n Hidden Hazards

he central Mississippi 

JL Valley has more earthquakes than any other 

part of the United States 

east of the Rocky Mountains. The geologic structures related to the causes 

of these quakes lie deeply 

buried by sedimentary 

deposits. Ongoing geophysical studies are revealing these hidden 

features, enabling residents of the region to 

better prepare for future 

earthquakes.

The winter of 1811-12 was extremely 

difficult for the European settlers of the Mississippi Valley. While Tecumseh, the Shawnee 

chief and visionary, was attempting to unite 

the tribes of the valley in an effort to drive out 

the settlers, the region was struck by three of 

the most powerful earthquakes in United 

States history. These magnitude 8 quakes, centered near the town of New Madrid (Missouri), 

devastated the surrounding region and rang 

church bells 1,000 miles away in Boston. The 

scars that those great earthquakes made on the 

landscape remain the quakes locally 

changed the course of the Mississippi River

r

^St.Louis

Memphis Mjss Tenn

in the Mississippi Valley

The dramatic Gateway Arch in St. Louis, Missouri, towers over the gentle landscape of the central 

Mississippi Valley. In the past, this region has been struck by very powerful earthquakes. The geologic 

structures related to the causes of earthquakes in the region are buried by deep sedimentary deposits. 

Using geophysical techniques, U.S. Geological Survey scientists are revealing these hidden earthquake 

hazards. (Photograph copyright Hichard Sprengeler. courtesy National Park Service.)

and created Reelfoot Lake, which covers an 

area of more than 10 square miles in northwestern Tennessee.

The great earthquakes of 1811-12 were 

not freak events. In recent decades, earth scientists have collected evidence that strong 

earthquakes in the central Mississippi Valley 

have occurred repeatedly in the geologic past. 

Small earthquakes happen in the region frequently. The area in which most of these 

quakes occur is referred to by scientists as the 

New Madrid seismic zone (NMSZ).

In 1811, the central Mississippi Valley 

was sparsely populated and there were few 

man-made structures. Today, this region is 

home to millions of people, including the 

populations of large cities, such as St. Louis, 

Missouri, and Memphis, Tennessee. A repeat 

today of the earthquakes of 1811-12 would 

cause widespread loss of life and billions of 

dollars in property damage.

The central Mississippi Valley is the most earthquake 

prone region of the United States east of the Rocky 

Mountains. The part of the region in which most 

quakes occur is referred to by scientists as the New 

Madrid seismic zone (NMSZ). The topography of the 

central Mississippi Valley is nearly level the higher 

areas, shown here in warm colors, are only a few 

hundred feet above the broad floor of the Mississippi 

Valley, shown in blue. Studies of surface geology offer 

few clues to the causes of quakes in the region.

Another earthquake as powerful as the 

great quakes of 1811-12 may not occur for 

many years. However, scientists estimate that 

there is a 9-in-10 chance of a magnitude 6 to 

7 temblor occurring in the NMSZ within the 

next 50 years. Because of differences in the 

geology east and west of the Rocky Mountains, the effects of a magnitude 7 quake in 

the midcontinental United States could be far 

worse than those of the 1989 magnitude 7 

Loma Prieta, California, earthquake. That 

quake, which struck the San Francisco Bay region during the World Series, killed 63 people 

and caused $6 billion of property damage.

In response to this threat, the U.S. Geological Survey (USGS) has been spearheading an effort to understand the causes of earthquakes in the Mississippi Valley. Begun in the 

1980's, this ongoing cooperative endeavor 

among universities, state governments, and 

Federal agencies has two goals to evaluate 

the level of the earthquake hazard and to help 

reduce the risk to lives and property from 

future quakes in the region.

The effort to assess the seismic risk in 

the Mississippi Valley is, however, hampered 

because the faults and other geologic structures related to earthquakes there have been 

deeply buried over hundreds of millions of 

years by thick layers of sediment. Therefore,

UNITED STATES GEOLOGICAL SURVEY
few clues to the causes of earthquakes in the 

NMSZ can be found at the Earth's surface.

To unmask these hidden geologic structures related to earthquakes, scientists are 

using geophysical techniques, such as mapping variations in the strength of Earth's 

magnetic field. A magnetic map of the central Mississippi Valley region made by geophysicists with the USGS shows with exceptional clarity a major buried feature known 

as the Reelfoot Rift. Most earthquakes in the 

central United States occur within this northeast-trending structure, which formed more 

than 500 million years ago.

