Probabilistic Seismic Hazard Mapping of the Mississippi Embayment

Seismic hazard is calculated for a grid of sites on the Mississippi embayment and adjacent areas. The seismic zonations, seismicity parameters, and maximum magnitudes for these calculations were synthesized from the EPRI (Electric Power Research Institute) and LLNL (Lawrence Livermore National Laboratory) interpretations, augmented by information from recent relevant studies. Attenuation functions developed for this study include the effects of source size and crustal structure. The soil amplification factors include the effects of surficial geology, soil-column thickness, and soil nonlinearity. Seismic hazard maps, including the effect of site conditions, are presented for spectral accelerations at 1 and 10 Hz and for peak acceleration, for average return periods of 100, 500, and 1000 years. These maps show significantly lower ground-motion amplitudes than those reported by Algermissen et al. (1991). Because of these large differences, it is suggested that code- development decisions be made after consideration of both studies.