Section R655-11-5A - Geological and Seismic StudyA review of the seismic or earthquake history of the region will be performed to establish the relationship of the site to known faults and epicenters. This will be based primarily on review of existing maps and technical literature and should include major earthquakes during historic time, epicenter locations and magnitudes, and the location of any major or regional fault traces. Geologic conditions at or near the dam site that might indicate recent fault or seismic activity should be included. Resulting design earthquakes and associated site ground motion parameters will be selected considering all available evidence including tectonic and seismological history. The ground motion parameters to be selected for the site will consist of those that are needed by the analyses that are appropriately selected for design and may include peak accelerations, velocities, displacements, response spectra, and acceleration time histories. Both the Maximum Credible Earthquake (MCE) and the Operating Basis Earthquake (OBE) will need to be investigated for all projects. The MCE should be evaluated using both deterministic and probabilistic methods.
1. A deterministic analysis from active faults in the region surrounding the dam will be performed to estimate magnitude and ground motion parameters. High and moderate hazard dams will be evaluated using ground motion parameters that are at least equal to mean plus 1 standard deviation predictions (84th percentile). At the discretion of the State Engineer, these values may be reduced to mean (50th percentile) for moderate hazard dams. Low hazard dams will be evaluated using ground motion parameters that are at least equal to mean (50th percentile) predictions. Evaluation of the impacts on the dam from more than one source, including the potential for multi-segment rupture for segmented faults may be necessary.2. A probabilistic analysis will be performed. The most recent United States Geological Survey (USGS) Interactive Deaggregation tool found on the USGS website, using a 5,000 year return interval, can be used to identify magnitude and ground motion parameters for high and moderate hazard dams. At the discretion of the State Engineer, a 2,500 year return interval can be used for moderate hazard dams. A 1,000 year return interval can be used for low hazard dams. Site specific evaluations may be performed to define ground motions for these events if the methods used and assumptions made are acceptable to the State Engineer. Unless waived by the State Engineer, the minimum earthquake magnitude shall be 6.5.3. The OBE will be determined by probabilistic methods acceptable to the State Engineer and may include the use of the Deaggregation tool on the USGS website with a 200 year return interval. An OBE evaluation is not necessary for a low hazard dam.4. Regardless of the assigned hazard rating, the seismic design parameters for flood control dams may be reduced at the discretion of the State Engineer, in consideration of unique operating conditions.Utah Admin. Code R655-11-5A
Amended by Utah State Bulletin Number 2016-8, effective 3/24/2016