You might be familiar with Pangea, a prehistoric supercontinent formed when all of Earth’s significant landmasses merged about 300 million years ago.

Before Pangea, there was the supercontinent Rodinia, which assembled itself between 950 million and 1.3 billion years ago. Then around 750 million years ago, Rodinia started to break apart. Rifts (or faults) emerged between — and within — the various continental plates. Some 600 million years ago, a point of division called the Reelfoot Rift appeared in what’s now the southeastern United States.

On occasion, rifts split continents in half. That didn’t happen here: The Reelfoot Rift was stopped in its tracks. Geologically speaking, it failed — but not before it weakened part of the continental plate and created faults that still exist today.
“[It] left something of a scar in the otherwise old and relatively homogeneous crust,” Susan Hough, a seismologist with the U.S. Geological Survey, says via email. The New Madrid Seismic Zone lies within the Reelfoot Rift. To earthquake researchers like Hough, this region is an enigma. As her email notes, there are a lot of other failed mid-continent rifts in North America. (A massive one extends beneath Lake Superior.) Yet these “have not produced significant earthquakes in recent or historic times.”

“So why has the Reelfoot lit up with earthquakes but not the [others]?” asks Hough. “There are some ideas but no definitive answer.”

https://science.howstuffworks.com/environmental/earth/geology/earthquakes-on-mississippi-new-madrid-seismic-zone.htm