U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3142

July 2006

U N D E R S T A N D I N G E A R T H Q U A K E H A Z A R D S I N U R B A N A R E A S

Urban Seismic Hazard Mapping for Memphis, Shelby County, 

Tennessee

Printed on recycled paper

Who Uses Urban Seismic Hazard 

Maps?

A number of public and private groups 

use hazard maps to minimize losses 

resulting from earthquake shaking. The 

maps are used to guide city planners and 

engineers in designing safe structures and 

determining where they should be located. 

Emergency responders use hazard maps 

in planning post-earthquake relief efforts. 

Insurance companies use hazard maps 

to examine the risk to their portfolios in 

order to ensure financial recovery after 

damaging earthquakes.

What is Seismic Hazard?

Seismic hazard refers to a measure of the shaking, or ground motion, 

from earthquakes that can damage our 

man-made environment. The hazard 

depends on the magnitudes and locations 

of likely earthquakes, how often they 

occur, and the properties of the rocks and 

sediments that earthquake waves travel 

through.

In the central U.S., earthquakes occur 

along the New Madrid seismic zone; 

active faults in this region are buried by 

the thick sedimentary deposits of the 

Mississippi embayment (fig. 2).

Why are there Different Map 

Types?

Just as weather forecast maps come in 

a variety of types to suit different needs, so 

do seismic hazard maps. There are maps 

showing the shaking expected for waves 

of different frequencies. This is because 

different structures are tuned to respond 

to particular seismic waves, much like 

different musical instruments resonate at 

particular sound frequencies. One seismic 

hazard map may show shaking from long 

period (for example, 1.0 second or longer) 

waves that cause the ground to move 

slowly over a large area. A long bridge 

or tall building might be most affected by 

these waves. Shorter period waves (for 

example, 0.2-second period) are more 

likely to damage single-family houses.

Earthquakes cannot be predicted, 

but scientists can forecast how 

strongly the ground is likely to shake 

as a result of an earthquake. Seismic 

hazard maps provide one way of 

conveying such forecasts. The U.S. 

Geological Survey (USGS), which 

produces seismic hazard maps 

for the Nation, is now engaged in 

developing more detailed maps for 

vulnerable urban areas. The first set 

of these maps is now available for 

Memphis, Tenn. (fig. 1).

Figure 1. Photograph of part of the mapped 

area: the view from downtown Memphis, 

Tenn., of Mud Island between the Wolf and 

Mississippi Rivers, and the I-40 bridge crossing from downtown to West Memphis, Ark.

Figure 2. Map of the region surrounding Memphis, Tenn. The darker orange area is covered 

by thick sediments, called the Mississippi embayment. These sediments significantly affect 

how the ground shakes during earthquakes. White lines indicate likely locations of major 

faults (buried beneath the sediments), and black dots show the locations of earthquakes 

recorded since the mid-1970s. Blue line is Mississippi River.

92°W 90°W 88°W

34°N

36°N

38°N

Mississippi

Tennessee Alabama

Arkansas

Missouri

Kentucky

Illinois

MEMPHIS

Mississippi River

0 100 KILOMETERS
Seismic hazard maps may also be 

either probabilistic or deterministic. In 

an earthquake-prone area, an engineer 

might assume a particular earthquake will 

occur and needs to know how hard it is 

likely to shake the ground. These needs 

call for “deterministic,” or “scenario,” 

forecast maps. Such maps might be used 

when conservative estimates and worstcase scenarios are called for, such as 

when building critical facilities. Deterministic maps do not take into account the 

likelihood of such a scenario occurring.

When considerations of likelihood 

become important, a “probabilistic” 

forecast map may be more appropriate. 

Probabilistic seismic hazard maps 

attempt to account for all earthquakes 

that might affect an area. They incorporate information about seismic activity 

and the locations of active faults, as well 

as estimates of the time interval between 

earthquake recurrences. 

320+

240-320

160-240

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40-80

20-40

0-20

%g

Highest hazard

Lowest hazard

Figure 4. The Memphis maps show a reduction in short-period (0.2 second) shaking by as 

much as 30 percent (TOP RIGHT INSET MAP) 

and an increase in longer period shaking, 

nearly doubling the motions in the 1.0-secondperiod map (TOP LEFT INSET MAP), compared 

to the National Seismic Hazard map (BOTTOM 

MAP and TOP BACKGROUND MAPS). These 

differences result directly from including 

the effects of the sediments, which are not 

accounted for in the National Seismic Hazard 

maps. The color scale shows the acceleration 

level, measured in percent of gravity or “g” 

units; 100 percent g is the acceleration of a 

freely falling object due to gravity.

