The 1811-1812 New Madrid Earthquakes (pronounced /njuː ˈmædrɨd/) were an intense intraplate earthquake series beginning with an initial pair of very large earthquakes on December 16, 1811. These earthquakes, as well as the seismic zone of their occurrence, were named for the Mississippi River town of New Madrid, Louisiana Territory, now Missouri.

There are estimates that the earthquakes were felt strongly over roughly 130,000 square kilometers (50,000 square miles), and moderately across nearly 3 million square kilometers (1 million square miles). The historic 1906 San Francisco earthquake, by comparison, was felt moderately over roughly 16,000 square kilometers (6,000 square miles).

Earthquakes

• December 16, 1811, 0815 UTC (2:15 a.m.); (M ~7.2 – 8.1^[1]) epicenter in northeast Arkansas. It caused only slight damage to man-made structures, mainly because of the sparse population in the epicentral area. The future location of Memphis, Tennessee experienced level IX shaking on the Mercalli intensity scale. A seismic seiche propagated upriver and Little Prairie was heavily damaged by soil liquefaction^[2]

• December 16, 1811, 1415 UTC (8:15 a.m.); (M ~7.2–8.1) epicenter in northeast Arkansas. This shock followed the first earthquake by six hours and was similar in intensity.^[1]

• January 23, 1812, 1500 UTC (9 a.m.); (M ~7.0–7.8^[1]) epicenter in the Missouri Bootheel. The meizoseismal area was characterized by general ground warping, ejections, fissuring, severe landslides, and caving of stream banks. Johnson and Schweig attributed this earthquake to a rupture on the New Madrid North Fault. This may have placed strain on the Reelfoot Fault.^[2]

• February 7, 1812, 0945 UTC (4:45 a.m.); (M ~7.4–8.0^[1]) epicenter near New Madrid, Missouri. New Madrid was destroyed. At St. Louis, Missouri, many houses were severely damaged, and their chimneys were toppled. This shock was definitively attributed to the Reelfoot Fault by Johnston and Schweig. Uplift along a segment of this reverse fault created temporary waterfalls on the Mississippi at Kentucky Bend, created waves that propagated upstream, and caused the formation of Reelfoot Lake by obstructing streams in Lake County, Tennessee.^[2]

Susan Hough, a seismologist of the United States Geological Survey (USGS), has recently estimated the earthquake magnitudes as "right around magnitude 7. Possibly a bit below, possibly a bit above, but not as big as 7.5."^[3]

Effects

Some sections of the Mississippi River appeared to run backward for a short time.^[2] Sand blows were common throughout the area, and can still be seen from the air in cultivated fields. The shockwaves propagated efficiently through midwestern bedrock. Residents as far away as Pittsburgh and Norfolk were awakened by intense shaking.^[4] Church bells were reported to ring as far as Boston, Massachusetts and York, Ontario (now Toronto) and sidewalks were reported to have been cracked and broken in Washington, D.C.^[5] There were also reports of toppled chimneys in Maine.^{[citation needed]}

Eliza Bryan^[6] in New Madrid, Territory of Missouri, wrote the following eyewitness account in March, 1812.

On the 16th of December, 1811, about two o'clock, a.m., we were visited by a violent shock of an earthquake, accompanied by a very awful noise resembling loud but distant thunder, but more hoarse and vibrating, which was followed in a few minutes by the complete saturation of the atmosphere, with sulphurious vapor, causing total darkness. The screams of the affrighted inhabitants running to and fro, not knowing where to go, or what to do — the cries of the fowls and beasts of every species — the cracking of trees falling, and the roaring of the Mississippi — the current of which was retrograde for a few minutes, owing as is supposed, to an irruption in its bed — formed a scene truly horrible.

The Shaker diarist Samuel Swan McClelland described the effects of the earthquake on the Shaker settlement at West Union (Busro), Indiana, where the earthquakes contributed to the temporary abandonment of the westernmost Shaker community.^[7]

Geology

Reelfoot Rift

The underlying cause of New Madrid earthquakes is not well understood, but modern faulting seems to be related to an ancient geologic feature buried under the Mississippi River alluvial plain, known as the Reelfoot Rift

The New Madrid Seismic Zone is made up of reactivated faults that formed when what is now North America began to split or rift apart during the breakup of the supercontinent Rodinia in the Neoproterozoic Era (about 750 million years ago). Faults were created along the rift and igneous rocks formed from magma that was being pushed towards the surface. The resulting rift system failed but has remained as an aulacogen (a scar or zone of weakness) deep underground. Another unsuccessful attempt at rifting 200 million years ago created additional faults, which made the area weaker. The resulting geological structures make up the Reelfoot Rift, and have since been deeply buried by younger sediments. But the ancient faults appear to have made the rocks deep in the Earth's crust in the New Madrid area mechanically weaker than much of the rest of North America.

