zone environments. Attenuation relationships have
h.
Developing Response Spectra from the
also been developed for different broad categories of
PSHA.
subsurface conditions, particularly for the categories
Described below are two alternative
of rock and firm soils. In some cases, attenuation
approaches for obtaining response spectra based on
relationships have distinguished the effects of
PSHA: Approach 1 - anchoring a response spectrum
different types of faulting (e.g., strike-slip vs. reverse
shape to the PGAs determined from PSHA;
faulting). It is important to select a set of attenuation
Approach 2 - developing equal-hazard spectra
relationships that are most applicable to the site
directly from the PSHA. The two approaches are
under consideration.
Several recently developed
schematically illustrated in Figures 3-3 and 3-4,
relationships are summarized in Seismological
respectively.
Research Letters (1997).
(1)
Approach 1 - Anchoring Response
g.
Conducting Probabilistic Seismic Hazard
Spectrum Shape to PGA Determined from PSHA. In
Analyses
(PSHA).
The
seismic
source
this alternative, the hazard analysis is carried out
characterization and ground motion attenuation
only for PGA, and the PGAs for the design ground
characterization are combined in a probabilistic
motions are obtained from the hazard curve
model
to
develop
relationships
between
the
developed for the site. The response spectra are then
amplitude of a ground motion parameter and the
constructed by anchoring appropriate response
probability or frequency of its exceedance (diagram c
spectrum shapes to the PGA values.
Typically,
of Figures 3-3 and 3-4).
These relationships are
spectrum shapes for the appropriate category of
termed hazard curves. A hazard curve for PGA is
subsurface condition, such as the shapes contained in
illustrated in Figure 3-12. Appendix E describes the
the 1994 Uniform Building Code (UBC), are used.
mathematical formulation for the seismic hazard
It should be noted, however, that widely used
model, and provides examples of its usage in
spectrum shapes, such as those in the 1994 UBC,
obtaining hazard curves.
The appendix also
were developed on the basis of predominantly WUS
discusses the quantification of uncertainty in hazard
shallow crustal earthquake ground-motion data, and
curves as related to the uncertainty involved in the
they may not be appropriate for EUS earthquakes or
relationships and parameters of the model (i.e.,
subduction zone earthquakes.
Furthermore, such
uncertainty in seismic source parameters such as
spectrum shapes are considered to be most applicable
to moderate-magnitude earthquakes (magnitude . 6
maximum earthquake magnitude, frequency of
earthquake occurrence, etc., and uncertainty in the
1/2) and close to moderate distances (distance < 100
choice of appropriate attenuation relationships). It is
km).
For larger magnitudes and distances, the
important to incorporate these uncertainties in a
shapes may be unconservative in the long-period
PSHA in order to provide robust estimates of the
range; conversely, for smaller magnitudes, the
mean hazard, and evaluate the uncertainties in the
shapes may be overly conservative for long periods.
hazard.
To assess the appropriateness of the spectrum
3 - 36
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