Feb. 7, 2012

More than 2 million people in nine states planned to participate in the second Great Central United States ShakeOut on Feb. 7, 2012, the 200th anniversary of the last of at least three major earthquakes that struck the New Madrid fault in southern Illinois in late 1811 and early 1812. The earthquakes were the largest to strike the United States since the arrival of European settlers in North America. A shakeout is a drill designed to help teach people in seismic zones how to prepare for an earthquake and how to protect themselves when an earthquake occurs. A seismic zone is an area on the surface that is or could be affected by shifts in interconnected faults (fractures) in Earth’s lithosphere (crust and upper portion of the mantle). Drills based on the slogan “Drop, Cover and Hold On” were planned in states in and around the New Madrid Seismic Zone (NMSZ), including Alabama, Arkansas, Illinois, Kentucky, Mississippi, Missouri, Oklahoma, and Tennessee. The slogan reminds people to “Drop” to the floor, take “Cover” under a sturdy piece of furniture, and “Hold On” until the shaking stops.

The New Madrid quakes, all of which had an estimated moment magnitude of at least 7.5, affected an area about 10 times as large as that affected by the stronger San Francisco earthquake of 1906 and Haiti earthquake of 2010. The first quake, which took place on Dec. 16, 1811, was widely felt across the Eastern United States. The shaking woke people in New York City and Washington, D.C., and rang bells in Boston. The second quake occurred on Jan. 23, 1812. The third quake, which may have been the most violent of the three, destroyed the town of New Madrid and damaged many buildings in Saint Louis. Large waves called seiches swept northward in the Mississippi River, giving the impression that the river was flowing backwards. The death toll from the New Madrid quakes is unknown. However, if any deaths occurred, the number was probably very small. The  population of the area at that time included only about 100,000 non-Indians and a somewhat greater number of Indians. St. Louis had an estimated population of 5,700 in 1811. A major earthquake along the New Madrid fault today could affect about 40 million people, 12 million of whom live in the NMSZ itself.

An earthquake occurs when Earth's rock suddenly breaks and shifts, releasing energy in vibrations called seismic waves. The point on Earth where the rock first breaks is called the focus. The point on the surface above is known as the epicenter. World Book illustration

The NMSZ remains an area of high earthquake activity. It is, in fact, the most active seismic area in the United States east of the Rocky Mountains. According to the United States Geological Survey (USGS), large earthquakes in this zone could be more devastating than those in California for a number of reasons. Because of the nature of the crust in the Midwestern United States, shaking from an earthquake would travel farther. The region also has many buildings that have not been built or reinforced to limit earthquake damage. In addition, most people in the region are unprepared to cope with a major earthquake. A recent study estimated that a magnitude-7.7 quake along the New Madrid fault would cause 3,500 deaths and 80,000 injuries as well as the displacement of at least 2 million people in the NMSZ alone. The USGS has estimated that there is a 7 to 10 percent chance that a major NMSZ earthquake (7.5 to 8.0 magnitude) could occur in any 50-year period.

Additional World Book articles:

• Geology
• Plate tectonics
• Richter magnitude
• San Andreas Fault
• When the Earth Moves (a Special Report)