Reelfoot Rift

Reelfoot Rift
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Reelfoot_Rift".

This article is about the seismic zone in southeastern Missouri.
For the zone in Illinois/Indiana, see Wabash Valley Seismic Zone.
For the April 2008 quake in southern Illinois, see 2008 Illinois earthquake

Seismic map of the New Madrid Seismic Zone.

Reelfoot Rift and the New Madrid Seismic Zone in a 3D topographic image

The New Madrid Seismic Zone, also known as the Reelfoot Rift or the New Madrid Fault Line, is a major seismic zone in the Southern and Midwestern United States stretching to the southwest from New Madrid, Missouri.

The New Madrid fault system was responsible for the 1812 New Madrid Earthquake and has the potential to produce damaging earthquakes on an average of every 300 to 500 years. Since 1812 frequent smaller intraplate earthquakes (earthquakes within a tectonic plate) were recorded for the area.^[1]

The seismic zone covers parts of six U.S. states: Illinois, Missouri, Arkansas, Kentucky, Tennessee, and Mississippi.

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Geology

750 million years ago

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 resulting rift system failed but remained as a scar or zone of weakness. The area was then flooded by an ancient ocean, depositing layers of sediment on the rift.

200 million years ago

Geological structure of Reelfoot Rift

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 3 to 15 mi (5 to 25 km) beneath the Earth's surface.

Modern seismic activity

Since the 1970s, thousands of earthquakes have been recorded in the New Madrid seismic zone.

Seismic monitoring

Quakes in the New Madrid and Wabash Valley seismic zones over several decades.

Instruments were installed in and around the area in 1974 to closely monitor seismic activity. Since then, more than 4,000 earthquakes have been recorded, most of which are too small to be felt. On average, one earthquake per year is large enough to be felt in the area.

Epicenter locations

More than 4000 earthquake reports since 1974

The red zones on the map indicate the epicenter locations of hundreds of minor earthquakes recorded since the 1970s. Two trends are apparent. First is the general northeast-southwest trend paralleling the trend of the Reelfoot Rift.

The second is the intense cross trend (northwest to southeast) 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.

Geographic distribution

The 150 mi (240 km) long fault system, which extends into five states, stretches southward from Cairo, Illinois, through Hayti, Missouri, Caruthersville and New Madrid, through Blytheville, Arkansas, to Marked Tree. It also covers a part of west Tennessee, near Reelfoot Lake, extending southeast into Dyersburg. Extreme western Kentucky, notably the Kentucky Bend also overlies the zone.

History

The zone had four of the largest North American earthquakes in recorded history, with magnitudes estimated to be greater than 7.0 on the Richter scale, all occurring within a 3 month period between 1811 and 1812. 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 quake can be difficult.

1811/1812 earthquake series

View to the southwest along the former riverbed of the Mississippi River, just south of the Tennessee/Arkansas state line near Reverie, Tennessee and Wilson, Arkansas (2007)

December 16, 1811, 0815 UTC (2:15 a.m.); 8.1 magnitude; epicenter in northeast Arkansas; Mercalli XI. It caused only slight damage to man-made structures, mainly because of the sparse population in the epicentral area. However, landslides and geological changes occurred along the Mississippi River, and large localized waves were caused by fissures opening and closing below the Earth's surface.

December 16, 1811, 1415 UTC (8:15 a.m.); 7.0 magnitude; epicenter in northeast Arkansas; Mercalli X-XI. This shock followed the first earthquake by six hours.

January 23, 1812, 1500 UTC (9 a.m.); 7.8 magnitude; epicenter in the Missouri Bootheel. The meizoseismal area was characterized by general ground warping, ejections, fissuring, severe landslides, and caving of stream banks.

February 7, 1812 (the New Madrid Earthquake), 0945 UTC (4:45 a.m.); ~8 magnitude; epicenter near New Madrid, Missouri. New Madrid was destroyed. At St. Louis, Missouri, many houses were severely damaged, and their chimneys were toppled. The seismic area was characterized by general ground warping, ejections, fissuring, severe landslides, and caving of stream banks.

