NEW MADRID COUNTY, Mo. — The Midwest’s most infamous earthquake zone was busy overnight, producing a handful of small earthquakes in less than nine hours.

Earthquake trackers recorded five earthquakes between 7:11 p.m. Monday and 3:40 a.m. Tuesday in northwest Tennessee, just east of the Missouri Bootheel.

The University of Memphis Center for Earthquake Research and Information documented the incidents as follows:

• 7:11 p.m. — 1.5 magnitude (south of Ridgely, Tenn.)
• 9:58 p.m. — 1.6 magnitude (south of Ridgely, Tenn.)
• 3:00 a.m. — 2.6 magnitude (southeast of Ridgely, Tenn.)
• 3:34 a.m. — 2.1 magnitude (southeast of Ridgely, Tenn.)
• 3:40 a.m. — 3.1 magnitude (northwest of Newbern, Tenn.)

This area is known as the New Madrid Seismic Zone.

The New Madrid Seismic Zone, located in southeast Missouri and adjacent states (Tennessee, Kentucky, Arkansas and Illinois) is the most seismically active area in North America east of the Rocky Mountains. 

This zone generated a massive family of quakes between 1811 and 1812 that changed the course of the Mississippi River. Experts believe the New Madrid Zone has been responsible for magnitude 7 to 8 intensity earthquakes about every 500 years over the past 1,200 years.

Earthquakes everywhere occur on faults within bedrock, usually miles deep. The earthquakes of the New Madrid Seismic Zone occur within a large network of faults called the Reelfoot rift. The rift formed about 500 million years ago when the region was stretched in the northwest-southeast direction.

Along a northeast-southwest zone at least 40 miles wide and 300 miles long, the rocks in the rift were slowly dropped down about 1 mile along some of the faults.

That region is undergoing east-west shortening, and the ancient faults of the Reelfoot rift are being reactivated to generate earthquakes. Presently, the Reelfoot rift and the New Madrid Seismic Zone are 1,200 miles from the nearest plate boundary, which is in the Caribbean Sea.

The network of faults in the seismic zone is buried beneath hundreds to thousands of feet of sand and mud. Four of the largest faults are recognized as alignments of abundant small earthquakes, and movements along two of these faults dammed rivers and created lakes during the earthquakes of 1811-1812.

A few more deeply buried faults were detected during oil and gas exploration, and a few small faults are known from geologic mapping. However, many earthquakes occur away from the few known faults, so there must be additional, unknown faults that can generate earthquakes in the seismic zone.

Unfortunately, the best overall guide to seismic hazards in the New Madrid Seismic Zone is the earthquakes themselves.