effects when such two- and three-dimensional effects
response analysis (Approach 2).
Approach 2 can
could significantly modify ground motions in
always be considered as an alternative or supplement
comparison to the one-dimensional site response
to approach 1.
effects that are usually modeled.
Special Characteristics of Ground Motion
(4) Soil amplification effects are stronger for
for Near-Source Earthquakes. At close distances to
soft clay soils than for stiff clays or dense sands,
the earthquake source, within approximately 10 to
amplification is also increased by a large change in
15 km of the source, earthquake ground motions
stiffness or shear wave velocity between the soils and
often contain a high energy pulse of medium- to
underlying bedrock; therefore, it is particularly
long-period ground motion (at periods in the range
appropriate to conduct site response analyses when
of approximately 0.5 second to 5 seconds) that
these conditions are present at a site.
occurs when fault rupture propagates toward a site.
It has also been found that these pulses exhibit a
Site response analysis methodology is
strong directionality, with the component of motion
schematically illustrated in Figure 3-15. The soil
perpendicular (normal) to the strike of the fault
profile between the ground surface and underlying
being larger than the component parallel to the strike
rock is modeled in terms of its stratigraphy and
(see, for example, Somerville et al., 1997). These
characteristics of near-source ground motions are
that are representative of the estimated rock motions
illustrated in Figure 3-16, which shows the
are selected, and are propagated through the
modeled soil profile using nonlinear or equivalent
histories and response spectra of the Rinaldi
linear response analytical methods, and top-of-soil
recording obtained during the 1994 Northridge
motions are obtained.
As in other types of
These ground-motion characteristics
theoretical modeling and numerical analyses, site
should be incorporated in developing design
response analyses are sensitive to the details of the
analytical procedures, soil dynamic properties, and
time histories for near-source earthquakes.
input motions. The sensitivities should be carefully
Vertical Ground Motions. For the design of
examined when these analyses are conducted.
some structures, it may be necessary to analyze the
(6) In certain cases, it may be appropriate to
structure for vertical, as well as horizontal, ground
consider other types of site effects in developing site-
motions. Generally, vertical design response spectra
specific ground motions.
These include surface
are obtained by applying vertical-to-horizontal ratios
topographic effects when the surface topography is
to horizontal design response spectra. Recent studies
very irregular and could amplify ground motions,
(e.g., Silva, 1997) indicate that vertical-to-horizontal
and subsurface basin or buried valley response
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