Geologic effects

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 km²). 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.^[2]

The earthquakes were felt as far away as New York City and Boston, Massachusetts, where churchbells rang.^[3]

Mississippi River course changes

View to the northeast along the former riverbed of the Mississippi River

This series of temblors caused permanent changes in the course of the Mississippi River, giving the illusion that it was flowing backward.^[4]

Because of the change in the course of the Mississippi River, land was cut off from counties by the river and wound up on the other side of the new riverbed, on the other side of the Mississippi. The settlement of Reverie, Tennessee, in Tipton County was cut off and placed on the western bank of the Mississippi River on the Arkansas side.^[5]

Along the Tennessee/Arkansas state line, geological features are still present almost 200 years after the events, showing the former course of the Mississippi River as it was before the 1811/1812 earthquakes.

Focal depth of the earthquakes

From what is known about the present seismicity of the area, it can be inferred that their focal depths were probably between 3 to 12 mi (5-20 km). The fault plane—or planes—on which the Earth rupture occurred are inferred to have had a north-northeast to south-southwest strike direction, more or less parallel to the Mississippi River.

Aftershocks

Contemporary woodcut of the effects of the New Madrid earthquakes

Hundreds of aftershocks followed over a period of several years. Aftershocks strong enough to be felt occurred until the year 1817. 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, nine events of magnitude 5.0 or greater have occurred in the area.

Commemorating the earthquakes

From the early years of the 19th century until well after the American Civil War, the citizens of Union City, Tennessee, would gather every February 7 for an all-night "vigil and fish fry" on the site currently occupied by the Cumberland Presbyterian Church, praying, singing, and beseeching the Almighty to "spare the land over" for another year.

More quakes predicted

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.

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. Establishing the probability for an earthquake of a given magnitude is an inexact science. By studying evidence of past quakes and closely monitoring ground motion and current earthquake activity, scientists attempt to understand their causes, recurrence rates, ground motion and disaster mitigation.

Probability of future earthquakes

The probability of magnitude 6.0 or greater in the near future is considered significant; a 90% chance of such an earthquake by 2040 has been given. In the June 23, 2005, issue of the journal Nature, the odds of another 8.0 event within 50 years were estimated to be between 7 and 10 percent.^[6]

Because of the unconsolidated sediments which are a major part of the underlying geology of the Mississippi embayment, as well as the river sediments along the Mississippi and Ohio River valleys to the north and east (note the red fingers extending up these valleys in the image above), large quakes have the potential for more widespread damage than major quakes on the west coast. Additionally, the area affected will be larger since beyond the rift zone itself there are few other faults to attenuate the seismic waves.

Media coverage

The consequences of an earthquake in this region have been featured on television shows specializing in natural disasters. An eruption from the seismic zone and its potential consequences was the subject of an episode of the History Channel show Mega Disasters. The potential danger to Memphis, Tennessee, from another mega New Madrid earthquake was featured on The Weather Channel's It Could Happen Tomorrow.

Walter Jon Williams novel The Rift depicts what would happen were a New Madrid Fault earthquake the size of the 1812 quake to happen today.

References

^ http://www.ceri.memphis.edu/ Center for Earthquake Research and Information at the University of Memphis.
^ Fuller, Myron (Contributor David Stewart) (1993). The New Madrid Earthquake. Care Publications. ISBN 0-93442-649-X.
^ http://quake.wr.usgs.gov/prepare/factsheets/NewMadrid/ United States Geological Survey USGS
^ http://earthquake.usgs.gov/regional/states/events/1811-1812.php#december_16 United States Geological Survey USGS
^ http://www.tnhistoryforkids.org/local/tipton TNHistoryForKids.org
^ http://www.livescience.com/forcesofnature/050622_new_madrid.html LiveScience.com

External links

Center for Earthquake Research and Information at the University of Memphis. Recent central U.S. earthquakes

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