active faults, design ground motions shall not be
50 years for Ground Motion B.
lower than ground motions that have a 10 percent
probability of exceedance in 50 years for Ground
conducting a probabilistic ground motion analysis.
Motion A, or 5 percent probability of exceedance in
Overview of Methodology.
development of site-specific response spectra using a
probabilistic approach involves the following basic
(1) characterizing earthquake sources in
terms of their locations and geometrics, maximum
occurrence; (2) characterizing source-to-site ground
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
spectrum is then constructed by anchoring a selected
response spectrum shape to the value of PGA
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
The effects of local soil conditions on
response spectra are incorporated either directly
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.
following paragraphs summarize the different steps
involved in developing site-specific response spectra;