A rift structure is created when powerful geologic forces begin to pull the Earth's 

crust apart. If this process continues long 

enough, the crust separates to form an ocean 

basin, as happened to form the Atlantic Ocean 

basin. The Reelfoot Rift is called a failed rift 

because the Earth's crust did not separate 

enough to create a new ocean basin. However, the crust was disturbed enough to form 

major faults that mark the axis and margins 

of the rift and now contribute to the occurrence of earthquakes in the NMSZ.

The locations of earthquakes within the 

NMSZ also appear to be strongly influenced 

by the presence of large bodies of igneous 

rock, which were formed by the cooling and 

solidification of molten rock beneath the 

Earth's surface. These igneous rock bodies 

show up as areas of high magnetic intensity 

on the magnetic map of the region. The southern part of the NMSZ is a narrow belt of 

quake activity that trends northeast, probably 

following a fault along the center of the 

Reelfoot Rift. However, this trend abruptly 

changes direction to north-northwest where 

it intersects and follows the major lobe of a 

large mass of igneous rock. Within this lobe, 

the belt of quake activity broadens to closely 

match the shape of the igneous body on the

Geologic structures related to earthquakes 

in the central Mississippi Valley region 

have been deeply buried over hundreds 

of millions of years by thick layers of 

sediment. Geophysical studies have 

revealed a major buried northeasttrending feature known as the 

Reelfoot Rift (here shown in 

cross section), which formed 

more than 500 million years 

ago when powerful geologic 

forces began to pull the Earth's 

crust apart. Most quakes in the 

central Mississippi Valley appear to be related to the major 

faults and large bodies of igneous rock within the rift.

magnetic map. The dispersed pattern of earthquakes associated with igneous rock bodies 

may be an important new clue to the causes 

of quakes in the NMSZ.

One of the few places in the central 

Mississippi Valley where faults break the 

Earth's surface is near the town of Commerce, Missouri. Trenches dug there reveal 

sedimentary deposits, as young as about 

12,000 years old, that have already been broken by movement on the faults. This site lies 

along a linear magnetic feature called the 

Commerce geophysical lineament. Geophysicists believe that this northeast-trending lineament may represent a zone of faulting more than 150 miles long. Although it is 

still poorly understood, this geologic feature 

could pose a significant additional seismic 

threat to the region.

Earthquakes generated by buried geologic features will continue to shake the Mississippi Valley in the future. Using geophysical techniques and other tools, earth scientists are striving to better understand the

The shaded-relief 

magnetic map of the 

region surrounding the 

New Madrid seismic 

zone (NMSZ), shows 

areas of high magnetic 

intensity as "hills" and 

those of low intensity as 

"valleys." The deeply 

buried Reelfoot Rift is 

expressed as a smoother 

appearing area between 

the yellow lines, and the 

Commerce geophysical 

lineament (CGL), 

paralleling the rift, is 

traced. Red dots show 

the locations of the many 

earthquakes recorded in 

the NMSZ since 1974, 

and major igneous 

bodies, which show up 

as prominent "hills," are 

outlined in black.

causes of these quakes. The knowledge they 

have gained is being used to increase public 

awareness of the region's hidden seismic hazards. This knowledge is also being applied 

by engineers to make new and existing structures in the Mississippi Valley more resistant to 

earthquakes. In these ways earth scientists and 

engineers are helping to protect residents of 

the central United States from loss of life and 

property in future quakes.

Thomas G. Hildenbrand, Victoria E.

Langenheim, Eugene Schweig, Peter H.

Stauffer, and James W. Hendley II

Graphic design by 

Sara Boore and Susan E. Mayfield

COOPERATING ORGANIZATIONS

Central United States Earthquake Consortium

Federal Emergency Management Agency

Missouri Dept. of Natural Resources, Division of

Geology and Land Survey

Saint Louis University

Southeast Missouri State University

The University Of Memphis

University of Kentucky

For more information contact:

U.S. Geological Survey (901) 678-2007

Center for Earthquake Research and Information

The University of Memphis, Memphis, TN 38152

http://www.ceri.memphis.edu

See also The Mississippi Valley "Whole Lotta 

Shakin'Goin'On" (\JSGS Fact Sheet 168-95).

U.S. Geological Surve\ Fad Sheet 200-96 

1996

Reelfoot Rift

U.S. DEPARTMENT OFTHE INTERIOR - U.S. GEOLOGICAL SURVEY