Memphis
is located along the New Madrid fault zone. The zone includes a large series
of faults beneath the continental crust in an area known as the Reelfoot
Rift. This fault system extends some 150 miles in length, from near
Marked Tree, Arkansas, through northwest Tennessee, the boot heal of Missouri,
extreme west Kentucky and to the southern part of Illinois.
The rift formed about 500 million years ago when plate
tectonics cause the region to be stretched in a northwest-south east direction.
In reaction to this stretching, rocks slowly 
dropped
down (approximately 1 mile) in an area 40 miles wide and 300 miles long,
running northest-southwest.
As seen in the diagram right, the Reelfoot Rift was then covered by approximately 3000 feet of sediment. This sediment was deposited by an ancient sea which covered the central part of the United States near the end of the Cretaceous period. Stretching of the earth's crust beneath the central portion of the United States caused the crust to begin to subside. This allowed the ocean to reach what is southern Illinois today. For more historical geology information follow this link to the Memphis Aquifer.
The New Madrid fault system has the greatest potential
for earthquake damage of any area east of the Rocky Mountains. And, although
these damaging earthquakes are much less frequent than in 
California,
they hold the potential for much greater damage. This potential for damage
is due to the sediment covering of the faults of the area. Earthquake waves
are amplified as they travel through sediments and therefore travel a much
greater distance. These waves can cause damage and earthquakes to be felt
many hundreds of miles farther than earthquakes in the California area.
For example, the picture (right) shows the San Francisco,
California, earthquake of 1906 (magnitude 7.8). It was felt 350 miles away
in the middle of Nevada. Whereas the New Madrid earthquake of 
December 1811 (magnitude 8.0) rang church bells in Boston, Massachusetts,
1,000 miles away. As described above, differences in geology east and west
of the Rocky Mountains cause this strong contrast.
Today, the crust in this same geographic region is going
through east-west shortening and the ancient Reelfoot Rift has been reactivated
again causing earthquakes in the area.
| Back to CERI.
Next site on the trip. Back to Reelfoot Lake.
Back to New Madrid Museum.
|
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 Questions/Comments? |
Graphics
CERI Logo, 2003, as Retrieved
from the World Wide Web, (May, 5, 2003).
USGS, Fact Sheet 131-02: Earthquake Hazard in the Heart of the Homeland-page 4, as Retrieved from the World Wide Web, sediment graphic, (June 4, 2003).
USGS, Uncovering Hidden Hazards in the Mississippi Valley, New Madrid map, as Retrieved from the World Wide Web, (June 4, 2003).
Photos
Thomas, Donna (May 2003).
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