U.S. Department of the Interior - U.S. Geological Survey
Earthquake exposure varies across the world. Seismic hazard maps one of the most recognized ways to illustrate this variation. Building code requirements are partially determined on the information that is represented in these maps.
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).
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.
- 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