1953 Seismic Hazard Maps and Ground Motion Parameters

The NEHRP Provisions incorporate a series of national seismic hazard maps for the United States and territories, developed by the United States Geologic Survey (USGS), specifically for this purpose (available at http://geohazards.cr.usfs.gov/eq/index.html). Two sets of maps are presented. One set presents contours of MCE, 5% damped, elastic spectral response acceleration at a period of 0.2 s, termed SS. The second set presents contours of MCE, 5% damped, elastic spectral response acceleration at a period of 1.0 s, termed S1. In both cases, the spectral response acceleration values are representative of sites with subsurface conditions bordering between firm soil or soft rock. Contours are presented in increments of 0.02g in areas of low seismicity and 0.05g in areas of high seismicity. By locating a site on the maps and interpolating between the values presented for contours adjacent to the site, it is possible to rapidly estimate the MCE level shaking parameters for the site, given that it has a soft rock or firm soil profile Figure 19.10 shows, for a portion of the western United States, contours of the 0.2 s spectral acceleration with a 90% probability of not being exceeded in 50 years. As indicated in the figure, in zones of high seismicity these contours are quite closely spaced, making use of the maps difficult. Therefore, the USGS has furnished software, available both over the internet (at the URL indicated above) and on a CD-ROM, which permits determination of the MCE spectral response acceleration parameters based on longitude and latitude.

Since many sites are located neither on soft rock nor on firm soil sites, it is necessary to correct the mapped values of spectral response acceleration to account for site amplification and de-amplification effects. To facilitate this process, a site is categorized into one of six site class groups, labeled A through F. Table 19.3 summarizes the various site class categories.

0.2 s spectral accel. (%g) with 10% probability of exceedance in 50 years site: NEHRP B-C boundary -124° -122° -120° -118° -116° -114°

0.2 s spectral accel. (%g) with 10% probability of exceedance in 50 years site: NEHRP B-C boundary -124° -122° -120° -118° -116° -114°

FIGURE 19.10 MCE Seismic Hazard Map (0.2 s spectral response acceleration) for western United States.

TABLE 19.3 Site Categories

Site class Description

Shear wave velocity, vs

Penetration resistance, N

Unconfined shear strength, su

0 0

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