I am a Geologist. I wanted to give the short answer as to why the New Madrid Fault complex is dangerous. And this IS the short answer.
- It is Still Active and always will be. Central-Cratonic faults never completely de-stresses.
- New Madrid quakes Destroyed the Pre-Colombian Mississippi Mound Builder Civilization.
- New Madrid quakes Destroyed the region in 1811-2 with aftershocks all the way through the Civil War.
- The New Madrid Fault Complex is around 9 different faultlines connected together, each capable of 7.5+ quakes. A quake on one shifts the stress to trigger a quake on another. They don’t all happen at once.
- Quakes are felt on the entire continent sharing what’s called the Cratonic Shield, which is basically continental shield rock that runs from the Rockies to the Atlantic down to Florida and Mexico, and all the way to the Arctic Ocean. So most of the USA’s Midwest, East, South and Canada. The South and Midwest will feel it most, but that whole shield will feel it.
- During the 1811-12 quakes church steeples swayed so much that their bells rang… in Boston. The whole East and middle of the country will feel these quakes, except the West which is isolated by very different geology and many intervening faultlines.
- Last time the ground reshaped so severely that it changed the course of the Mississippi River, making it flow backwards before it found its way around the new high ground.
- Shaking caused liquefaction along several hundred miles of the River.
- Sand geysers erupted 150 feet into the air, complete with coal and natural gas that caught fire. Sulfur dioxide gas (more poisonous than Cyanide) was also described in journals by survivors.
- Entire log cabins flipped over, sank into the ground, or both. In modern times, most levees, bridges, roads build across water saturated ground, and probably earthen dams are doomed in quakes over 7.5 magnitude.
- Shaking was described as over 3 minutes.
- Where you are affects what the shaking does. On hard rock you’ll feel it badly, however your house won’t sink into the ground. This is greatly complicated by things called Nodes, as you find with sound waves. This is why some buildings collapse and others near them are largely undamaged.
- What your home is built from matters. Unreinforced masonry (brick) fares BADLY in an earthquake. Stick lumber with foundation anchor bolts has the best survival in a quake, as shown in Western homes.
- In the Loma Prieta (Bay Area 1989) quake, many people with private wells saw them cut off due to ground shifting and had to be redrilled and recased and a new pump installed. Factor that cost in.
- Ground waves from that LP quake also destroyed underground water and sewer lines and the state funded and quietly taxed for the replacement which took 10 years. During that time millions were exposed to contaminated drinking water full of cholera since sewer and water lines are often close to each other under the streets. Those $$ billions are still being paid off as municipal bonds.
- The cost of repairing utilities and roads destroyed by multiple 7.5+ quakes affecting the Entire East Coast, South, and Midwest is $$ trillions. They will have to be done several times due to major aftershocks years later breaking them again. The repairs cost more than the country can pay. Giving up public utilities is a major step down in civilization and will lead to disease outbreaks.
- Unchecked flooding from ruptured levees and dams allows for many more mosquitoes, and malaria should be expected to make a comeback, along with cholera and typhoid fever and new infections like Dengue.
That’s the short version. For some idea of what severe ground shaking can do, see the Anchorage 1964 quake. Sincerely, – I.K.