Seismic Hazard Evaluation of the Magna Tailings Impoundment

Abstract A state-of-the-art seismic hazards evaluation was performed for a tailings impoundment near Magna, Utah. The objective was to develop seismic design criteria in accordance with State of Utah requirements for dam safety using both deterministic and probabilistic approaches. Seismic sources in and around Salt Lake Valley such as the Wasatch, West Valley, Oquirrh and Stansbury fault zones were considered in the hazard analysis. The East Great Salt Lake fault, whose southern end extends to within 1-3 km of the project site, and which could generate a maximum earthquake of moment magnitude (MW) 7 was selected as the source of the Maximum Credible Earthquake (MCE). Using empirically-based attenuation relationships, a design peak horizontal ground acceleration of 0.52 g was calculated at the project site for the MCE and for deep soil conditions. Based on a probabilistic seismic hazard analysis, this value has a return period of 3,000-4,500 years depending upon the location along the tailings impoundment. Attenuation relationships were also used to develop the 5% damped MCE design acceleration response spectrum. Due to the proximity of the site, geologic investigations of the southern termination of the East Great Salt Lake fault and state-of-the-art numerical ground motion modeling were performed. The latter was carried out to evaluate possible near-field source as well as site effects on ground motions. The Operating Basis Earthquake (OBE) was defined based on a peak horizontal acceleration of 0.29 g calculated from the probabilistic analysis and assuming a return period of about 500 years. At this return period, the background earthquake is the dominant contributor to hazard at the site. Thus, the maximum background earthquake of MW 6 1/2 at a source-to-site distance of 8 km, which would produce 0.29 g at the site, was selected as the OBE.