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   About the New Madrid Seismic Zone  

The New Madrid Seismic Zone (NMSZ) is a 150-mile long fault zone spanning four states in the Midwestern United States (see Figure 1). The NMSZ lies within the central Mississippi Valley, extending from northeast Arkansas, through southeast Missouri, western Tennessee, and western Kentucky to southern Illinois. Historic earthquakes in the region, such as the 1811-1812 earthquakes are believed to have had magnitudes (M) of approximately 8.0 or greater that were felt throughout the eastern United States as well as Canada. Hundreds of aftershocks also followed the 1811-1812 quakes over a period of several years. Other significant quakes were recorded in 1843 and 1895. The 1895 quake was the worst of the two and was determined to be 6.8 (M) causing immense damage. Since the Central United States geology is susceptible to soil liquefaction, earthquake damage is magnified over a potentially wider area and is the U.S. Region most vulnerable to a catastrophic earthquake.

The greatest earthquake risk east of the Rocky Mountains is along the NMSZ. Damaging earthquakes are not as frequent as in California, but when they do occur, the destruction covers more than 15 times the area because of the underlying geology and soil conditions prevalent in the region. The zone is active, averaging about 200 earthquakes per year, though most of them are too small to be felt. With modern seismic networks, the capability to detect earthquakes has greatly increased, and many more very small earthquakes are being detected now than in the past. There is a common misconception that the number of earthquakes has increased over the years, but the increase is due to more sophisticated recording methods that can detect earthquakes that were previously unrecorded. The history of the region tells us, however, that the earthquake risk is the most serious potential disaster we could face.


Figure 1: New Madrid Seismic Zone 1

An earthquake is a phenomenon that results from the sudden release of stored energy in the earth's crust that creates seismic waves. At the earth's surface, earthquakes may manifest themselves by a shaking or displacement of the ground, which may lead to loss of life and destruction of property. Earthquakes may occur naturally or as a result of human activities. In its most generic sense, the word earthquake is used to describe any seismic event, whether a natural phenomenon or an event caused by humans that generates seismic waves.

This sudden release results in ground shaking, surface faulting, and/or ground failures. Most earthquakes result in little or no damage. In fact, there are hundreds of earthquakes that occur every year along the NMSZ, and most are of magnitudes less than 2.0. However, an earthquake has the potential to be the most dangerous of all natural hazards that could impact the State of Illinois.

The NMSZ saw four of the largest North American earthquakes in recorded history, with magnitude estimates greater than 8.0 on the Modified Mercalli Scale. These earthquakes all occurred over a three-month period. Many of the published accounts describe the cumulative effects of all the earthquakes, known as the New Madrid Sequence; thus, finding the individual effects of each earthquake can be difficult.

The first earthquake occurred on December 16, 1811, at 2:15 a.m., and had a severity of magnitude 8.1 to 9.0. It caused only slight damage to man-made structures, mainly because of the sparse population in the epicenter area. However, landslides and geological changes occurred along the Mississippi River, and large localized waves occurred due to fissures opening and closing below the earth's surface.

The second major earthquake occurred on January 23, 1812, at 8:15 a.m. with a magnitude 7.8 and another at 9:00 am. Although the intensity was slightly less, these earthquakes were just as violent as the earlier earthquakes.

The last of this series of earthquakes occurred on February 7, 1812, at 4:45 a.m. with an estimated magnitude of 8.0 to 11.0. As the epicenter for this quake was near New Madrid, Missouri, the town was destroyed. At St. Louis, many houses were severely damaged and their chimneys were toppled. The meizoseismal area was characterized by general ground warping, ejections, fissuring, severe landslides, and caving of stream banks.

This three-month period of earthquakes caused permanent changes in the course of the Mississippi River, which flowed backwards temporarily. The effects of these earthquakes were felt as far away as New York City and Boston, Massachusetts, causing church bells to ring. Large areas sank into the earth, fissures opened, lakes permanently drained, new lakes were formed, and forests were destroyed over an area of 150,000 acres. Hundreds of aftershocks followed over a period of several years.

In terms of response, it has been reported that the probability of a repeat of the 1811-1812 earthquakes (magnitude 7.5-8.0) is from 7-10% and the probability of a magnitude 6.0 or larger is from 25-40%. However, it is understood that a large magnitude event grows more probable with each passing day. A catastrophic seismic event on the NMSZ could directly impact more than 50% of the state's population and could trigger a national response on a larger scale than any recorded earthquake event in modern United States history.

 

 

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