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The New Madrid Seismic Zone

 

Historical Large Magnitude Events

 

The New Madrid Seismic Zone (NMSZ) lies within the central Mississippi Valley and extends through northeast Arkansas, southeast Missouri, and western Tennessee. Historically, this area has been the site of some of the largest earthquakes in North America.

 

In the winter of 1811-1812, there was a series of strong magnitude earthquakes in the central U.S. The last major earthquake in this area was on February 7, 1812, and had an estimated magnitude of ~8.0. Shaking during the 1812 event was felt over thousands of square miles (right), ringing bells in Charleston, rattling furniture in the White House, and causing the Mississippi River to run backwards.

 

An isoseismic map (right) shows shaking intensities of the 1812 earthquake in the NMSZ. The roman numerals represent shaking intensities on the Modified Mercalli Intensity scale. From historical records such as journal entries and newspaper articles, geologists can estimate the strength of the shaking based on descriptions of observations during the shaking.

 

 

Isoseismic map of the 1811 New Madrid earthquake.

 

Shaking was strong enough to force sand to erupt at the surface, trigger landslides, cause large areas to be uplifted or dropped down in elevation creating sunk lands such as Reelfoot Lake that later filled with water.

 

 

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To learn more about the historical earthquakes of 1811-1812, click the arrow to play the video above, or click here. And check out the 20 Cool Facts about the New Madrid Seismic Zone poster on the USGS website.

 

 

 

 

Block diagram of the New Madrid Seismic Zone and Reelfoot Rift area.

Tectonic Setting

 

The NMSZ coincides with an ancient rift complex that formed during the late Precambrian and was reactivated during the Mesozoic (~160 million years ago) when the supercontinent Pangaea was breaking apart to form the Atlantic Ocean and the separate continents we now have. During that time, scientists believe that a mantle plume (an upwelling of hot magma similar to that beneath Hawaii) developed under the New Madrid. The upward push and the extensional pull thinned the crust and allowed magma to push its way up through faults, forming igneous plutons. As North America drifted past the hot spot, the New Madrid crust began to cool and sink, warping downward. Today, the region is undergoing compressional stress which is further allowing movement along the faults (left).

 

 

The NMSZ is comprised of multiple fault segments. Seven faults have been identified: four buried faults interpreted from seismicity, and three visible at the surface. The New Madrid region has more earthquakes than any other part of the U.S. east of the Rocky Mountains 100 to 200 quakes detected per year. Data recorded from these earthquakes is used to learn more about the structure and faults underground. To learn more about how the subsurface faults were identified, watch the video below (or click here).

 

 

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Seismic Hazard

 

It is generally accepted that earthquakes can be expected in the future as frequently as in the recent past.

 

Given this assumption, and interpretations of research collected over the past 15 years, USGS estimates that for the next 50-year period:

 

      The probability of a repeat of the 1811-1812 earthquakes is 7-10%

 

      The probability of a magnitude 6.0 or greater is 25-40%

 

To read the USGS publication (USGS FS-131-02) that contains the above probabilities and additional information, click here.

 

2008 national map showing earthquake hazard probability.

 

The map above is a 2008 USGS hazard probability map. Colors show levels of horizontal shaking that have a 2-in-100 chance of being exceeded in a 50-year period. Shaking is expressed as a percentage of g (gravitational force).

 

 

Disaster Planning

 

Due to the potential for a large magnitude event occurring in the NMSZ in the future, many groups have been conducting research and planning associated with response to a large New Madrid earthquake. Some of the most active New Madrid research groups include the Center for Earthquake Research and Information, USGS, and the Mid-America Earthquake Center. Groups such as the Central U.S. Earthquake Consortium (CUSEC), of which Alabama is a member, and state agencies across the CUSEC eight-state region have been actively participating in planning and exercises related to coordination and preparation for a large magnitude event. To learn more about CUSEC and this effort, click on the video to the right.

 

 

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