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New Madrid Seismic Zone

From Academic Kids

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Seismic map New Madrid Seismic Zone. Credit: USGS
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Earthquakes in the New Madrid seismic zone since 1974. Credit: USGS

The New Madrid Seismic Zone, also known as the Reelfoot Rift or the New Madrid Fault Line, is a major seismic zone, located in the mideastern United States. Largely inactive during the 20th century, the New Madrid fault system was responsible for the 1812 New Madrid Earthquake and has the potential to produce damaging earthquakes in coming decades.

Contents

Geography

The New Madrid Seismic Zone is made up of reactivated faults that formed when North America began to split or rift apart during the breakup of the supercontinent Rodinia in the Neoproterozoic Era (about 750 million years ago). The rift failed, but remained as a scar or zone of weakness. During the Mesozoic Era (about 200 million years ago), as the Atlantic Ocean was opening in the east, rifting was once again re-activated and intrusive igneous rocks were emplaced. But again the rifting failed and the continent remained intact, although with a significant zone of weakness. This rift is known as the Reelfoot Rift and coincides with the northernmost portion of the Mississippi embayment. Most of the seismicity is located from 5 to 25 km beneath the Earth's surface.

The 150-mile long fault system, which extends into five states, stretches southward from Cairo, Illinois, through Hayti-Caruthersville and New Madrid, Missouri, through Blytheville, to Marked Tree, Arkansas. It also covers a part of Tennessee, near Reelfoot Lake, extending southeast into Dyersburg.

The red zones on the map above indicate the epicenter locations of hundreds of minor earthquakes recorded since the 1970s. Two trends are apparent. First is the general NE-SW trend paralleling the trend of the Reelfoot Rift. The second is the intense cross trend, NW-SE, that occurs just southwest of New Madrid. This second trend coincides with an intrusive igneous body which lies deeply buried beneath the sediments of the rift zone. Several other bodies of deeply buried intrusive rock are known to exist within the seismic zone. The depths of these igneous rock bodies closely corresponds to the depth of the seismic activity.

History

The zone has seen four of the largest North American earthquakes in recorded history, with magnitude estimates greater than 7.0 on the Richter scale, all within a 3 month period:

These catastrophic earthquakes occurred during a three-month period in December 1811 and early 1812. They caused changes in the course of the Mississippi River, which rolled backwards temporarily, and were felt as far away as New York City and Boston, Massachusetts, where churchbells rang. 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 (600 km&sup2). Many houses at New Madrid were thrown down. "Houses, gardens, and fields were swallowed up" one source notes. But fatalities and damage were low, because the area was sparsely settled. Hundreds of aftershocks followed over a period of several years. All three major quakes are generally believed to have exceeded 8.0 on the Richter Scale, and some seismologists believe the largest was 9.0 or larger.

The largest earthquakes to have occurred since then were on January 4, 1843 and October 31, 1895 with magnitude estimates of 6.0 and 6.2 respectively. In addition to these events, seven events of magnitude >= 5.0 have occurred in the area. Instruments were installed in and around this area in 1974 to closely monitor seismic activity. Since then, more than 4000 earthquakes have been located, most of which are too small to be felt. On average one earthquake per year will be large enough to be felt in the area.

More quakes predicted

The potential for the recurrence of large earthquakes and their impact today on densely populated cities in and around the seismic zone has generated much research devoted to understanding earthquakes. By closely monitoring the earthquake activity, scientists can hope to understand their causes, recurrence rates, ground motion and disaster mitigation. The probability for an earthquake of magnitude 6.0 or greater is significant in the near future, with a 90% chance of such an earthquake by the year 2040. ("The odds of another 8.0 event within 50 years are between 7 and 10 percent" [1] (http://www.livescience.com/forcesofnature/050622_new_madrid.html)) Because of the unconsolidated sediments which are a major part of the underlying geology of the Mississippi embayment, large quakes here can affect as much as 20 times the land area of major quakes on the west coast.

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Comparison: the 1895 Charleston, Missouri, earthquake in the New Madrid seismic zone with the 1994 Northridge, California, earthquake. Red indicates area of structural damage, yellow indicates area where shaking was felt.

See also

References

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