In the winter of 1811–1812 a series of three great earthquakes occurred in the New Madrid seismic zone. In addition to the three principal shocks, at least 15 other earthquakes, I_o ≥ VIII, occurred within a year of the first large earthquake on December 16, 1811. The three main shocks were felt over the entire eastern United States. They were strong enough to cause minor damage as far away as Indiana and Ohio on the north, the Carolinas on the east, and southern Mississippi on the south. They were strong enough to cause severe or structural damage in parts of Missouri, Illinois, Indiana, Kentucky, Tennessee, Mississippi, and Arkansas. The section of this poster titled "Seismic history of the New Madrid region" describes what happened in the epicentral region. Fortunately, few people lived in the severely shaken area in 1811; that is not the case today. What would happen if a series of earthquakes as large and numerous as the "New Madrid" earthquakes were to occur in the New Madrid seismic zone today?

Photographs 1-10 show typical damage to structures that occurred during various earthquakes in the United States. Structural damage to buildings in the Modified Mercalli intensity scale, a scale used for assigning numbers to earthquake effects, begins at intensity VIII. Minor or architectural damage (cracked plaster, windows, and chimneys) occurs at intensities VI and VII, and effects on people and small objects predominate at intensities below VI (earthquake felt, direction and duration noted, dishes broken). Photographs 1-10 illustrate damage caused by intensity VIII and above. None of the damage shown in these photographs occurred in earthquakes larger than the 1811–1812 New Madrid shocks, and most of the examples are from considerably smaller shocks. Photographs 1-4 and 7 show damage to masonry buildings, mostly old and unreinforced, none designed to be earthquake resistant. How many such buildings are in use in your community? Photographs 5 and 6 show damage to modern structures close to the epicenter of a magnitude 6.5 earthquake, a small shock compared to the magnitudes (8.4–8.7) of the New Madrid earthquakes. Photographs 8–10 are typical of damage that can occur at large distances from great earthquakes. Tall buildings are particularly susceptible to ground motions with relatively long periods (greater than 1 second), while short, rigid buildings are generally more likely to be damaged by ground motions with periods of less than 1 second. Shorter period ground motions attenuate more rapidly with distance from the epicenter than do the longer period motions.

The 1811-1812 earthquakes produced extensive ground effects. Earthquakes can cause many different kinds of ground effects, ranging from minor fissures, slumps, and rockslides to major landslides and disturbances of the ground surface. Photographs 1117 show typical earthquake ground effects and the kinds of damage they can cause to manmade structures.

Is another earthquake as large as the 1811-1812 shocks really likely to occur in New Madrid seismic zone? Great earthquakes such as the 1811-1812 series are estimated to occur in the zone on the average every 500 years. However, earthquakes strong enough to cause structural damage (VIII in the Modified Mercalli intensity scale) are estimated to occur in the seismic zone on the average every 50 years. The last such shock occurred in 1843.

How do you prepare for a possible earthquake and what do you do if one occurs? The section on "Earthquake safety tips" explains what to do before, during, and after an earthquake.

Want to know more? See "Sources of additional information." Other reports of interest are listed in "References cited."