This weakness, possibly combined with focusing effects from mechanically stronger igneous rocks nearby, allows the relatively small east-west compressive forces that exist in the North American plate to reactivate old faults, making the area prone to earthquakes.^[8]

Since other rifts are known to occur in North America's stress environment but not all are associated with modern earthquakes, (for example the Midcontinent Rift System that stretches from Minnesota to Kansas), other processes could be at work to locally increase mechanical stress on the New Madrid faults. Stress changes associated with bending of the lithosphere caused by the melting of continental glaciers at the end of the last Ice Age, has been considered to play a role,^[9] as well as downward pull from sinking igneous rock bodies below the fault.^[10] It has also been suggested that some form of heating in the lithosphere below the area may be making deep rocks more plastic, which concentrates compressive stress in the shallower subsurface area where the faulting occurs.^[11] There may be local stress from a change in the flow of the mantle beneath the NMSZ, caused by the sinking Farallon Plate, according to one model.^[12]

When epicenters of modern earthquakes are plotted on a map, three trends become apparent. First is the general northeast-southwest trend paralleling the trend of the Reelfoot Rift, in Arkansas, south of where the epicenters turn northwest. This is a right-lateral strike-slip fault system parallel to the Reelfoot Rift.

The second is the southeast to northwest trend that occurs just southwest of New Madrid. This trend is a stepover thrust fault known as the Reelfoot Fault, associated with the Tiptonville dome and the impoundment of Reelfoot Lake. Epicenter locations on this fault are more spread out because the fault surface is inclined and dips into the ground, towards the south, at around forty degrees. Slip is towards the northeast. Motion on this fault in the 1811-1812 series created waterfalls on the Mississippi.

The third line, extending northeast from the northwestern end of the Reelfoot Fault is another right-lateral strike-slip fault, termed New Madrid North.

Seismic Zone

The epicenters of over 4,000 earthquakes can be identified from seismic measurements taken since 1974. It can be seen that the earthquakes originate from the seismic activity of the Reelfoot Rift. The zone which is colored in red on the map is called the New Madrid Seismic Zone.

Recent earthquakes

4000 earthquake reports since 1974

The zone remains active today. In recent decades minor earthquakes have continued.^[5] New forecasts estimate a 7 to 10 percent chance, in the next 50 years, of a repeat of a major earthquake like those that occurred in 1811–1812, which likely had magnitudes of between 7.5 and 8.0. There is a 25 to 40 percent chance, in a 50-year time span, of a magnitude 6.0 or greater earthquake.^[13]

Recurrence potential

In a report filed in November 2008, The U.S. Federal Emergency Management Agency warned that a serious earthquake in the New Madrid Seismic Zone could result in "the highest economic losses due to a natural disaster in the United States," further predicting "widespread and catastrophic" damage across Alabama, Arkansas, Illinois, Indiana, Kentucky, Mississippi, Missouri and particularly Tennessee, where a 7.7 magnitude quake or greater would cause damage to tens of thousands of structures affecting water distribution, transportation systems, and other vital infrastructure.^[14]

The potential for the recurrence of large earthquakes and their impact today on densely populated cities in and around the seismic zone has prompted research devoted to understanding in the New Madrid Seismic Zone. By studying evidence of past quakes and closely monitoring ground motion and current earthquake activity, scientists attempt to understand their causes and recurrence intervals.

The lack of apparent land movement along the New Madrid fault system has long puzzled scientists. In 2009 two studies based on eight years of GPS measurements indicated that the faults were moving at no more than 0.2 millimetres (0.0079 in) a year.^[15] This contrasts to the rate of slippage on the San Andreas Fault which averages up to 37 millimetres (1.5 in) a year across California.^[16]

• New Madrid Seismic Zone
• List of earthquakes in the United States
• List of earthquakes
• Postglacial rebound
• 1865 Memphis earthquake
• 1886 Charleston earthquake

The New Madrid Seismic Zone (pronounced /njuː ˈmædrɨd/), sometimes called the New Madrid Fault Line, is a major seismic zone and a prolific source of intraplate earthquakes (earthquakes within a tectonic plate) in the southern and midwestern United States, stretching to the southwest from New Madrid, Missouri.

The New Madrid fault system was responsible for the 1811–1812 New Madrid earthquakes and may have the potential to produce large earthquakes in the future. Since 1812 frequent smaller earthquakes were recorded in the area.^[1]

Earthquakes that occur there potentially threaten parts of seven U.S. states: Illinois, Indiana, Missouri, Arkansas, Kentucky, Tennessee, and Mississippi.

Reelfoot Rift and the New Madrid Seismic Zone in a 3D topographic image