Dingee Reservoir Final Seismic Report - East Bay Municipal Utility ...

The next Generation Attenuation relations (NGA, 2008) developed by Abrahamson and Silva, Boore
and Atkinson, Campbell and Bozorgnia, and Chiou and Youngs were used to compute estimates of
median plus 1-sigma ground motions. These four relations were selected as they have characterized site
conditions in terms of V
S
(30m) in a consistent manner. Additionally, several of the relations have
included depth to 1 km/sec (or 2.5 km/sec) material as independent variables to more accurately
characterize strong ground motions at sites with shallow depths to bedrock materials, as occurs at
Dingee Dam. The NGA relations (NGA, 2008) provide median as well as median plus 1 standard
deviation estimates of 5% damped pseudo absolute response spectra over the period range of 0.01 sec to
10.00 sec. The estimates are for an average horizontal component termed GM roti which was developed
to be independent of orientation of the horizontal component (NGA, 2008) and very closely reflects a
geometrical average horizontal component. Figure 1 compares median plus 1 sigma estimates
computed with the four selected relations for M 7.25 at a rupture distance of 0.3 km, a V
S
(30m) of 600
m/sec, and depth to 1 km/sec material of 5m. Also shown is the weighted average (log average)
spectrum (Table 1), assuming equal weights. It should be noted the range in 84 th percentile estimate
(epistemic variability) is not large and may be underestimated as a result of the close cooperation
between the NGA developers. This consideration typically does not impact deterministic assessments
of 84 th percentile motions as it is usually neglected in the averaging process. For probabilistic hazard
analyses, the lack of adequate epistemic variability for source and site location conditions poorly
reflected in the strong motion database can result in motions at a higher exceedence frequency than
desired. In computing the average (log) median plus 1 sigma spectrum shown in Figure 1, the epistemic
variability (differences between the four estimates of the 84 th percentiles) has been included. This
process results in an average 84 th percentile motion that includes both aleatory (randomness defined by
the standard deviation of each ground motion relation) and epistemic variability.
To develop time histories, the spectral matching criteria prescribed in ASCE 43-05 (2005) was
followed. Input (basis) time histories (Table 2) were selected which displayed strong forward
directivity pulses, considered appropriate for 84 th percentile motions. The spectral matching used a
frequency
PACIFIC ENGINEERING and ANALYSIS 2