geotechnical specialist work closely with the design
exceedance, or probability of ground motion
engineer to ensure a common understanding of
exceedance in a certain exposure time (or design
design earthquakes, approaches to be followed in
time period). The probability of exceeding a certain
developing site-specific response spectra, and the
level of ground motion at a site is a function of the
nature and limitations of the ground motion outputs
locations of seismic sources and the uncertainty of
developed from the geotechnical studies. The peer
future earthquake locations on the sources, the
review prescribed for Seismic Use Group III
frequency of occurrence of earthquakes of different
buildings in Paragraph 1-9a shall apply to the site-
magnitudes on the various sources, and the source-
specific determination of ground motion for those
to-site ground motion attenuation, including its
buildings.
uncertainty.
c.
General Approaches. There are two general
(3)
In this document, site specific Ground
approaches to developing site-specific response
Motions A and B are determined using both
spectra:
deterministic approach, and probabilistic
probabilistic and deterministic parameters.
approach.
(a) In regions where active faults have not
(1) In the deterministic approach, site ground
been identified, design ground motions shall be
motions are estimated for a specific, selected
determined using a probabilistic approach as two-
earthquake; that is, an earthquake of a certain size
thirds (for Ground Motion A) and three-fourths (for
occurring on a specific seismic source at a certain
ground Motion B) of ground motions having a 2
distance from the site.
Often, the earthquake is
percent probability of exceedance in 50 years.
selected to be the largest earthquake judged to be
capable of occurring on the seismic source, or the
(b) In regions where active faults have been
maximum earthquake, and is assumed to occur on
identified, ground motions shall be determined using
the portion of the seismic source that is closest to the
both a probabilistic and a deterministic approach.
site. After the earthquake magnitude and distance
Design ground motions may be the lesser of: (1) two-
are
selected,
site
ground
motions
are
then
thirds (for Ground Motion A) or three-fourths (for
deterministically estimated using applicable ground-
Ground Motion B) of ground motions having a
motion attenuation relationships (see Paragraph 3f
probability of exceedance of 2 percent in 50 years;
below), statistical analyses of ground motion data
and (2) two-thirds (for Ground Motion A) or three-
recorded
under
similar
conditions,
or
other
fourths (for Ground Motion B) of ground motions
techniques.
determined deterministically as one- and one-half
times the median (50th percentile) ground motions
(2) In the probabilistic approach, site ground
estimated assuming the occurrence of maximum
motions are estimated for selected values of annual
magnitude earthquakes on portions of active faults
frequency or return period for ground motion
closest to the site. Furthermore, in regions having
3-20
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