Modeling of the Sedimentary Interbedded Basalt Stratigraphy for the Idaho National Laboratory Probabilistic Seismic Hazard Analysis
This 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
... 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 different facility areas. In 2000, seismic hazard calculations for rock were made for 5% damped response spectral accelerations at periods from 0.02 to 2.0 sec for INL facility areas: Idaho Nuclear Technology and Engineering Center (INTEC); Reactor Technology Complex (RTC); Power Burst Facility (PBF); Naval Reactor Facility (NRF); Radioactive Waste Management Complex (RWMC); and Test Area North (TAN). The mean hazard and the 5th-through 95th-percentile hazard were computed for the input parameter distributions defined in the hazard model logic trees. TAN has the highest mean hazard because of its proximity to the active Basin and Range faults and regional source zone. PBF has the lowest mean hazard because of its relatively high kappa value of 0.033 sec. The mean hazard for INTEC, RTC, NRF, and RWMC are generally similar. The uncertainty about the mean hazard is greater at short periods (0.02 and 0.1 sec) than at longer periods (1.0 sec) for comparisons of the mean hazard of the stochastic and empirical attenuation models. The hazard estimated from the empirical attenuation relationships exceeds the hazard using the site-specific stochastic attenuation relationships at each facility area. This difference is attributed to the damping effects of the sedimentary interbeds and the relatively low velocity gradient in the basalts compared to a typical western U. S. rock site. These effects were captured in the parameter kappa. Use of empirical attenuation relationships in the PSHA incorporates uncertainties in source and path effects. The development of site-specific attenuation relationships using a well-validated stochastic numerical ground motion model that incorporates the unique INL geology with its sedimentary interbedded basalt stratigraphy captures the epistemic uncertainties in site properties. These properties include the site-specific S-wave velocity profiles and the near-surface attenuation (kappa). Also, multiple stochastic attenuation relationships were computed using input parameters that have a range of values bounded by site-specific data. The approach taken for INL PSHA still represents the state-of-the-art and is currently being used at other DOE sites. The methodology used in the INL PSHA is consistent with current DOE Standards and recommendations of the Nuclear Regulatory Commission (such as NUREG/CR-6728). Since 2000 no new data have been collected that would invalidate the ground motion estimates of the INL PSHA, which forms the basis for current rock design basis earthquake (DBE) levels at each INL facility area. v FOREWORD The purpose of the report is to summarize how the effects of the sedimentary intebedded basalt stratigraphy were modeled for several seismic hazard analyses conducted for the Idaho National Laboratory (INL). The modeling of the sedimentary interbedded basalt stratigraphy has evolved through independent peer reviews of the input parameters and results of the INL seismic hazard analyses, which occurred from 1990 through 2001. Two independent panels have reviewed INL seismic hazard analyses. The first panel was commissioned to review seismic hazard analyses conducted in 1990 and 1992 for the New Production Reactor site characterization program. The second review was commissioned by the State of Idaho Oversight office to review the 1996 probabilistic seismic hazard analysis (PSHA) and modeling of the sedimentary interbeds. In 1999, the Nuclear Regulatory Commission (NRC) reviewed the INL PSHA in support of granting a license for the TMI-2 Independent Fuel Storage Installation at the Idaho Nuclear Technology and Engineering Center (INTEC). This report references the relevant aspects of the past seismic analyses that were incorporated into the most recent computations of the PSHA completed in 2000.