U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3125

January 2007

UNDERSTANDING EARTHQUAKE HAZARDS IN THE CENTRAL UNITED STATES

Earthquake Hazard in the Heart of the Homeland

Printed on recycled paper

Evidence that earthquakes threaten the Mississippi, 

Ohio, and Wabash River valleys of the Central United 

States abounds. In fact, several of the largest historical 

earthquakes to strike the continental United States occurred 

in the winter of 1811–1812 along the New Madrid seismic 

zone, which stretches from just west of Memphis, Tenn., into 

southern Illinois (fig. 1). Several times in the past century, 

moderate earthquakes have been widely felt in the Wabash 

Valley seismic zone along the southern border of Illinois and 

Indiana (fig. 1). Throughout the region, between 150 and 200 

earthquakes are recorded annually by 

a network of monitoring instruments, 

although most are too small to be felt by 

people. Geologic evidence for prehistoric 

earthquakes throughout the region has 

been mounting since the late 1970s. 

But how significant is the threat? How 

likely are large earthquakes and, more 

importantly, what is the chance that the 

shaking they cause will be damaging? 

Like many natural phenomena, how 

and why earthquakes happen, and their 

effects, depend on many factors. Studying earthquakes is especially challenging 

because they happen infrequently and 

without any advanced warning. Also, the 

processes that cause earthquakes work 

deep beneath the Earth’s surface and 

over many thousands of years. Nonetheless, research during the last 20 years has 

led to new understanding, particularly 

with respect to earthquakes in the New 

Madrid seismic zone. Not surprisingly, 

as the understanding of earthquakes 

evolves, so do the estimates of the 

hazard that they pose. 

Estimating Earthquake Hazards

Earthquake hazard refers to the 

consequences of an earthquake that may 

disrupt the normal activities of people 

or cause them loss. Most damage is due 

to ground shaking caused by waves that 

emanate from the abrupt fault movement 

during an earthquake. Seismic hazard 

maps depict the ground shaking that is 

expected to be exceeded at a selected 

probability (or chance) over a specific 

time period. Estimates of this “probabilistic” ground shaking, or hazard, at any 

given location must account for many 

factors including the possible shaking 

from all likely earthquakes and the types 

of rocks and soil in the region.

The USGS produces earthquake (seismic) hazard maps on 

a national scale (fig. 2). Hazard maps are also now produced in 

select urban areas. At either scale there are a suite of maps for 

different probabilities and time periods; the choice of which to 

use depends upon the needs of the user. Builders of a dam, for 

instance, might want to consider longer periods of time and less 

likely levels of shaking than a home builder. This is because a 

dam is built to have a longer life, and damage to it would have a 

greater impact on society. 

V

Figure 1.  This map of the New Madrid and Wabash Valley seismic zones shows

earthquakes as circles. Red circles indicate earthquakes that occurred from 1974 to

2002 with magnitudes larger than 2.5 located using modern instruments (University

of Memphis). Green circles denote earthquakes that occurred prior to 1974 (USGS

Professional Paper 1527). Larger earthquakes are represented by larger circles.
Earthquake Hazards in the Central United States

One of the most prominent features on the national seismic 

hazard maps is a zone of high hazard surrounding the New 

Madrid region in the Central United States. By some measures, 

the hazard in this region is as high as for places in California. 

Buried sand-blow deposits within the Mississippi River valley 

are believed to be the by-product of strong ground shaking 

associated with large past earthquakes (see “Digging Up the 

Past”). These deposits and the major earthquakes that caused 

them have been dated at about A.D. 900 and A.D. 1450.

A five-hundred-year interval between magnitude 7 or 

greater earthquakes in the Mississippi River valley is much 

more frequent than expected. Elsewhere scientists use the 

frequency of small and moderate earthquakes to estimate the 

likelihood of larger events, but in the New Madrid region, this 

rule of thumb does not appear to apply. In other words, in the 

New Madrid region, the rate of occurrence of major earthquakes and the high hazard they pose is comparable to that in 

California, whereas smaller earthquakes occur much less often. 

These smaller events remind us about the less frequent, but 

more hazardous major earthquakes. In the New Madrid region,

we lack these reminders, but not the hazard associated with the 

major earthquakes. Additionally, large earthquakes in the New 

Madrid region seem to happen much more frequently than predicted from extremely accurate satellite-based measurements of 

ground deformation. These measurements show that the Earth’s 

crust is being deformed, or strained, extremely slowly, if at all. 

This is in contrast to California, where the rate of major earthquakes is predictable based on measured deformation.

Figure 3.  On this shaded topographic relief map, the New

Madrid seismic zone is shown by a star, and the boundaries of

the North American and surrounding plates are indicated by red

lines. Mountainous regions tend to occur near plate boundaries,

resulting from the motions between plates. The nearest plate

boundary to the New Madrid seismic zone is many thousands of

kilometers away.

%g

64 +

48-64

32-48

16-32

8-16

4-8

0-4

Highest hazard

Lowest hazard

Figure 2.  The U.S. Geological Survey shaking-hazard maps for the United States are based on

current information about the rate at which earthquakes occur in different areas and on how far

strong shaking extends from earthquake sources. Colors on this particular map show the levels

of horizontal shaking that have a 1-in-50 chance of being exceeded in a 50-year period. Shaking

is expressed as a percentage of g (g is the acceleration of a falling object due to gravity).
beneath sediments

 on top of sediments

SEDIMENT

ROCK

time

Figure 4.  When waves of a small earthquake impinge on the boundary

between hard rock and overlying sediments from below (lower seismogram),

they become amplified as they travel through the sediments to the surface

(upper seismogram). A seismogram records the wave motion over time. The

amplifying effects of thick sediments above the New Madrid seismic zone

results in motions at the surface that are many times larger than below.

