The Greater St. Louis area is located at the northernmost tip of the New Madrid Seismic Zone, also known as the New Madrid Fault Line. It is a major seismic zone and a major cause of intraplate earthquakes (NOTE: you may want to formally define “intraplate”) in the Midwest and Southern United States. This is why earthquake insurance is so important in the St. Louis area.
The New Madrid Seismic Zone
The New Madrid Seismic Zone produced the 1811-12 New Madrid Earthquakes, three of the most powerful earthquakes ever recorded in North America, and still has the potential to produce even larger earthquakes than those in the future. While no scientific instruments were available at the time to properly measure those earthquakes, historical eyewitness accounts and analyses of the present-day landscape have led seismologists to estimate that these earthquakes ranged between 7.3 and 7.5 magnitudes on the Richter Scale.
For comparison purposes, the 2011 earthquake in Japan that produced the devastating tsunami registered 9.1 magnitude. The Alaskan Earthquake of 1964 was also a 9.1 – the largest recorded earthquake in the United States. Meanwhile, the infamous 1989 earthquake in San Francisco that interrupted the World Series measured at 6.9 magnitude.