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
incorporate the effects of tectonic environment,
magnitude, distance, and probability level on
Ground Motion B than for Ground Motion A); this is
response spectral shape.
illustrated in Appendix E.
Accounting for Local Site Effects on
(2) Approach 2 - Developing Equal Hazard
Response Spectra Directly From PSHA.
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
relationships), as well as for PGA.
spectrum shapes for rock motions.
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
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
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)
by developing rock response spectra at the site from
equal to or less than 0.03 second).
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
ground motions. The choice between Approaches 1
needed for Approach 2 are available for both EUS
and WUS, and are as reliable as attenuation
are available that are sufficiently applicable to the
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