The preponderance of evidence leads us to conclude that earthquakes can be expected in the future as frequently and as severely as in the past 4,500 years. The resulting slurry of water and sediment tends to flow towards the ground surface along cracks and other weaknesses. Many structures in Memphis, Tenn., St. Louis, Mo., and other communities in the central Mississippi River Valley region are vulnerable and at risk from severe ground shaking. Three earthquakes in this sequence had a magnitude (M) of 7.0 or greater. This assessment is based on decades of research on New Madrid earthquakes and related phenomena by d… 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. 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. The Reelfoot rift is identified today as a subsurface system of fractures and faults in the earth's crust. 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. 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. If we use the data on historical seismicity combined with the new information on recurrence of large earthquakes, and make the same assumptions that go into the National Seismic Hazard maps, we would estimate a 25-40% chance of a magnitude 6.0 and greater earthquake in the next 50 years and about a 7-10% probability of a repeat of the 1811-1812 earthquakes in the same time period.However, it is VERY important to note that these estimates alone do not include information about WHERE the earthquakes might occur and therefore what shaking might affect any given location. 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.Photograph of sand blow deposit and related feeder dike exposed in excavation. (Credit: Martitia Tuttle. Scientists at some workshops evaluated the New Madrid GPS results of the past 12 years and debated their meaning. 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. Flood deposits bury other sand blows. Diagnostic artifacts found in association with sand blows provide a preliminary estimate of the age of the causative earthquake. )When people think of earthquakes in the United States, they tend to think of the west coast. The experts did not find the GPS data to be a convincing reason to lower the assessment of earthquake hazard in the New Madrid region, especially in light of the many other types of data that are used to construct the hazard assessment, several of which are described here.There are historical accounts of major earthquakes in the New Madrid region during 1811-12. Public domain. Bringing back that encounter with those map’s in the 1960’s in a prophetic warning from The Lord. )The New Madrid seismic zone is located in the northern part of what has been called the Mississippi embayment. 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.
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. Both cultural periods are subdivided into early, middle, and late intervals.
Three earthquakes had a magnitude of 7.0 or greater. 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. One of the things we have learned is that coming up with probabilities is much more difficult than we used to think. New Madrid dwindled to insignificance and decay; the people trembling in their miserable hovels at the distant and melancholy rumbling of the approaching shocks.Woodcut by Henry Howe, from Historical Collections of the Great West (Cincinnati, 1854, p.239).
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… When they are, the USGS sometimes has held workshops to try to come to a consensus and at other times has announced our own internal consensus.