can be used to date associated sand blows. Radiocarbon dating of plant remains is the most commonly used dating technique in paleoseismology. It is preferable to have radiocarbon dates from both overlying and underlying horizons to bracket the age of the sand blow.
Paleoseismology is the study of the timing, location, and magnitude of prehistoric earthquakes preserved in the geologic record. Knowledge of the pattern of earthquakes in a region and over long periods of time helps to understand the long-term behavior of faults and seismic zones and is used to forecast the future likelihood of damaging earthquakes. In eastern North America, where near-surface faulting is uncommon or difficult to identify, paleoseismology often employs liquefaction features to learn about prehistoric earthquakes. Earthquake-induced liquefaction features are distinctive and form as the result of strong ground shaking.
Liquefaction features include sand blows, dikes, and sills. Sand blows are deposits that form on the ground surface as the result of venting of water and sand. Sand dikes are sediment-filled cracks through which water and sand flowed. Sand sills usually take the form of lenses intruded below clay layers and are connected to sand dikes. Most large earthquakes around the world have induced liquefaction.
Over the past decade, paleoseismic studies have begun to unravel the earthquake history of the New Madrid seismic zone. Studies focusing on earthquake-induced liquefaction features utilized archeology and radiocarbon dating to estimate the ages of liquefaction features, and thus, the timing of the earthquakes that caused them. In this way, sand blows across the New Madrid region were found to have formed during earthquakes about 1450 A.D., 900 A.D., 300 A.D., and 2350 B.C.
In addition, the size and spatial distributions of historic and sand blows that formed about 1450 A.D. and 900 A.D. were determined to be strikingly similar to each other, suggesting that the prehistoric earthquakes had similar locations and magnitudes to the 1811-1812 earthquakes. Furthermore, sand blows attributed to the 1450 A.D., 900 A.D., and 2350 B.C. earthquakes are composed of multiple, fining upward layers similar in thickness to those that formed in 1811-1812. These observations support the interpretation that the prehistoric events were similar in location and magnitude to the 1811-1812 earthquakes and also suggests that they too were earthquake sequences. Paleoseismic studies concluded that the New Madrid seismic zone generated magnitude 7 to 8 earthquakes about every 500 years during the past 1,200 years.