A 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.
