New Madrid Seismic Zone
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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 United States and Midwestern United States. 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.
The New Madrid 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 portion of the Mississippi embayment. Most of the seismicity is located from 3 to 15 miles (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 (240 km) long fault system, which extends into four 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 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.
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[edit] History
The zone had four of the largest North American earthquakes in recorded history, with magnitude estimates 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.
- 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 were caused by 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 the 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, Missouri, 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.
This series of temblors caused permanent changes in the course of the Mississippi River, which flowed backward temporarily, and they 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.
Because of the change in the course of the Mississippi River land was cut off their original counties by the river, placing them 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.[1]
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.
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 miles (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 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 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.
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.
[edit] 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 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.[2] 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.
The consequences of an earthquake in this region has 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.
[edit] See also
- Earthquake
- Intraplate earthquake
- Fault mechanics
- Disaster preparedness
- Household seismic safety
- Plate tectonics
- Seismic retrofit
- Soil liquefaction
[edit] References
- USGS New Madrid
- U.S. Geologic Survey site discusses the New Madrid seismic zone.
- Uncovering Hidden Hazards in the Mississippi Valley
- Central US Earthquake Consortium
- U. Memphis, TN, Center for Earthquake Research and Information (CERI) comprehensive references for the 1811–1812 earthquakes
- Links to eyewitness descriptions of the three 1811-12 earthquakes, and results of the earthquakes as seen in photographs of 1904.
- Your Fault, My Fault, and the New Madrid Fault