MISSISSIPPI R

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GERMANTOWN

NONCONNAH CREEK

WOLF RIVER

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BARTLETT

MEMPHIS

g

1.0

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BARTLETT

MEMPHIS

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72

61 78

14

MISSISSIPPI R

VI ER

NONCONNAH CREEK

GERMANTOWN

51 14

0 10 KILOMETERS

90°5'W

35°5'N

35°10'N

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35°10'N

90°W 89°55'W 89°50'W

Figure 3. Two of the probabilistic urban seismic hazard maps for the Memphis, Shelby County, Tenn., area. These maps show accelerations with 

oscillation periods of 1.0 second, with a 2 percent (LEFT) and 10 percent (RIGHT) chance of being exceeded in 50 years. The color scale shows 

the acceleration level, measured in gravity, “g” units; 1 g is the acceleration of a freely falling object due to gravity.
The USGS has developed a series 

of probabilistic seismic hazard maps 

for the Nation. These maps consider a 

time period of 50 years, a typical building lifetime. At all locations the maps 

answer the question, “At some specific 

probability, what level of shaking is likely 

to be exceeded?” (that is, the probability 

of exceedence). For example, the maps 

of the Memphis area (fig. 3) show the 

ground motions that have a 2 percent (left) 

and a 10 percent (right) chance of being 

exceeded within the next 50 years. The 

predicted motions shown on the 2 percent 

maps are larger because they allow for 

the more infrequent, large earthquakes.

The Memphis Maps

Although the USGS National Seismic 

Hazard maps cover the entire Nation, 

they are not sufficiently detailed to 

include near-surface geologic conditions, 

which can greatly affect ground shaking. 

Therefore, the USGS is also developing 

complementary urban hazard maps for 

earthquake-prone metropolitan areas. 

These urban hazard maps are much better 

predictors of local variations in ground 

shaking because they take into consideration the near-surface geology. The first 

set of these urban maps is now available 

for Memphis, Tenn.

Memphis is a densely populated urban 

area, built on a very thick layer (nearly 1 

km) of sediments that extends over much of 

the broad Mississippi Valley. Near-surface 

sediments are variable from place to place 

beneath the city, but in general the sediment 

layer thickens toward the northwest.

A comparison of the Memphis and 

the National Seismic Hazard maps (fig. 

4) highlights the importance of including the influence of shallow geologic 

structure when estimating ground motion. 

The national maps show an increase in 

hazard toward the northwest because the 

faults of the New Madrid seismic zone lie 

northwest of Memphis. As a result of the 

enhanced dissipation of waves traveling 

through thick sediments, the Memphis

maps show reductions in short-period 

shaking (right map, fig. 4), implying that single family homes and other 

small buildings are not as vulnerable as 

previously thought. However, the thick 

sediments also resonate at some frequencies and cause increases in longer period 

shaking (left map, fig. 4); this implies a 

greater vulnerability of tall buildings or 

long bridges to earthquake shaking. The 

average motions for both maps (about 

0.7 g each; 1.0 g is the acceleration of 

a falling object due to gravity) are large 

enough to cause significant damage. 

The Memphis seismic hazard maps 

show estimated values of ground acceleration (an indicator of the force of the 

seismic waves). These maps display 

ground acceleration in two ways: (1) peak 

ground acceleration (PGA), the largest 

value of the total motion, consisting of 

many types of seismic waves, and (2) 

spectral accelerations (Sa), consisting of 

waves with periods of either 0.2 or 1.0 

second. Peak ground accelerations of as 

little as 0.1 g may be strongly felt and 

do light damage, and severe shaking and 

moderate to heavy damage may occur for 

values above about 0.3 g.

The USGS Memphis urban seismic 

hazard maps include probabilistic maps 

(fig. 3) showing ground motions expected 

to be exceeded during the next 50 years 

with probabilities of 2, 5, and 10 percent 

(5 percent maps are not shown here but are 

available online at http://earthquake.usgs.

gov/regional/ceus/urban_map/memphis/

index.php). Products also include scenario 

maps for magnitude 7.7 (left, fig. 5) and 

magnitude 6.2 (right, fig. 5) earthquakes. 

