Yesterday, a magnitude 3.8 earthquake shuddered across west central Indiana just north of Terre Haute.

Reports arrived from locations as far east as Peoria, Illinois, and as far north as Crown Point, Indiana. Although geologists classify a magnitude 3.8 earthquake as Minor (felt by many, but resulting in little or no damage), it was jarring nonetheless. After all, the Midwest is about as far away as you can get from the terrifying potential of California’s San Andreas Fault. The Midwest worries about tornadoes and flooding, not earthquakes.

But we should worry…or at least be wary. The Midwest has not one, but TWO seismic zones right in our backyard: the Wabash Valley Seismic Zone and the New Madrid Seismic Zone. Both are very much active.

The New Madrid Seismic Zone

The most famous, or more correctly the most infamous, is the New Madrid Seismic Zone (abbreviated to NMSZ). Named after the small city of New Madrid, Missouri, this massive fault zone tears through five states and potentially threatens eight: Illinois, Oklahoma, Tennessee, Arkansas Mississippi, Missouri, Kentucky…and Indiana. This area has produced frequent, but small-ish earthquakes for centuries, but it was four dark months two centuries ago that has geologists concerned.

Between December 1811 and February 1812, a series of cataclysmic earthquakes rattled the Midwest, affecting over one million square miles of the continental United States. Estimates of its strength and frequency vary slightly, but most geologists agree that in less than three months, four major earthquakes struck the NMSZ, each ranging between magnitude 7.2 to 8.2, framed by numerous aftershocks.

At the time, the American population resided primarily in the Eastern United States, but enough settlers and Native-Americans were present in the Midwest to provide hundreds of written accounts, including this frightening journal entry by Missouri settler Eliza Bryan…

On the 16th of December, 1811, about two o’clock, a.m., we were visited by a violent shock of an earthquake, accompanied by a very awful noise resembling loud but distant thunder, but more hoarse and vibrating, which was followed in a few minutes by the complete saturation of the atmosphere, with sulphurious [sic] vapor, causing total darkness. The screams of the affrighted inhabitants running to and fro, not knowing where to go, or what to do—the cries of the fowls and beasts of every species—the cracking of trees falling, and the roaring of the Mississippi— the current of which was retrograde for a few minutes, owing as is supposed, to an irruption in its bed— formed a scene truly horrible.

~Eliza Bryan, Territory of Missouri

The cataclysmic earthquake coincided with the appearance of the Great Comet of 1811. Both imprinted themselves in Native-American folklore across the continent. The most famous interpretation came from Tecumseh, the Shawnee chief that created a confederacy of Native-American tribes to combat the intrusion of pioneers in the American West (at the time, the Midwest WAS the West). He and his people took it as a positive omen, portending their victory against the settlers. Obviously, it didn’t work out that way.

Geologists aren’t just interested in the New Madrid Seismic Zone for its role in American history or Native-American mythology. What had then been thinly-populated prairies and forests of the unexplored continent are now home to millions of American citizens, along with some of its largest cities. In 2011, to mark the bicentennial of the New Madrid earthquake, AIR Worldwide (Applied Insurance Research), a Boston data analysis firm specializing in catastrophe and risk modeling, projected a cost range of $40-400 billion if it occurred today, with a “best data” estimate of $110 billion.

Let me step back a moment here: In an effort to bring you more practical data, I read through the 2009 report “Impact of New Madrid Seismic Zone Earthquakes on the Central USA” compiled by the Mid-America Earthquake Center at the University of Illinois. I read through the damage and fatality projections and then didn’t sleep most of the night. Feel free to follow the embedded link above if you’d like to learn why, but I suggest you don’t. Some things are better left unknown.

It earthquake like that would be very, very bad.

Should you be worried? Yes and no. Yes, because the United States Geologic Service reported “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.” That’s better odds than you winning a $1000 lottery scratch card.

You also needn’t worry too much. While the American population might not be level-headed at anticipating or reacting to unexpected disasters (See COVID-19), seismologists and geologists are. You can be sure that as you read this, hundreds of well-educated eyes are paying close attention to the New Madrid Seismic Zone, ready to alert the populace at a moment’s notice.

The Wabash Valley Seismic Zone

The June 17th earthquake originated in the Wabash Valley Seismic Zone, a line of faults one running almost 200 miles up the southwestern spine of Indiana, with its hypocenters buried miles under the surface (the surface directly above a hypocenter is the epicenter).

Although technically considered its own seismic zone, the Wabash Valley Zone’s proximity to the NMSZ has some geologists believing the two might be related. The most recent major earthquakes likely originating in the Wabash Valley Zone were the 1968 Illinois Earthquake (magnitude 5.4) and the 2008 Illinois Earthquake (magnitude 5.2).

While neither earthquake resulted in fatalities and caused only slight damage, they concerned seismologists. In geologic time, those earthquakes were basically back-to-back. While a magnitude 5 earthquake might all shake and no substance, it’s not insubstantial either, and anything stronger could result in significant damage or injuries. The deep location of the Wabash Valley Zone hypocenters also makes them much more difficult to study.

Like the New Madrid Zone, the older, denser bedrock of the Wabash Valley Zone also makes it more dangerous. Young bedrock, like that on the West Coast, slows down and weakens the seismic wave of an earthquake, but the older, harder bedrock of the Midwest allows waves to travel faster without a dramatic loss of energy*.

*To illustrate this, try a fun experiment: take any stick and slap a pile of gravel, then take the same stick and slap a steel pot. Which one is louder? 

Our best defenses against seismic catastrophe are education, attention, and preparation. By reading this humble article, you’ve learned a little something about these seismic zones. If you were in Indiana yesterday, might have gained the best education of all—firsthand knowledge. By taking earthquakes seriously, I don’t think attention will be an issue. The network of emergency alerts on our smartphones makes inattention almost impossible. The final step is preparation.

The Federal Emergency Management Agency (FEMA) has provided a wealth of earthquake resources on its website HERE. No matter how you feel about that agency or the government, FEMA offers sound advice, especially when it comes to preparing your home and family for disasters.

With earthquakes, like meteorites or contagious diseases, it’s never a question of IF but WHEN. Some day, there will be a cataclysmic seismic event in the Midwest. It could be next year or it could be five centuries from now, but it will happen.

Want to Know More? 

Learn more about the New Madrid Zone in this short but informative packet “Earthquake Hazard in the New Madrid Seismic Zone Remains a Concern” published by the Department of the Interior in 2009. If you’re REALLY interested, you’ll find the 255-page 2014 update HERE.

Examine the USGS data on the June 17th, 2021, Indiana earthquake HERE. This page provides a treasure trove of raw data for science junkies, geology enthusiasts, or conspiracy theorists, anyone concerned about the next Big One.