The New
Madrid Seismic Zone (NMSZ) lies within the central MississippiValley 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.
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 ReelfootLake
that later filled with water.
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.
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-AmericaEarthquakeCenter. 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.
