The New Madrid Seismic Zone

Plate tectonics explains why California shakes. It does not easily explain the New Madrid Seismic Zone, a cluster of faults buried beneath the cotton fields of the central Mississippi Valley, far from any plate boundary. Over the winter of 1811 and 1812, three earthquakes of about magnitude 7.3 to 7.5 struck here, toppling chimneys, collapsing riverbanks, and throwing the Mississippi River into waves that seemed to make it run backward. Shaking was felt across much of the eastern United States. Because the old, cold crust of the continent carries seismic waves far, a repeat today would affect Memphis, St. Louis, and a region that was never built for earthquakes.

The zone runs about 200 kilometers from near Marked Tree, Arkansas, to Cairo, Illinois, crossing the bootheel of Missouri, western Kentucky, and western Tennessee. It follows the Reelfoot Rift, a failed continental rift that formed when an ancient supercontinent began to tear apart more than 500 million years ago, now buried beneath thick river sediments. Today the old rift faults are being reactivated by stresses transmitted across the North American Plate. The Reelfoot fault, a thrust that lifts the Lake County uplift, connects strike-slip arms to the northeast and southwest. GPS measures very little strain across the zone, which keeps scientists debating how often it can produce large quakes. Small earthquakes, recorded almost daily, outline the faults.

The main shocks struck on December 16, 1811, January 23, 1812, and February 7, 1812, which the USGS rates at about magnitude 7.5, 7.3, and 7.5. The February quake on the Reelfoot fault dropped the land enough to form Reelfoot Lake in Tennessee. Few people lived in the region, and only one death was confirmed at New Madrid itself. Buried sand blows show similar sequences around 1450 and 900 CE, suggesting large events roughly every 500 years. The USGS estimates a 7 to 10 percent chance of a repeat of the 1811–1812 sequence and a 25 to 40 percent chance of a magnitude 6 or larger quake within any 50-year period. The Central United States Earthquake Consortium, formed in 1983, coordinates preparedness among the affected states.

• The San Andreas Fault
  California's 1,200-kilometer seam between the Pacific and North American plates contrasting plate-boundary fault
• The Tectonic Plates
  The seven great slabs and dozens of smaller ones that make up Earth's moving shell intraplate zone
• Mississippi River
  North America's great central artery, from Minnesota to the Gulf lies beneath the valley of
• United States
  A continental superpower of fifty states and astonishing variety located in
• North America
  The third-largest continent, from the Arctic to the tropics within the plate of