Soil liquefaction, in which certain types of soil
(W.U.S. and E.U.S.) and subsurface conditions (soil
Figure 3-17 illustrates trends for these ratios as a
substantial amount of strength due to strong
function of period of vibration, source-to-site
earthquake ground shaking, potentially resulting in
distance, and subsurface conditions for shallow
crustal W.U.S. earthquakes of moment magnitude
spreading, settlement, and other adverse effects.
approximately equal to 6.5. The figure illustrates
Soil differential compaction, which refers to the
spectral ratio of two-thirds is generally conservative
densification of soils and resulting settlements that
for longer-period ground motions, but is generally
may occur due to strong ground shaking.
unconservative for short-period ground motions from
near-source earthquakes. In fact, these ratios may
Landsliding of soil and rock masses on hillside
significantly exceed 1.0 in some cases, as shown in
slopes, due to earthquake-ground-shaking-induced
Ratios such as those presented in
inertia forces in the slope.
Figure 3-17 may be used to construct design vertical
response spectra of ground motions. However, the
longer period (greater than 0.2 second) spectral
includes the phenomena of tsunami, seiche, and
values should be carefully examined, and it may be
dam, levee, and water tank failures.
desirable to adopt for design vertical-to-horizontal
spectral ratios for longer periods that are higher than
The sites of all new buildings shall be evaluated to
the ratios shown in Figure 3-17.
minimize the possibility that a structure which is
adequately resistant to ground shaking could fail due
to the presence of a severe site geologic hazard.
Guidelines for conducting a geologic hazards study
are described in Appendix F.
As described in
Although, the hazard of strong ground shaking is
Appendix F, a screening procedure may be applied
generally the principal cause of damage to buildings
initially to ascertain whether the possibility of one or
and other structures during earthquakes, other
more geologic hazards can be screened out for a
seismic-geologic hazards have caused catastrophic
For those hazards that cannot be
damage to structures during earthquakes.
screened out, more detailed procedures should be
used to evaluate whether a significant hazard exists,
Surface fault rupture, which is the direct,
measures. Guidelines for more detailed evaluations
shearing displacement occurring along the surface
trace of the fault that slips during an earthquake.
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