Tuesday, May 23, 2006

New Madrid Seismic Zone

[The U.S.G.S. is worried that Arkanas may see a major earthquake at some point in the (near?) future. Yes, Arkansas.]
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.

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.

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

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.

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. Many of the published accounts describe the cumulative effects of all the earthquakes, 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....

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.[1] 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.


No comments: