% g 16-32

32-48

48-64

64+

8-16

4-8

0-4

Highest hazard

Lowest hazard

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3018

April 2008 Printed on recycled paper

2008 United States National Seismic Hazard Maps

T he U.S. Geological Survey’s National Seismic Hazard 

Maps are the basis for seismic 

design provisions of building 

codes, insurance rate structures, 

earthquake loss studies, retrofit 

priorities, and land-use planning. Incorporating these hazard 

maps into designs of buildings, 

bridges, highways, and critical infrastructure allows these 

structures to withstand earthquake shaking without collapse. 

Properly engineered designs not 

only save lives, but also reduce 

disruption to critical activities 

following a damaging event. By 

estimating the likely shaking 

for a given area, the maps also 

help engineers avoid costs from 

over-design for unlikely levels of 

ground motion. 

The Update Process

The U.S. Geological Survey recently updated the 

National Seismic Hazard Maps by incorporating new seismic, 

geologic, and geodetic information on earthquake rates and 

associated ground shaking. These 2008 maps supersede versions released in 1996 and 2002. Updating the maps involved 

interactions with hundreds of scientists and engineers at 

regional and topical workshops. USGS also solicited advice 

from working groups, expert panels, State geological surveys, 

Federal agencies, and hazard experts from industry and academia. The Pacific Earthquake Engineering Research Center 

developed new crustal ground-motion models; the Working 

Group on California Earthquake Probabilities revised the 

California earthquake rate model; the Western States Seismic 

Policy Council submitted recommendations for the Intermountain West; and three expert panels were assembled to provide 

advice on best available science.

Changes to the Maps

The most significant changes to the 2008 maps fall into 

two categories, as follows:

1. Changes to earthquake source and occurrence rate models:

•	 In California, the source model was updated to account 

for new scientific information on faults. For example, 

models for the southern San Andreas Fault System 

were modified to incorporate new geologic data. The 

source model was also modified to better match the 

historical rate of magnitude 6.5 to 7 earthquakes.

•	 The Cascadia Subduction Zone lying offshore of 

northern California, Oregon, and Washington was modeled using a distribution of large earthquakes between 

magnitude 8 and 9. Additional weight was given to the 

possibility for a catastrophic magnitude 9 earthquake 

that ruptures, on average, every 500 years from northern California to Washington, compared to a model that 

allows for smaller ruptures.

Colors on this map show the levels of horizontal shaking that have a 2-in-100 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).
For Further Information

•	 The Wasatch fault in Utah was modeled to include the 

possibility of rupture from magnitude 7.4 earthquakes 

on the fault.

•	 Fault steepness estimates were modified based on global 

observations of normal faults.

•	 Several new faults were included or revised in the 

Pacific Northwest, California, and the Intermountain 

West regions.

•	 The New Madrid Seismic Zone in the Central U.S. was 

revised to include updated fault geometry and earthquake information. In addition, the model was adjusted 

to include the possibility of several large earthquakes 

taking place within a few years or less, similar to the 

earthquake sequence of 1811–1812.

•	 Source models for the region near Charleston, S.C., have 

been modified to include offshore faults that are thought 

to be capable of generating earthquakes.

•	 A broader range of earthquake magnitudes was used for 

the Central and Eastern U.S.

•	 Earthquake catalogs and seismicity parameters were 

updated.

2. Changes to models of ground shaking (that show how 

ground motion decays with distance from an earthquake’s 

source) for different parts of the U.S., based on new published studies: 

•	 New ground-motion prediction models developed by the 

Pacific Earthquake Engineering Research Center were 

adopted for crustal earthquakes beneath the Western 

U.S. These new models use shaking records from 173 

global shallow crustal earthquakes to better constrain 

ground motion in western States.

•	 Several new and updated ground-shaking models for 

earthquakes in the Central and Eastern U.S. were implemented in the maps. One of the new ground-shaking 

models accounts for the possibility that ground motion 

decays more rapidly from the earthquake source than 

was previously considered.

•	 New ground-motion models were applied for earthquake sources along the Cascadia Subduction Zone.

The new National Seismic Hazard Maps show, with some 

exceptions, similar or lower ground motion compared with the 

2002 edition. For example, ground motion in the Central and 

Eastern U.S. has been generally lower by about 10–25 percent 

due to the modifications of the ground-motion models. Ground 

motion in the Western U.S. is as much as 30 percent lower for 

shaking caused by long-period (1-second) seismic waves, which 

affect taller multistory buildings, and ground motion is similar 

(within 10–20 percent) for shaking caused by short-period 

(0.2-second) waves, which affect structures of one or a few 

stories.

The new 2008 maps represent the best available science 

as determined by the USGS from an extensive informationgathering and review process. Changes will be made in future 

versions of the maps as new information on earthquake sources 

and resulting ground motion is gathered and processed.

To learn more about the National Seismic Hazard Mapping Project, go to URL http://earthquake.usgs.gov/hazmaps/;

Working Group on California Earthquake Probabilities, go to URL http://pubs.usgs.gov/of/2007/1437/

Or you may also contact Mark Petersen: mpetersen@usgs.gov.

San Francisco, Calif., Earthquake, April 18, 1906. Fault trace 2 miles 

north of the Skinner Ranch at Olema. View is north. 1906. Plate 10, U.S. 

Geological Survey Folio 193; Plate 3-A, U.S. Geological Survey Bulletin 

324. (USGS Photo Library). Photograph by G.K. Gilbert.

Significance of Results