Earthquake Ground Shaking Levels for the Wasatch Front

2003 International Building Code, 0.2 and 1 second spectral response acceleration maps

The Utah Seismic Safety Commission compiled a series of eight maps showing 0.2 and 1 second spectral response acceleration contours for Weber, Davis, Salt Lake, and Utah Counties.

These maps were created by the Utah Geological Survey (UGS) using the gridded data from the 2003 International Building Code (IBC) Seismic Design Parameters CD-ROM, prepared by the U.S. Geological Survey. The maps show values for an IBC site class B, so adjustments to map values must be made depending on the actual site class as outlined in the IBC.

Generalized site class maps for Salt Lake Valley and the Wasatch Front are available as UGS Open-File Report 424 (CD) and Report of Investigation 248, respectively, at the Natural Resources Map and Bookstore.

These maps are intended for use by building officials and engineers to illustrate earthquake ground-shaking levels, but are not for use in building design. The IBC Seismic Design Parameters CD-ROM included with the IBC (available from International Code Council; www.iccsafe.org) should be used for design.

The following information is for those other than building officials and engineers who are interested in viewing these maps to understand relativeground shaking hazards.

Larger values shown on contour lines in the maps indicate relatively greater levels of ground shaking expected during a given period of time, in this case, 2,500 years (equivalent to a 2% probability of exceedance in 50 years).

The Ss (0.2 second) maps indicate levels of ground shaking at high frequencies (or short periods) that are particularly damaging to 1-2 story structures such as houses. The S1 (1.0 second) maps indicate levels of ground shaking at lower frequencies (or longer periods) that are more damaging to tall structures (around 10 stories or more).

For example, if you are interested in the relative ground-shaking hazard to houses in Salt Lake Valley, the Ss (0.2 second) maps indicate that the hazard (relative strength of ground shaking) is highest in the East Bench area (1.7-1.8), and relatively lower along the west side of Salt Lake Valley (1.1-1.3).

Keep in mind these maps do not take into account local geologic conditions at a site, which may either amplify or dampen the motions shown on the map.