(Courtesy of C. Langston, The University of Memphis.)

How Likely is a New Madrid Earthquake? 

In much the same way as meteorologists forecast rain, earth scientists present forecasts of earthquakes as 

the chance or “probability” of an earthquake occurring in a specific time interval. Although we are still learning 

why the New Madrid region experiences large earthquakes, research during the last 15 years in the Central United 

States has led to new discoveries that, not surprisingly, have led to new estimates of the likelihood of damaging 

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

past. We determine how often earthquakes recur from historical and geological studies (see “Digging Up the Past”). 

Using these data, which were also used to produce the National Seismic Hazard Maps, the USGS and the Center 

for Earthquake Research and Information of the University of Memphis now estimate that for a 50-year 

time period:

Probability of a repeat of the 1811–1812 earthquakes

(magnitude 7.5–8.0) = 7–10%

Probability of a magnitude 6.0 or larger = 25–40%

Far from a Plate Boundary

Why does the Central United States have more 

large earthquakes than expected? Why is it so different from California? The answers to these questions are still unknown, but scientists now ask 

whether there might be fundamental differences 

between the geologic processes at, and away 

from, plate boundaries (fig. 3). The New Madrid 

region is far away from any tectonic plate boundary, and thus applying what has been learned 

from studies of places like the San Andreas fault 

in California to the New Madrid region may not 

be appropriate.

Factors Controlling Ground Shaking

In addition to earthquake recurrence, hazard 

also depends on how amplitudes of earthquake 

waves die out as they move away from the earthquake source to the affected site. It is well known 

that wave energy decreases much more slowly in 

the Central and Eastern United States than in the 

West. For the same size earthquake, this leads to 

greater shaking over larger areas, or higher hazard, in the Central and Eastern United States.

Earthquake shaking also may be significantly amplified or damped by the soils immediately beneath a site (fig. 4). This is particularly true 

for the thick sediments that underlie most of the New 

Madrid seismic zone. Recent research has highlighted the potential amplification of ground shaking 

by these thick sediments and has begun to shed light 

on the physical processes that cause this. Because 

characterization of this amplification requires 

detailed mapping of the soils and their properties, 

amplification characteristics are not included in the 

National Seismic Hazard Maps. Efforts are underway, however, to produce maps that do show these 

amplification characteristics on scales appropriate 

for use in urban planning, earthquake hazard mitigation, and response planning efforts. 
Buried sand blow

Artifacts

Charcoal, sticks, etc.

Buried prehistoric sand blow

Modern sand blow

Liquefied sand

Earthquake waves

Silt and clay layers

Filled fissure

(sand dike)

(after Sims and Garvin, 1995,

Bulletin Seismological Society 

of America, v. 85, p. 51–65.)

For more information, contact:

Eugene Schweig 

U.S. Geological Survey

3876 Central Avenue

Suite 2

Memphis, TN 38152-3050

901-678-4974

schweig@usgs.gov

http://earthquake.usgs.gov/regional/ceus/

Written by Joan Gomberg

and Eugene Schweig

Publication layout by Sharon Powers

This fact sheet is available online at

http://pubs.usgs.gov/fs/2006/3125

Earthquakes in the approximate range of magnitude 7.5 

to 8.0 are capable of causing widespread damage over a large 

region. Magnitude 6.0 earthquakes can cause serious damage 

in areas close to the earthquake’s location. It is important to 

remember that earthquake damage mostly results from the shaking associated with the waves emitted by the earthquake. The 

probability of damaging shaking, or hazard, depends on much 

more than earthquake size. These other factors, such as how far 

away the earthquakes occur and local soil conditions, must also 

be considered when assessing the hazard posed by earthquakes.

Digging Up the Past

To determine the earthquake hazard of an area, scientists 

need to know how often the largest earthquakes occur. This is 

difficult because the time between major earthquakes is much 

longer than the time period for which we have modern instrumental measurements or even historical accounts of earthquakes. Fortunately, scientists have found a sufficiently long 

record of past earthquakes that is preserved in the rock and soil 

beneath our feet. The unraveling of this record is the realm of 

a field called “paleoseismology.” In the Central United States, 

abundant sand blows are studied by paleoseismologists. These 

patches of sand erupt onto the ground when waves from a large 

earthquake pass through wet, loose sand. The water pressure 

increases, forcing the sand grains apart until the sand starts to 

behave like a liquid (“liquefies”), and a slurry of sand and water 

is forced to the ground surface. If the age of material buried 

by the erupted sand can be determined (for example, by using 

Native American artifacts or plant remains), then we know the 

earthquake must have occurred after this date. If the sand blow 

is itself buried by something that can be dated, then we know 

the earthquake happened before this date. We now know that a 

series of large earthquakes, similar in size and location to those 

in 1811–1812, have occurred twice in the past 1,200 years, once 

between A.D. 800 and 1000, and again between A.D. 1300 and 

1600, or about once every 500 years.