The magnitude 6.2 scenario map shows 

ground motions expected for a repeat of 

an earthquake that occurred near Marked 

Tree, Ark., in 1843. The magnitude 7.7 

scenario map was chosen to show the 

expected effect of a repeat of an earthquake 

that occurred on December 16, 1811. This 

Figure 6. Effective strengthening of existing 

structures, like the recent retrofitting of the 

I-40 bridge crossing the Mississippi River, 

requires accurate information about likely 

earthquake ground shaking. (Photo by G. 

Patterson, CERI, University of Memphis.)

MISSISSIPPI R

VI ER

GERMANTOWN

NONCONNAH CREEK

WOLF RIVER

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BARTLETT

MEMPHIS

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72

61 78

14

MISSISSIPPI R

VI ER

NONCONNAH CREEK

GERMANTOWN

51 14

0 10 KILOMETERS

90°5'W

35°5'N

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35°10'N

90°W 89°55'W 89°50'W

g

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0.8

0.7

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0.0

Figure 5. Scenario maps for the Memphis, Shelby County, Tenn., area. The map on the LEFT is for a magnitude 7.7 earthquake, a possible repeat of 

the December 16, 1811, earthquake, and the map on the RIGHT is for a magnitude 6.2 earthquake, like the Marked Tree, Ark., event in 1843. The ground 

motions shown correspond to shaking with an oscillation period of 1.0 second.
earthquake occurred along the southern 

fault shown on the map in figure 2. Urban 

seismic hazard maps like these provide 

needed information for the design of new 

structures and retrofitting (fig. 6) of existing ones.

Related Products

The investigations done to produce the 

Memphis hazard maps resulted in a variety of other products. Among these are 

maps of the surficial geologic materials 

(fig. 7) and maps showing the likelihood 

of liquefaction (not shown). Liquefaction 

occurs when water-saturated sediments 

lose their cohesive strength during violent 

ground shaking.

An online database of sediment 

properties determined from well and 

boring logs can be found at http://gwidc.

memphis.edu/website/introduction/. Also, 

a variety of technical and nontechnical 

reports are available at http://earthquake.

usgs.gov/regional/ceus/urban_map/memphis/index.php.

Making Urban Seismic Hazard Maps

0 1 MILE

90°5'W

35°5'N

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Shaking amplification Geologic maps and

subsurface measurements

Figure 7. Geologic map of the southeastern portion of the Memphis mapping 

area (by D. Moore and S. Diehl). Colors show different sediment types; most of 

this area (beige color) is covered by sediments (called loess) carried by wind and 

deposited during the last glacial period, and by river flood-plain sediments (yellows 

and tans). This and other new geologic maps for Memphis are available online at 

http://earthquake.usgs.gov/regional/ceus/urban_map/memphis/index.php. 

Figure 8. Geologists map the soils and sediments at the surface. Properties of these surficial materials, how deeply they extend, and 

properties of the materials below are characterized from measurements made in boreholes 

drilled for construction projects and in water 

wells. Much like X-rays or ultrasound imaging, 

information about the subsurface also comes 

from subsurface imaging techniques. These 

data are then integrated to construct a 3-D 

model of the layering of soils and sediments.

The soil and sediment layers change the seismic waves that travel upward through them, 

from an earthquake below. The 3-D model 

describes the layering beneath any particular 

location, and is used to calculate how the 

shaking from the waves will vary from place to 

place. For example, the sediments in the river 

flood plain beneath site B may amplify some 

types of shaking more than at site A located 

on the bluffs.

For More Information Contact:

Joan Gomberg or Eugene Schweig

Earthquake Hazards Program

U.S. Geological Survey

3876 Central Avenue, Suite 2

Memphis, TN 38152

901-678-4974

gomberg@usgs.gov or

schweig@usgs.gov

Central U.S. Hazard Maps are at:

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

ceus/urban_map/index.php/

Earthquake Hazards Program:

http://earthquake.usgs.gov/

Partners: Regional Seismic Networks, 

Central U.S. Earthquake Consortium 

(CUSEC), Mid-America Earthquake 

Center, and university collaborators 

Written by Joan Gomberg

Graphics and layout design by

Lisa Wald and Richard Dart

This Fact Sheet is available online at: 

http://pubs.usgs.gov/fs/2005/3142/