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 mideastern United States.
Situated 25 miles below the surface, it could be considered a
"failed rift", because it does not split the North American
plate.
Seismic map showing the
New Madrid Seismic Zone - USGS
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.
Structure
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. 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. 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 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.
In New Madrid
The zone is named after the town of New Madrid, Missouri, the site
of three of the most severe North American earthquakes in recorded
history, including:
- December 16, 1811
- January 23, 1812
- February 7, 1812 was the strongest, with estimated magnitude
exceeding 8
In the fault zone
The New Madrid fault zone lies within the central Mississippi River
valley, extending from northeast Arkansas, through southeast
Missouri, western Tennessee, western Kentucky to southern Illinois.
Historically, this area has been the site of some of the largest
earthquakes in North America. Between 1811 and 1812, four
catastrophic earthquakes, with magnitude estimates greater than 7.0
on the Richter scale, occurred during a 3 month period. Hundreds of
aftershocks followed over a period of several years. 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. 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.
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. By closely monitoring the earthquake activity,
scientists can hope to understand their causes, recurrence rates,
ground motion and disaster mitigation. The probability for an
earthquake of magnitude 6.0 or greater is significant in the near
future, with a 90% chance of such an earthquake by the year
2040.
Historical catastrophes
These catastrophic earthquakes occurred during a three-month period
in December 1811 and early 1812. They caused changes in the course
of the Mississippi River, which rolled 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.
Earthquakes in the New Madrid
seismic zone since 1974. Credit: USGS
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. Scientists estimate that there is a 90%
probability of a magnitude 6.0 to 7.0 quake on this fault system
before 2040. Because of the unconsolidated sediments which are a
major part of the underlying geology of the Mississippi embayment,
large quakes here can affect as much as 20 times the land area of
major quakes on the west coast.
Since 1812
The largest New Madrid seismic zone 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. The last major
earthquake to occur in this region occurred in Charleston,
Missouri, in 1895, and is estimated to have had a magnitude of 6.7
on the Richter Scale.
Comparison: the 1895 Charleston,
Missouri, earthquake in the New Madrid seismic zone with the 1994
Northridge, California, earthquake. Red indicates area of
structural damage, yellow indicates area where shaking was
felt.
A request dated January 13th, 1814, by the Territorial Governor,
William Clark, asked for Federal relief for the "inhabitants of New
Madrid County". This was possibly the first example of a request
for disaster relief, which would later become the job of the
Federal Emergency Management Agency (FEMA).
Issue #32
