distances will, in some cases, differ significantly for
relationships for PGA. Also, by using Approach 2,
different probability levels.
Usually the dominant
the resulting response spectrum will directly
magnitudes increase as the probability of exceedance
incorporate the effects of tectonic environment,
decreases (e.g., larger dominant magnitudes for
magnitude, distance, and probability level on
Ground Motion B than for Ground Motion A); this is
response spectral shape.
illustrated in Appendix E.
i.
Accounting for Local Site Effects on
Response Spectra.
(2) Approach 2 - Developing Equal Hazard
Response Spectra Directly From PSHA.
In
(1)
If the site is a rock site, local soil
Approach 2, the hazard analysis is carried out for
amplification effects are not applicable, and the
response spectral values at a number of periods of
response spectrum is directly obtained from the
vibration
(using
response
spectral
attenuation
PSHA using attenuation relationships and response
relationships), as well as for PGA.
For the
spectrum shapes for rock motions.
probability levels for the design ground motions, the
response spectral values are obtained from the
(2) If the site is a soil site, it is important to
hazard curves, and are then plotted versus period of
account for soil amplification effects on response
vibration. A smooth curve is then drawn through the
spectra. Such effects can be very strong in many
response
spectral
values
obtained
for
each
cases, such as the case illustrated in Figure 3-14, in
earthquake, resulting in an equal-hazard response
which ground motions recorded on a soft soil site
spectrum for each earthquake; that is, a spectrum
(Treasure Island) during the 1989 Loma Prieta
having the same probability of exceedance at each
earth- quake were amplified greatly in comparison to
period of vibration.
The process of constructing
motions recorded on an adjacent rock site (Yerba
equal-hazard response spectra from hazard curve
Buena Island).
results is illustrated in Figure 3-13 for the same site
for which the PGA hazard curve was constructed in
(3) Two approaches for incorporating soil
Figure 3-12. The example in Figure 3-13 is for a
amplification effects are: (1) by directly
return period of 1,000 years, which is approximately
incorporating soil amplification effects in the PSHA
equal to the return period for Ground Motion B.
through the use of attenuation relationships
(Note in Figure 3-13 that PGA is identically equal to
applicable to the soil conditions at the site; and (2)
zero-period response spectral acceleration at periods
by developing rock response spectra at the site from
equal to or less than 0.03 second).
In general,
a PSHA using rock attenuation relationships, and
response
spectra
should
be
developed
using
then carrying out site response analyses to assess the
Approach 2 rather than Approach 1. This is partly
modifying influence of the soil column on the
because response spectral attenuation relationships
ground motions. The choice between Approaches 1
needed for Approach 2 are available for both EUS
and 2 depends on whether attenuation relationships
and WUS, and are as reliable as attenuation
are available that are sufficiently applicable to the
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