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NEW MADRID SEISMIC ZONE

The New Madrid Seismic zone lies within the central Mississippi Valley, extending from northeast Arkansas, through southeast Missouri, western Tennessee, and western Kentucky to southern Illinois. Historically, this area has been the site of some of the largest earthquakes in North America.

Instruments were installed in the New Madrid area in 1974 to closely monitor seismic activity. Since then more than 4,000 earthquakes have been located, most of which are too small to be felt. On average one earthquake per year will be felt in the area. By closely monitoring the earthquake activity, scientists hope to understand their causes, recurrence rates, ground motion, and disaster mitigation.

The potential for the recurrence of a major earthquake in the region today has generated much research because this area is now densely populated and a major quake could result in great loss of life and property damage in the billions of dollars. The probability for an earthquake of magnitude 6.0 or greater is significant in the near future, with a 50% chance by the year 2000 and a 90% chance by the year 2040. Therefore, it is important to understand the geologic causes of the earthquakes and the relationship of these quakes to plate tectonics.

Sometimes the convection currents that cause the plates to move will cause them to rip apart. If a continent happens to be sitting over the tear, then it too will be torn, or rifted, apart. Such rifting apparently occurred nearly 600 million years ago and then again about 100 million years ago beneath the present Mississippi River valley. Although the rift, called the Reelfoot Rift, failed each time before it completely ruptured the crust, the pulling apart of the rocks caused the formation of many faults. These faults remain as a zone of weakness in the earth's crust called the New Madrid Seismic Zone. Compressional forces acting on the North American plate are causing movement (earthquakes) along the faults. 

Development of the Reelfoot Rift began nearly 600 million years ago (A) as the forces that drive the motion of the earth's plates began to tear the continent apart. Later, the rifting stopped (B) and the area sank and was buried deeply by sediments that accumulated in a shallow inland sea that covered the area. Then, rifting was activated (C) and may have been continued as late as about 70 million years ago. Since then the area has been compressed (D), resulting in numerous earthquakes as the squeezed bodies of rock grind past each other along the deeply buried faults.

New Madrid Earthquakes of 1811-12 

On the night of December 15, 1811, people living in the vicinity of New Madrid, Missouri, were jarred from sleep by a shock that rippled through the earth with such force that buildings collapsed, trees toppled, and the Mississippi River changed course. This explosive force was one of the most powerful earthquakes ever recorded in North America during historical times. For the next three months the area was rocked by at least two more powerful shakes as strong as the first and by hundreds of smaller shocks. The larger quakes were so strong they caused the church bells to ring in Washington, D. C. They were also felt a thousand miles away in Boston. The force of the large quakes was so powerful that sand and water blew out of cracks in the land surface by means of a process called "liquefaction" (see below).

When strong earthquakes release their energy, the violent shaking may cause underground layers of saturated sand and gravel to behave like a fluid under pressure. This fluid mass may then shoot up through cracks in the overlying soil and blow out over the surface (above). The resulting liquefaction feature is preserved in the soil as evidence of the earthquake's occurrence and strength.

Sandblow dike indicating paleoliquefaction in the New Madrid area
(photo by USGS)

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