earthquake
tectonic
environment
Soil liquefaction, in which certain types of soil
(W.U.S. and E.U.S.) and subsurface conditions (soil
and rock).
deposits below the groundwater table may lose a
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
reduced
foundation-bearing
capacity,
lateral
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
that
the
commonly
used
vertical-to-horizontal
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
Figure 3-17.
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
induced
by
earthquakes,
which
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.
3-5.
Geologic Hazards.
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
facility site.
For those hazards that cannot be
damage to structures during earthquakes.
These
screened out, more detailed procedures should be
hazards include:
used to evaluate whether a significant hazard exists,
and if necessary, to develop hazard mitigation
Surface fault rupture, which is the direct,
measures. Guidelines for more detailed evaluations
shearing displacement occurring along the surface
of hazards and for hazard mitigation are also
trace of the fault that slips during an earthquake.
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