active faults, design ground motions shall not be
50 years for Ground Motion B.
The
lower than ground motions that have a 10 percent
following
paragraphs
provide
guidance
for
probability of exceedance in 50 years for Ground
conducting a probabilistic ground motion analysis.
Motion A, or 5 percent probability of exceedance in
d.
Overview of Methodology.
The
development of site-specific response spectra using a
probabilistic approach involves the following basic
steps:
(1) characterizing earthquake sources in
terms of their locations and geometrics, maximum
earthquake magnitudes, and frequency of earthquake
occurrence; (2) characterizing source-to-site ground
motion attenuation; (3) carrying out a probabilistic
ground motion analysis (often termed a probabilistic
seismic hazard analysis, or PSHA) using inputs from
(1) and (2); and (4) developing response spectra
from the PSHA results.
These basic steps are
illustrated in Figures 3-3 and 3-4. Figure 3-3 is for
the case where a PSHA is carried out for peak
ground acceleration (PGA) only, and the response
spectrum is then constructed by anchoring a selected
response spectrum shape to the value of PGA
obtained from the PSHA for the selected probability
level. Figure 3-4 is for the case where a PSHA is
carried out for response spectral values as well as for
PGA, and an equal-probability-of-exceedance (equal-
hazard) response spectrum is directly determined
from the PSHA results for the selected probability
level.
The effects of local soil conditions on
response spectra are incorporated either directly
through the choice of appropriate attenuation
relationships or spectral shapes, or by supplemental
analyses of site effects in the case where the PSHA is
carried out for rock motions at the site.
The
following paragraphs summarize the different steps
involved in developing site-specific response spectra;
details
of
the
methodology,
including
the
3-21
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