ATTENUATION RELATIONSHIPS AND SEISMIC HAZARD ANALYSES IN THE UPPER MISSISSIPPI EMBAYMENT

FERNANDEZ, J.A., and RIX, G.J., School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0355, glenn.rix@ce.gatech.edu.

Ground-motion attenuation relationships have been developed for soil sites in the Upper Mississippi Embayment, which consists of soft sediments with thickness varying from a few meters to a maximum of about 1200 meters. The effects of epistemic and aleatory uncertainties in source, path, and site processes, and the effect of non-linear soil behavior have been incorporated into the attenuation relationships. Epistemic uncertainties were included by using alternative models for each aspect of the earthquake process, including three source models - Atkinson and Boore (1995), Frankel et al. (1996) and Silva et al. (2003) ; three stress drop values - median value, and high and low values using a 100% variation on the median case ; two soil shear wave velocity profiles for the Lowlands and Uplands geological units; one set of non-linear soil properties given by EPRI (1993); seven depth bins ranging from 6 to 1220 meters; eight magnitudes ranging from M 4 to M 7.5; and 550 epicentral distances ranging from 1 to 750 km. Aleatory uncertainties were included by considering random variations in parameter values including stress drop, source depth, crustal attenuation coefficient (Q₀), near-surface attenuation coefficient (kappa), crustal velocity, near-surface shear wave velocity, and modulus reduction and damping curves. The relationships were derived by regression analysis of spectral accelerations from a stochastic ground-motion model based on a point-source. An equivalent linear approach was used to account for non-linear soil behavior. These attenuation relationships have been used for deterministic and probabilistic seismic hazard analyses for the region.