It could happen again. The faults are still there, running a serpentine course for about 150 miles, from Marked Tree, Arkansas, through southeastern Missouri, to Cairo, Illinois, just as they have for millennia. The faults are known as the New Madrid Seismic Zone (NMSZ), and Rob Williams, a geophysicist with the US Geological Survey (USGS), says they constitute—along with faults in Oklahoma—one of the two most active seismic areas in the United States east of the Rocky Mountains.

New Madrid Earthquakes
Note: Modern city locations and state boundaries shown. Sources: Magnitude and time of earthquakes from U.S. Geological Survey, Earthquake Hazards Program, “New Madrid 1811-1812 Earthquakes.” Perceived shaking information from Ohio W. Nuttli, “The Mississippi Valley Earthquakes
of 1811 and 1812” (1973); and U.S. Geological Survey

Geologists have evidence of major quakes here dating back to 300 ad. A few hundred quakes of magnitude 2.0 and larger, have occurred in southeast Missouri since the early 1990s, extending from the Bootheel in the south to the center of the state. Rob, who is also a coordinator for the USGS Earthquake Hazards Program east of the Rockies, says the NMSZ experiences more than 200 earthquakes a year—an average of more than one every two days. Most are small, or microseismic, and generally not felt above the surface. However, once or twice every 18 months, they are large enough to shake southeastern Missouri’s taller buildings. A series of aftershocks usually follow quakes of any magnitude.

What Are The Chances?

The ability to scientifically evaluate an earthquake at the time of the devastating New Madrid quakes of 1811–12 was virtually nonexistent. Despite the tremendous advances in geological science over the past two centuries, it is still impossible to predict the zone’s future activity. USGS scientists suggest that the likelihood of another earthquake over the next 50 years with the magnitude of the 1811–1812 New Madrid quakes is around 7 to 10 percent. The probability of quakes exceeding a 6.0 magnitude is roughly between 25 and 40 percent. These figures, however, come with the caveat that forecasting earthquakes is still an uncertain science.

Rock Hard

An earthquake in Middle America would be felt over an area roughly 10 times larger than an earthquake of the same magnitude in California. That’s because the rocks that comprise a part of the earth’s crust in the central part of the United States are harder, cooler, and less fragmented than those in the western states; these rocks would transmit the shock farther.

Damage Would Be Worse Today

The damage caused by the cataclysmic New Madrid quakes of 1811–1812 would not compare to the destruction wreaked by a similar event in the 21st century.

At the time, only two Missouri settlements were in the worst zone of the quakes: Little Prairie and New Madrid. The structures then were mostly low, one-story log houses and barns, fairly flexible and resilient.

Today, the area’s tall, narrow buildings—many of which were constructed before considering earthquake resistance—are the structures of greatest concern, especially when they stand within close proximity to one another. Both St. Louis and Memphis lie within the New Madrid Seismic Zone, making the risk much more drastic for the older tall stone, brick, and concrete buildings and their dense populations. Earlier structures containing large open spaces such as auditoriums, arenas, churches, atria, schools, factories, and hospitals are also at risk.

Not all earthquake destruction is wrought on man-made structures. Because the Bootheel is primarily rural, the greatest disruption, both immediate and long term, would be to agriculture, brought about by damage to the earth itself.

Seismic Scales

The US Geological Survey monitors seismic activity in cooperation with a number of other agencies and organizations. USGS seismologists and geologists work closely with the University of Memphis Center for Earthquake Research and Information to oversee a series of more than 50 seismometers located in affected states. They coordinate data from various sources to determine magnitude, location, and position relative to known faults.

Richter Scale

A one-point increase on the Richter magnitude scale indicates a 10-time jump in an earthquake’s energy release. Therefore, an earthquake measuring 8.0 is not double the scope of a 4.0, but rather demonstrates a 10,000-times greater release of energy— roughly the difference in size between a basketball and a hot-air balloon. Seismologists rarely use the Richter scale and now employ the Moment Magnitude, which is similar, however, to the Richter in its continuum of magnitude values.

 

Richter Earthquake Magnitude Scale and Classes

Mercalli Intensity Scale

The Mercalli Intensity Scale gives a picture of the range of actual damage the quake does to the physical world around us: how many buildings were damaged, how much damage was inflicted, and so on. The measured intensity often varies depending on how far from the quake the measurements are taken, but it can also vary depending on the firmness of the geologic deposits below the measured site. The Mercalli Scale is one of the only methods available with which to measure the actual size of a historical quake, one that occurred before the introduction of instruments. The results, however, are not always accurate.

How to Prepare

Missourians need to educate themselves, says Jeff Briggs, earthquake program manager at Missouri’s State Emergency Management Agency. “You can’t learn what to do when the siren goes off,” he advises. “You have to know it well in advance.” The information is out there, with several state and federal agencies making it available and user-friendly for Missourians. For a list of websites with good information, visit MissouriLife.com.

The settlers in 1811 had no idea of the chaos they would face; we do.

Crowd-sourcing quakes

Should a quake occur, the US Geological Survey employs a software program called “Did-You-Feel-It,” to alert first responders to the areas most in need of immediate aid. People who have been affected by the shaking of the earthquake can tap into the program and answer several questions on the nature and intensity of their experience. As many as 100,000 people respond in a one-hour period, whereupon scientists build a map within minutes based on the input. The map is then distributed to police, firemen, and other emergency management facilities, telling them where to respond first and fastest.

To Insure or Not to Insure

Missouri is the third-largest earthquake insurance market in the country. Earthquake insurance is not your standard policy. While fire and water damage due to burst water or gas pipes is generally covered in most homeowners’ policies, the damage to a home that is directly impacted by a quake is not. A separate earthquake policy is required for both a home and its contents. According to Missouri Department of Insurance Director Chlora Lindley-Myers, “Knowing what to do during an earthquake is very important; but it is also important to protect your financial assets should you need to rebuild or recover. Missourians should add earthquake coverage to their homeowners’ policy if they live in areas that may be impacted by an earthquake.”

Illustrations by Andrew Barton, top photo courtesy of Brad Helmink