A "major" earthquake, classified as registering 7.5 or higher on the Richter scale, strikes the New Madrid seismic zone about every 200 to 300 years. The last major earthquakes to rock the region occurred in 1811 and 1812. During a five-month stretch, more than 2,000 tremors were noted -- five of which were estimated to top 8.0 on the Richter scale -- and 18 shockwaves reached the Eastern seaboard, some with enough force to cause church bells to ring.
Perhaps the most famous anecdote associated with the 1811 and 1812 earthquakes is that some of them caused the Mississippi River to flow backward.
Fortunately, in the early 19th Century modern-day Missouri was considered "out west," and this region of America was sparsely populated.
Today, a rupture of the New Madrid fault on the magnitude of the 1811-12 earthquakes could leave large parts of St. Louis and Memphis, Tennessee, uninhabitable. According to a 2010 study that the University of Illinois conducted and the Federal Emergency Management Agency commissioned, a 7.7-magnitude earthquake in this area could leave more than 7 million people homeless.
The cost of property damage would quickly soar to billions of dollars. Flooding along the Missouri and Mississippi Rivers would be likely, and parts of the central United States might be without power for weeks.
The FEMA study urges state and local governments to retrofit hospitals, fire stations, police stations, nuclear power plants and other essential facilities to improve their odds of holding up in a big earthquake.
The St. Charles County (Missouri) Division of Emergency Management puts the odds of another major New Madrid earthquake happening before 2040 at 25 percent.
On average, scientists record about 20 "events" in the New Madrid seismic zone registering more than 1.0 on the Richter scale each month.
A tremor strong enough to be felt (2.5 to 3.0 on the Richter scale) usually happens at least once a year.
A magnitude 5.0 or higher shock strikes about once per decade and generally causes damage in a multitude of states.
A "damaging" earthquake (6.0 or greater) occurs in this area about every 80 years, according to seismologists. The last one to hit the area was in 1895.
The 1811 and 1812 earthquakes that shook the New Madrid zone were several times more powerful than the infamous 1906 earthquake that left much of San Francisco in ruins.