National Seismic Hazard Maps
U.S. Geological Survey Updates Earthquake Rate Information
May 12, 2008
The U.S. Geological Survey (USGS) has updated national Seismic Hazard Maps for the contiguous portion of the United States. These maps combine the latest seismic, geologic, and geodetic data on earthquake rates and ground shaking. This effort replaces maps developed during 1996 and 2002.
How Are Seismic Hazard Maps Used? - Building officials, highway designers, architects, and land-use planners use seismic hazard maps to develop building codes, predict earthquake related losses, design critical infrastructure to withstand ground movement, and plan for public safety. Information on earthquake hazards helps planners reduce disruptions caused by a trembler, while assisting engineers in avoiding over-design of construction projects.
The Seismic Map Update Process - The U.S. Geological Survey updated Earthquake hazard maps by combining the most recent seismic, geologic, and geodetic data on seismic rates and associated ground shaking. This updating effort depended on input from hundreds of scientists and engineers participating in regional and national workshops. Other groups representing state geological surveys, federal agencies, universities, and industry helped in the seismic update. Several expert panels were retained to review the best available science used in the project.
Notable Changes to the Hazard Maps - Some significant changes were seen in the seismic hazard maps for California, the Intermountain West, and Eastern United States. Changes were also noted in ground shaking models that show how shaking motion decreases from the source of an earthquake.
Changes in Earthquake Source and Occurrence Rate Models:
- The California source model was updated to reflect new scientific information on earthquake producing faults
- The possibility of a catastrophic quake was given more weight on the Cascadia Subduction Zone offshore of the Pacific Northwest
- Several additional faults were added to the maps of the Pacific Northwest and Intermountain West
- Fault geometry and earthquake information was updated for the New Madrid Zone in the Central United States
- Offshore faults were added to source models in place for the Charleston, South Carolina region
- A greater range of earthquake magnitudes was considered for the Eastern United States
Changes in Ground Shaking Models:
- New models for the ground motion of crustal earthquakes developed by the Pacific Earthquake Engineering Research Center were adopted for tremblers beneath the Western United States
- A number of ground-shaking models were updated for the Central and Eastern United States that predict ground motion to decay more quickly than previously thought for the regions
- New ground-motion models for the Cascadia Subduction Zone were applied
Representing the best available science available for identifying earthquake hazards in the contiguous United States, the USGS anticipates the effort as a work in progress that will improve as earthquake data develops.
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