New Madrid Seismic Zone

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 Midwestern United States. The New Madrid fault system was responsible for the 1812 New Madrid Earthquake and has the potential to produce damaging earthquakes in coming decades.

Geology

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. The area was then flooded by an ancient ocean, depositing layers of sediment on the 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. The natural valley formed by collapsed rock allowed the Mississippi River to flow through, depositing more sediment on the rift. 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 depth of the rift allows earthquakes' seismic waves to travel long distances through the bedrock, making them destructive when they do happen.

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 saw four of the largest North American earthquakes in recorded history, with magnitude estimates greater than 7.0 on the Richter scale, all occurring in a 3 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 quake can be difficult.

First earthquake of December 16, 1811, 0815 UTC (2:15 a.m.); 7.7 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 occurred due to fissures opening and closing below the Earth's surface.

Second earthquake of 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.

Earthquake of January 23, 1812, 1500 UTC (9 a.m.); 7.6 magnitude; epicenter in Missouri Bootheel. The meizoseismal area was characterized by general ground warping, ejections, fissuring, severe landslides, and caving of stream banks.

Earthquake of February 7, 1812 (the New Madrid Earthquake), 0945 UTC (4:45 a.m.); 7.9 magnitude; epicenter near New Madrid, Missouri. New Madrid was destroyed. At St. Louis, many houses were damaged severely and their chimneys were thrown down. The meizoseismal area was characterized by general ground warping, ejections, fissuring, severe landslides, and caving of stream banks.

These catastrophic earthquakes occurred during a three-month period in December 1811 and early 1812. They caused permanent changes in the course of the Mississippi River, which flowed 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 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. 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.

From what is known about the present seismicity of the area, it can be inferred that their focal depths were probably between 5 and 20 kilometers. The fault plane—or planes—on which the Earth rupture occurred are inferred to have had a NNE–SSW strike direction, more or less parallel to the Mississippi River.

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, seven events of magnitude ≥ 5.0 have occurred in the area. From the early years of the Nineteenth Century until well after the American Civil War, the citizens of Union City, Tennessee would gather every February 7th 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.

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.

In the late 1980s Iben Browning predicted that a major earthquake on the New Madrid fault line in southern Missouri would occur on December 3, 1990; the predicted earthquake did not occur. The prediction created a stir that was followed by scorn and ridicule.

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. 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. The probability of magnitude 6.0 or greater in the near future is considered significant; a 90% chance of such an earthquake by the year 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.^* 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 here have the potential for more widespread damage than major quakes on the west coast. The possibility of a quake measuring 8.0 was even explored on the tv program It Could Happen Tomorrow, on The Weather Channel in early 2006.

Trivia

An eruption from the seismic zone and its potential consequences was the subject of an episode of the History Channel show Mega Disasters.

References

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Geology

History

More quakes predicted

Trivia

See also

References