At 2:15 in the morning on December 16, 1811, the ground beneath northeastern Arkansas lurched with a force estimated at magnitude 7.5. Trees snapped at their bases. Riverbanks collapsed into the Mississippi. In Washington, D.C., windows rattled. In Richmond, Virginia, church bells rang on their own. In Charleston, South Carolina, well water sloshed out of its containers. The quake was centered not along the Pacific Ring of Fire or any tectonic plate boundary, but deep in the interior of the North American continent, in a region that looks -- from the surface -- like the flattest, most geologically unremarkable farmland in America.
The New Madrid Seismic Zone stretches across parts of seven states -- Illinois, Missouri, Arkansas, Kentucky, Tennessee, Mississippi, and Indiana -- running southward from Cairo, Illinois through New Madrid, Missouri into Marked Tree, Arkansas. The faults responsible lie embedded in a subsurface geological feature called the Reelfoot Rift, a failed continental split likely dating to the Cambrian Period, over 500 million years ago. The rift never succeeded in tearing North America apart, but it left behind an aulacogen -- a permanent zone of weakness deep underground. As the North American plate continues drifting westward, even small compressive forces can reactivate these ancient fractures. The result is a landscape where the most powerful earthquakes in recorded North American history have occurred hundreds of miles from the nearest plate boundary.
Between December 1811 and February 1812, the New Madrid zone unleashed four earthquakes estimated at magnitude 7 or greater, along with thousands of aftershocks. The first struck on December 16; within hours, 27 aftershocks rattled New Madrid, some matching the original in intensity. Uplift along the faults created temporary waterfalls on the Mississippi River. Water waves moving upstream gave observers the impression the river was flowing backwards. The quakes dammed streams, and Reelfoot Lake in western Tennessee formed where land subsided and filled with water. The shaking knocked plaster off houses in Columbia, South Carolina, moved furniture in Jefferson, Indiana, and sent residents fleeing their homes in Lebanon, Ohio. Observers in Herculaneum, Missouri described the shaking as "severe" with a duration of ten to twelve minutes. The cumulative effects reshaped the topography of the central Mississippi Valley.
The 1811-12 sequence was not the first time these faults ruptured catastrophically. Archaeological studies of sand blows and soil layers reveal that similar earthquake clusters struck around AD 1450, AD 900, and AD 300, with evidence of still earlier events around 2350 BC. Near Marianna, Arkansas, liquefaction features suggest major quakes around 3500 and 4800 BC. Dendrochronology studies on ancient bald cypress trees growing in Reelfoot Lake confirmed the 1811-12 series through fracture patterns followed by rapid growth after inundation. Tree ring records extending back to 1321 in the St. Francis sunklands show no comparable signals between that date and 1811, suggesting the zone experiences intense bursts of activity separated by centuries of relative quiet.
A 2008 FEMA report warned that a major New Madrid earthquake could produce "the highest economic losses due to a natural disaster in the United States." A 2009 study modeled a scenario where all three fault segments ruptured simultaneously at magnitude 7.7: the projected toll was 86,000 casualties, 3,500 fatalities, 715,000 damaged buildings, 7.2 million displaced people, and at least $300 billion in direct economic losses. Memphis and St. Louis would sustain severe damage. Since monitoring instruments were installed in 1974, more than 4,000 earthquakes have been recorded in the zone, most too small to feel. The USGS estimates a 7 to 10 percent chance of a quake matching the 1811-12 events within the next 50 years, and a 25 to 40 percent chance of a magnitude 6 earthquake in the same timeframe. In 2014, the USGS raised the risk assessment for the area.
The zone presents a puzzle that divides scientists. In 2009, researchers from Northwestern University and Purdue University reported that GPS measurements showed almost no fault movement, suggesting the system might be "shutting down" with stress accumulating elsewhere. That same year, researchers proposed the continuing small quakes might merely be aftershocks of the 1811-12 events, still echoing after two centuries. The USGS counters that the small earthquakes are not diminishing over time as true aftershocks would, and that 4,500 years of archaeological evidence outweighs a decade of GPS data. The first known written account of ground shaking here came from a French missionary on Christmas Day, 1699, near present-day Memphis. Three centuries later, scientists still argue over whether the next big one is a matter of centuries away -- or already overdue.
The New Madrid Seismic Zone is centered at approximately 36.58N, 89.59W, stretching from Cairo, Illinois southward through southeast Missouri and into northeastern Arkansas. From cruising altitude, look for the Mississippi River floodplain, Reelfoot Lake in western Tennessee (formed by the 1811-12 earthquakes), and the flat alluvial landscape of the Mississippi embayment. No dramatic surface features mark the fault zone. Nearest airports include Memphis International (KMEM) approximately 60 nm south, Cape Girardeau Regional (KCGI) 30 nm north, and Jonesboro Municipal (KJBR) 50 nm west. Best observed at 5,000-10,000 ft where the river meanders and Reelfoot Lake are visible.