Earthquake Hazard in the Heart of the Homeland

Evidence that earthquakes threaten the

Mississippi, Ohio, and Wabash River 

valleys of the Central United States

abounds. In fact, one 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, Tennessee, into southern

Illinois. Several times in the past century,

moderate earthquakes have been widely

felt in southern

Illinois and southwestern Indiana.

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 an

earthquake affects

people depends on

many factors.

Studying earthquakes is especially

challenging because

they happen infrequently and without

U.S. Department of the Interior

U.S. Geological Survey

U.S. Geological Survey Fact Sheet FS-131-02

October 2002 

“. . . research

during the 

last 15 years

has led to new

understanding,

particularly

with respect 

to earthquakes 

in the 

New Madrid

seismic zone.”

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.

any advanced warning. Also, the processes that cause earthquakes work over many

thousands of years and deep beneath the

Earth’s surface. Nonetheless, research

during the last 15 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. 
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).

level of shaking being exceeded in 50

years, while a home builder is more likely to use a map with a 10-percent chance

of being exceeded in 50 years.

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. The hazard in this region is as

high as for places in California, despite a

lower level of modern-day earthquake

activity. Scientists have designated this as

a high hazard region based on studies of

buried sand blow deposits within the

Mississippi River Valley. Sand blow

deposits are believed to be the byproduct

of strong ground shaking associated with

large earthquakes (see box “Digging up

the past”). These deposits have been

dated at about A.D.

900 and A.D. 1450

and suggest that major

earthquakes (magnitude 7 or greater)

reoccur in the region

approximately every

500 years, with the

last sequence being in

1811-1812.

A five hundred-year

interval between magnitude 7 or greater

earthquakes is much more frequent than

expected, given the current level of earthquake activity in the New Madrid region.

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. As anyone who lives in the New Madrid region

would testify, earthquakes are felt in the

New Madrid region much less frequently

than in California, despite the similar

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 hazard arises from

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. 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 lower likelihood that shaking will be

exceeded 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. The bridge

designer would thus use a hazard map

with, say, a 2-percent chance of a given

%g

64 +

48-64

32-48

16-32

8-16

4-8

0-4

Highest hazard

Lowest hazard
To figure out the earthquake hazard of an area, scientists need to

know how often the largest earthquakes occur. Unfortunately (from

a scientific perspective), 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 (such as 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 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 one every 500 years. 

Digging up the past

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)

level of hazard. Additionally, the 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.

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 question

whether there might be fundamental differences between the geologic processes

at, and away from, plate boundaries. The

New Madrid region is far away from any

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.

tectonic plate boundary, and thus applying

what has been learned from studies of

places like the San Andreas system in

California to the New Madrid region may

not be appropriate.
For more information,

contact:

Joan Gomberg 

Eugene Schweig

U.S. Geological Survey

3876 Central Avenue

Suite 2

Memphis, TN 38152-3050

901-678-4858

gomberg@usgs.gov

Graphic design and layout 

by Pamela Svendsen Detra

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 reoccur from historical and geological studies (see box “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:

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

where the earthquakes occur and

local soil conditions, must also be

considered when assessing the

hazard posed by earthquakes.

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%

How likely is a New Madrid earthquake?

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. This is particularly true

for 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

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.)

beneath sediments

 on top of sediments

SEDIMENT

ROCK

time

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 complementary

maps that do show these amplification

characteristics on scales appropriate for

use in urban planning, earthquake hazard

mitigation, and response planning efforts.