Vibratory Ground Motions

DOE-STD-1022-94

5.4.2 Vibratory Ground Motions

After seismic sources in the region of a site are defined, vibratory ground motions at the site can be estimated

using criteria that are specified in DOE-STD-1023-95. The vibratory ground motions are generally defined

by the horizontal and vertical response spectra corresponding to the expected ground motion at the free-field

ground surface. The estimate of vibratory ground motions at the site should be based on a detailed evaluation

of the earthquake potential taking into account regional and local geology, tectonics, seismicity, as well as

specific local soil conditions.

In general, the factors that influence site ground motions include the characteristics of the earthquake source,

the travel path between the source and the site, and the local site conditions. Assessment of the influence

of local soil conditions is described in Section 5.5.2. The attenuation effect of the geological materials in the

travel path (e.g., Q factor) shall be estimated by regional seismology studies or based on the strong ground

motion data, if a sufficient data base is available. The effect of local geology and rock conditions on the

ground motions shall also be considered. With respect to the first two factors (i.e., earthquake source and

travel path), it is suggested that one or both of these factors can result in significant differences in earthquake

ground motions in three broad tectonic regimes in the United States -- the Central and Eastern U.S. (EUS),

Western U.S. (WUS), and areas in the vicinity of subduction zones. Precise geographic limits for the regions

are not defined, but the WUS and EUS are generally west and east, respectively of the Rocky Mountains,

while subduction zone earthquakes in the United States occur only along coastal northwest California,

Oregon, Washington, and southern Alaska. Based on recent ground motion study by Electric Power Research

Institute (EPRI, 1993), the EUS area can be further subdivided into the Mid-continent and the Gulf Coast

regions. Different attenuation relationships would thus be appropriate for each region. Thus, care must be

taken and uncertainties must be recognized in utilizing data bases, relationships, and methodologies

applicable to each region.

In the WUS, particularly coastal California, relatively well-constrained attenuation relationships (i.e.

relationships between earthquake magnitude, source-to-site distance, and ground motion amplitude, e.g.

Boore et al., 1993; Joyner and Boore, 1988; Campbell, 1985; Boore and Joyner, 1982, Abrahamson and

Silva, 1997, Boore et al., 1997, Campbell, 1997, Sadigh et al., 1997) have been developed from a data base

of abundant strong ground motion recordings from shallow crustal earthquakes. Estimates of site ground

motions for shallow crustal earthquakes in the WUS can be made using these attenuation relationships or

through appropriate statistical analyses of strong ground motion data sets. If ground motion estimates are

required for particular combinations of source geometry, earthquake size, and/or crustal structure that are

not represented in the empirical data, theoretical/numerical ground motion modeling techniques can be used

to evaluate various source and path effects and extend the empirical models beyond the limits of the recorded

data.

In the EUS, there are relatively few strong ground motion recordings, particularly for larger magnitude

earthquakes. Empirical ground motion attenuation relationships can be developed by using ground motion

recordings from the EUS enhanced by recordings from other regions of the world similar to the EUS.

Attenuation relationships can also be obtained on the basis of theoretical/numerical ground motion models

(Toro et al, 1997, Atkinson and Boore, 1995). Since there are limited calibration and verification from

ground motion data for these models, the greater uncertainty involved shall be recognized in using the

attenuation relationships. The WUS versus EUS correction factors may be developed using stochastic

ground motion models as well as using available EUS and WUS data for generic site categories (McGuire,

et. al., 2001). Comparison of empirical Western North America (WNA) amplification factors for deep stiff