performance of an area. Although such information
landsliding is shown in Figure F-7. Figure F-8
does not exist for all locations, it is available for
illustrates the hazard of landslide material (rockfall
numerous locations throughout the country; for
debris in this case) impinging on a structure below a
example, in Northern California (Youd and Hoose,
slope. Even a single large boulder dislodged from a
1978; Tinsley et al., 1994); in the New Madrid,
slope can cause considerable damage to a structure
Missouri region (Obermeier, 1989; Wesnousky et al.,
below.
1989); in the Charleston, South Carolina region
(Obermeier et al., 1986; Gohn et al., 1984); in the
e. Flooding. Earthquake-induced flooding at a site
northeastern United States (Tuttle and Seeber, 1989);
can be caused by a variety of phenomena including
among others. If a hazard has previously occurred at
seiche, tsunami, landsliding, and dam, levee, and water
the site, then the evaluations described in paragraph F-
storage tank failures. Seiches are waves induced in an
4 should be conducted; its absence, however, does not
enclosed body of water such as a bay, lake, or reservoir
preclude the occurrence of the hazard during future
by interaction of the water body with the arriving
seismic events. Second, a check should be made as to
seismic waves. Seiches can be caused by earthquakes
whether the site is included in an area for which a
that occur either in the region of a site or thousands of
regional earthquake hazard map has been prepared by a
miles away. Seiche waves may reach several feet in
federal or state agency. For example, under the U.S.
height and can be damaging to facilities located at or
Geological Survey's National Earthquake Hazard
very near the shoreline.
Reduction Program (NEHRP), liquefaction potential
maps have been prepared for several urban areas of the
(1) Tsunamis are ocean waves generated by vertical
United States. If the area containing the site has been
seafloor displacements associated with large offshore
mapped as having a high risk with respect to any
earthquakes. Tsunami waves at a site may be produced
geotechnical hazard (e.g., in an area of "high
by local or distant earthquakes; and wave heights may
liquefaction potential"), then evaluations described in
reach tens of feet at some coastal locations. Onshore
paragraph F-4 should be conducted.
tectonic movements accompanying earthquakes can
also cause flooding, such as crustal tilting causing
a. Surface fault rupture. The potential for
water to overflow a dam or uplift along a thrust fault
experiencing fault rupture (or not) at a site is controlled
causing damming of a river.
primarily by the regional and local tectonic
environment. For the hazard of surface fault rupture to
(2) Another source of tsunami waves is rapid
be present, an active fault or faults must pass beneath
landsliding into bodies of water, either from hillside
the site. A fault is considered to be active and capable
slopes above the water body or from submarine slopes
of producing surface rupture if the fault exhibits any of
within the water body. Another type of flooding hazard
the following characteristics indicative of recent
is that caused by earthquake-induced failure of a dam,
tectonic activity:
levee, or water storage tank.
$ It is a documented source of historical
F-3.
Screening Procedures
earthquakes or is associated spatially with a well-
defined pattern of microseismicity.
The following sections describe screening procedures
$ Its trace (the zone where the fault intersects the
for the geologic hazards described above. The possible
ground surface) is marked by well-defined
conclusions from screening for each hazard are: (1) a
geomorphic features like scarps, deflected
significant hazard potential does not exist; or (2)
drainages, closed depressions, etc. that are
further evaluation (described in paragraph F-4) is
suggestive of geologically recent faulting.
required to assess the hazard and its significance.
Because such features are easily modified or
There are two screening procedures that should be
destroyed by erosion and deposition, their
followed for all the hazards. First, a check should be
made as to whether a hazard has previously occurred at
the site (or in the near vicinity of the site in similar
geotechnical conditions) during historical earthquakes.
This check may involve review of the earthquake
history of an area, review of published post-earthquake
reconnaissance reports, and discussions with engineers
and geologists knowledgeable of the prior earthquake
F-9
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