Site-Specific Seismic-Hazard Analysis that is Completely Probabilistic

When a site-specific probabilistic ground-motion estimate is required, the full site-amplification distribution should be used instead of a single deterministic median value. A probabilistic methodology using site-amplification distributions to modify rock ground-motion attenuation relations into site-specific relations prior to calculating seismic hazard has been developed and applied at two selected sites in the central United States: Memphis, Tennessee, and Paducah, Kentucky. The use of a completely probabilistic approach can make about a 10% difference in ground-motion estimates over simply multiplying a bedrock probabilistic ground motion by a median site-amplification factor at a 1 in 2475 annual probability of exceedance and even larger differences at smaller probabilites of exceedance. The value of this approach is that a probabilistic answer incorporating the uncertainty in our knowledge of site amplification of ground motions can be calculated.