SUMMARYThis report summarizes how the effects of the sedimentary interbedded basalt stratigraphy were modeled in the probabilistic seismic hazard analysis (PSHA) of the Idaho National Laboratory (INL). Drill holes indicate the bedrock beneath INL facilities is composed of about 1.1 km of alternating layers of basalt rock and loosely consolidated sediments. Alternating layers of hard rock and "soft" loose sediments tend to attenuate seismic energy greater than uniform rock due to scattering and damping. The INL PSHA incorporated the effects of the sedimentary interbedded basalt stratigraphy by developing site-specific shear (S) wave velocity profiles. The profiles were used in the PSHA to model the nearsurface site response by developing site-specific stochastic attenuation relationships.The INL PSHA estimated the probability that ground motion levels will be exceeded at a site for a specified annual frequency or return period. Uncertainties in conceptual models and parameters were incorporated into the hazard analysis through the use of logic trees. The first two branches of the INL PSHA represented the selection of attenuation models that included 1) empirical attenuation relationships from principally the western U. S.; and 2) site-specific attenuation relationships derived from stochastic modeling using parameters specific to crustal properties of the eastern Snake River Plain (ESRP). The empirical attenuation relationships accounted for the uncertainty of estimating ground motions in an extensional tectonic region with limited empirical data. The stochastic modeling accounted for possible attenuation effects of the unique sedimentary interbedded basalt stratigraphy. A weight of 0.4 was assigned to the combined contributions of the empirical attenuation relationships and weight of 0.6 to the site-specific stochastic attenuation relationships. Greater weight was given to the stochastic attenuation relationships because they were site-specific incorporating the unique sedimentary interbedded basalt stratigraphy beneath INL facilities.Site-specific S-wave velocity profiles were developed for each INL facility area based on observed lithology and measured seismic velocities and densities from several shallow (<580 m) and four deep drill holes (580-3000 m). In the stochastic modeling to compute site-specific attenuation relationships, the Swave velocity profiles were used with recordings of regional earthquakes at INL facilities to determine the near-surface attenuation or kappa. At INL facility areas, kappa values ranged from 0.012 to 0.033 sec. The kappa values at INL are iv near the low end of typical western U.S. rock values that range from 0.01 to 0.06 sec. The lower kappa values at INL are attributed to the high-velocity basalts that allow more efficient transfer of high-frequency seismic energy through the sitespecific geologic profiles. Comparison of kappa among INL facility areas indicates this effect is offset by the damping due to the low velocity sedimentary interbeds within the basalt stratigraphy at ...