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. The 2008 versions supersede those released in 1996 and 2002. These maps are the basis for seismic design provisions of building codes, insurance rate structures, earthquake loss studies, retrofit priorities, and land-use planning. For some areas such as western Oregon and Washington, the new maps contain higher estimates for how hard the ground will shake compared to earlier versions of the maps released in 1996 and 2002. But for most of the United States, the ground shaking estimates are lower. This revision incorporates new seismic, geologic and geodetic information on earthquake rates and the manner in which the energy released in earthquakes dies off with distance from the rupture. Follow the link to the newest National Seismic Hazard Maps at http://pubs.usgs.gov/fs/2008/3018/ .
The map included the following regional changes:
- Several new faults were included or revised as a source of earthquake ground shaking in California, the Pacific Northwest and the Intermountain West.
- The Wasatch fault in Utah was modeled to include the possibility of a magnitude-7.4 earthquake, in addition to smaller earthquakes along the fault.
- The model for earthquakes along the New Madrid Seismic Zone in the Central United States includes a wider range of possible magnitudes and return periods between major earthquakes. The model was also adjusted to allow for sequences of earthquakes to occur in groups of three within a few years time, similar to what occurred in 1811 – 1812.
- Offshore faults were added as possible sources of earthquakes near Charleston, S.C.
- For the Cascadia Subduction Zone, more weight was given to a magnitude-9 earthquake that ruptures the length of the subduction zone, versus multiple smaller magnitude-8 earthquakes that fill the zone over the same 500-year time period.