It’s hard to believe that the most powerful earthquake in the “Lower 48” was in Missouri! They were 8.0 shakers near New Madrid in 1811 and 1812. This area is about 100 miles southeast of Taum Sauk. The area was sparsely populated at the time and so there was little loss of life. But among its features was making the Mississippi River flow backwards. Scientists are certain another earthquake of that magnitude will occur again. When it happens it will be unimaginably devastating for a region that does not consider itself earthquake country or takes suitable precautions. It is likely to cause major devastation up and down the Missississippi and Ohio Rivers with catastrophic devastation in St. Louis and Memphis. Cincinnati would feel the effects of Class 5 quake and Chicago would get a Class 4.
A major quake at New Madrid will actually be felt in a much bigger area than the traditional quake zones in the West because the soil is porous (while the west is rocky). The Virtual Times has the best site on New Madrid and its hazards. The USGS also has an excellent New Madrid site including a dramatic graphic in show on earthquakes affect a huge area while comprable west quakes affects relative tiny areas. There are about 150 quakes a year on the New Madrid fault.
The area is very complex geologically. The St. Francois Mountains (of which Taum Sauk is part) at 1.4+ billion years is said to be the oldest mountain range in North America and possibly on the planet! (by contrast the Applachians are 460 million years old and the Rockies 70 million years). It is of volcanic origin which is a rarity in the region. It also has the distinction of being the only mountain range never covered by glaciers or the ocean (ocean fossils can even be found on Everest). New Madrid by contrast was at the southern tip of the last glacier to cover North America and scientists believe that may have contributed to the instability. Plus just to make things interesting the Gulf of Mexico at one time reached it. Scientist say that water from wells in the area may be a million years old! Answers.com has an excellent resource on the Francois mountains.
Scientists can’t 100% explain why New Madrid is so active. Earthquakes along the coasts such as California and Alaska are the result of the movement of the continental plates. The most active other inland earthquake region in the “Lower 48” is “downstream” from the Yellowstone Supervolcano and that has led to dramatic uplifting of Idaho’s Borah Peak with last big quake there being in 1983.
New Madrid was actually the northern point of a collision of 286 to 320 million years ago between the North American Plate and the South American plate when the Gulf of Mexico was replaced by uplifted mountains such as the Ouachitas in Arkansas (and this also explains why mountains in this region run east-west while most mountains in the United States run north-south). At the conclusion of this collision, a rift developed that could have split North America in two along the Mississippi River (this rift is known as the “Reelfoot Rift” — Reelfoot Lake in Tennessee was formed by the 1811-12 quakes). Scientists say the rift is not contiuing although New Madrid remains active. Rockhoudingar.com has the best explanation I’ve seen of putting this geological history into the broad picture.
Scientists are uncertain when the next quake will occur. Iben Browning scared the Midwest when he predicted a Magnitude 7.0 quake would hit the fault on Dec. 3, 1990. Browning who was a zoologist and biologist but not a geologist believed the quake would occur because of a combination of tides and alignment of the sun and moon. He had argued that he had predicted many major quakes before (although I can’t find them at the moment) and so he was taken seriously enough that businesses and schools shut on the day. When the quakes did not occur there has much debate about whether he was a huckster. Browning was 71 at the time of the prediction and I do not know if he is alive now. About.Com has an article on the flim flam world of earthquake prediction. This course outline from the University of Michigan also gives you good background. It is important to note that although was wrong in making the specific prediction, he is not wrong on nature of the threat and the USGS has an extensive free publication (Circular 1083) saying this.
Some scientists say the 1811-12 quakes were a once in 500 or 1,000 year event. Scientists believe the previous quakes of similar magnitude occurred in the mid-1400’s and the 900’s. There have been quakes registering 6.0 or greater since 1811 in the Midwest. The strongest quakes on the fault since the event were Marked Tree, Ark., in 1843 (magnitude 6.3), and the northeastern end near Charleston, Mo., in 1895 (magnitude 6.6). There were no quakes greater than 6.0 during the 20th century.
One other thing of interest about the zone is that it lacks virtually any identifable scarps or fault signs on the fault jumps back and forth across the Mississippi River starting at the confluence of the Ohio and Mississippi at Cairo, Illinois, zigzagging to New Madrid, Missouri, on to Ridgely, TN, Caruthersville, MO, and winding up at Marked Tree, Arkansas.
The image below shows the 3.0 or greater quakes. Clicking on it will take you to more detail on the quakes.
Recent 3.0+ Quakes in Midwest
(Break out of frame – STRONGLY recommended)
Recent Quakes in Central US
(Break out of Iframe – STRONGLY recommended)