Central Regional Earthquake Risk in the U.S.

 December 16, 1811, residents of the small town of New Madrid, Missouri, where awakened by the roar and violent shaking of a large earthquake. It was the first of three large quakes, estimated to be between magnitude 7.5 and 8.0, that shook the area over two months, the last large one striking February 7, 1812. Some sources estimate the highest magnitudes to be 8.7 to 9.2.

Residents of the area reported cracks opening, the ground rolling in visible waves, and large areas of ground rose or sunk. Locations as far away as Charleston, South Carolina, and Washington, D.C. reported damage (New Madrid Seismic Zone). The December 1811 quake “rang church bells in Boston, Massachusetts, 1,000 miles away” (Insurance Information Institute, Background on: Earthquake insurance and risk). There were reports of the Mississippi River flooding, and even running backwards in some locations (Arkansas Department of Emergency Management, Earthquake). Other accounts saw the river rise “like a great loaf of bread to the height of many feet,” and fissures opening with geysers of sand, water, mud, and coal (The Great Midwest Earthquake of 1811). 

The following image is  an isoseismal map depicting estimated earthquake intensities throughout the central and eastern United States of the December 16, 1811 earthquake that struck at 08:15 UTC. Image is public domain, retrieved from https://www.usgs.gov/media/images/1811-1812-new-madrid-earthquakes-isoseismal-map, abridged from Seismicity of the United States, 1568-1989 (Revised), by Carl W. Stover and Jerry L. Coffman, U.S. Geological Survey Professional Paper 1527, United States Government Printing Office, Washington: 1993.

Biggest Central Earthquake Risk

The biggest threat to the Central U.S. is from the New Madrid Fault Zone. It stretches along the central Mississippi valley, extending from northeastern Arkansas to southeastern Missouri, out to western Tennessee, then Kentucky, and up to Illinois. 

Until a few decades ago, geologists assumed the 1811-12 ruptures were one-time events. Excavations of sand blows have discovered evidence of previous large earthquakes. Outside of the seismic zone other faults have been discovered including near Memphis, Tennessee, Commerce, Missouri, and in other places. All of the faults have evidence of having been active in the past few thousand to millions of years. As a result, scientists have warned these quakes were not freak occurrences.

The biggest threat to the Central U.S. is from the New Madrid Fault Zone

 

 Back in 1811 the population in St. Louis was 5,700. Now there are 11-12 million people in the St. Louis-Memphis region. With damage occurring over 232,000 square miles in the 1811-12 earthquakes, a similar event today would have catastrophic consequences.

Unlike the ground on the west coast, earthquakes in central and eastern states are felt over much larger areas. This is due to the geology of the area, where the softer soils allow for the seismic waves to travel further.

Central Earthquake Risk - New Madrid Seismic Zone

East of the Rocky Mountains, the New Madrid Seismic Zone is the most seismically active area. Over the next 50 years, there is a 7-10 percent probability of a magnitude 7.0 or larger earthquake. The likelihood of a 6.0 or larger is between 28 and 46 percent (Arkansas Department of Emergency Management, Earthquake).

From seismic activity, geologists have also learned there are several thrust faults are in the area, from Marked Tree, Arkansas, to Cairo, Illinois. Unlike the west, where many faults are visible, faults in the central (and eastern) U.S. are buried deep. Not knowing exactly where the faults are, they are categorized as seismic (or fault) zones. Besides the New Madrid zone, the Wabash Valley Seismic Zone can also produce damaging earthquakes. 

More information about the potential damage from a large earthquake in the New Madrid Seismic Zone can be found under the various states and cities in that area in my book. I'll also be posting that information in the future.

The following image shows the New Madrid and Wabash Valley Seismic Zones as the orange patches. The red circles are magnitude 2.5 or larger earthquakes that occurred from 1974 to 2002. Green circles are earthquakes prior to 1974. The larger the circle, the bigger the quake. Image from USGS, public domain, from https://www.usgs.gov/media/images/map-new-madrid-seismic-zone

Central Earthquake Risk - Wabash Valley Seismic Zone

North of the New Madrid Seismic Zone, in Southeastern Illinois and Southwestern Indiana, is the Wabash Valley Seismic Zone. On April 18, 2008, a magnitude 5.2 quake hit near Mt. Carmel, Illinois, and it was felt in at least 16 states. There were no injuries or fatalities, but it demonstrated the area is at risk for earthquakes. A smaller, magnitude 4.6, quake occurred several years earlier, on June 18, 2002 near Evansville, Indiana.

The Wabash Valley Seismic Zone is capable of magnitude 7.0 earthquakes. Geologists have found prehistoric evidence of liquefaction and sand dikes, and an estimated magnitude 7.1 earthquake created one formation. If a similar event happened today, there’d be widespread damage and destruction.