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Collateral Hazards
Collateral hazards include fault ruptures, landslides, liquefac-
tion, tsunamis, and seiches. As noted in an earlier section, collateral
hazards can cause very significant losses. For example, most of the
losses experienced in the Alaskan earthquake of 1964 resulted from
collateral hazards, with landslides causing major property loss and
tsunamis causing 119 of the 131 fatalities (National Research Coun-
cil, 1972~.
In principle, the overall method for predicting losses that might
result from these hazards is much the same as that for losses caused
by shaking of facilities. The first question is: How extensive and
severe knight collateral hazards be? For example, how large an area
might be affected by landsliding, and how far and how rapidly might
the earth move? These are questions for earth scientists, such as
geologists.
The next question is: What damage and losses would result?
These primarily are questions for engineers. Where structures are
impacted d*ectly by severe collateral hazards, losses may be quite
large. Sometimes the effect of a hazard may be indirect. For example,
the Hebgen earthquake in Montana in 1959 triggered a landslide that
blocked the Madison River, and a lake began to form behind this
earthen mass. Considerable effort was expended to alleviate concern
about potential downstream flooding when this potentially erodible
"dam" eventually overtopped.
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FAULT RUPTURE
A surface fault rupture involving several feet of movement will
cause major damage (50 percent loss ratio or greater) to almost all
houses or small structures sited directly on the fault, and generally
even greater damage to larger structures. However, well-built build-
ings located within only a few feet of a well-defined fault rupture
may experience little or no damage. Losses to buildings directly
attributable to surface rupture usually are a small fraction of total
losses associated with an earthquake, and can be predicted with sat-
isfactory accuracy once the path of the rupture is defined. In the
eastern United States, it appears that potential faults are deeply
buried and hence fault rupture probably does not extend to the sur--
face. Hence this particular hazard generally can be ignored in this
large portion of the country.
LANDSLID1:S AND LIQUEFACTION
Landslides and liquefaction are consequences of ground shaking.
A rational procedure for evaluating liquefaction and landslide-caused
losses as a result of a scenario earthquake is described in ATC-13 and
involves the following steps:
1. Identify soil, geologic, and topographic conditions potentially
susceptible to such failures as a result of an earthquake.
2. Select relations expressing the likelihood of failure, or the
fraction of susceptible area expected to fail, as a function of the
intensity of ground shaking.
3. Select additional relations giving loss ratios for buildings~and
other facilities located at failed areas.
These steps parallel those for the direct effect of ground shaking upon
buildings (Figure 1-2~.
Identification of areas where these hazards may occur is a major
inventory-type problem. The USGS has prepared, or helped prepare,
detailed geological hazard maps for several metropolitan areas (e.g.,
Borcherdt, 1975; Youd et al., 1978; Ziony 1985), and is collaborat-
ing in the development of such maps for additional areas. Generally
these maps showed areas within which there is a significant likelihood
that landsliding or liquefaction might be triggered by an earthquake,
and it is not necessarily expected that actual hazards will occur in
all such areas or over all of a given area. Preparing a good map of
collateral geological hazards is a time-consuming and expensive task.
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If these maps are not already available, it is necessary to construct
an approximate hazards map using the expertise of experienced local
geotechnical engineers and geologists. ATC-13 summarizes the gen-
eral principles to be followed, and more specific guidance is available
in a recent compilation of papers by USGS (Ziony, 1985) and a report
on liquefaction by the National Research Council (1986~.
Liquefaction
The word liquefaction has been used to cover several differ-
ent types of phenomena associated with the increase in pore water
pressures in cohesioniess soils during earthquake shaking, with a
resulting decrease of strength and/or stiffness. One common mani-
festation is lateral spreading of nearly level ground toward adjacent
dips or other low points. Such lateral spreading can disrupt road-
ways, canals, pipelines, and so on, and will also damage buildings in
the area affected by the spreading.
Liquefaction can also cause more dramatic flow slides from
steeper slopes, which occurred at the lower Van Norman Dam dur-
ing the 1971 San Fernando earthquake (Seed et al., 1975), at mine
tailings dams in Chile in 1965 (Dobry and Alvarez, 1967), and at the
waterfront in Seward, Alaska in 1964 (Seed, 1973~. Obviously such
slides are extremely damaging both to any structures on the slicle
area or in the path of the slide.
Still another manifestation of liquefaction is the appearance of
sand boils (small volcanoes emitting sand and water) on the surface
of level ground. Where such sand boils occur, there can be excessive
settlements of facilities. There may also be large differential horizon-
tal movements between only slightly distant points on the surface,
resulting in damage to highway, pipelines, and so on.
Liquefaction has been observed repeatedly during earthquakes
in California, but it has also been observed elsewhere. An enormous
area was affected by massive liquefactions during the 1811-1812 New
Madrid, Missouri earthquakes, and the phenomenon was very evident
during the ISS6 Charleston, South Carolina earthquake. Liquefac-
tion is a potential problem in any area where a loss estimation is
being made.
The liquefaction hazard maps are based on general indicators of
liquefaction susceptibility: geological age, the manner in which cohe-
sioniess soil is deposited, and the depth to the water table. Detailed
procedures involving drilling and sampling have been developed for
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evaluating the liquefaction susceptibility of specific sites; such efforts
are beyond the scope of a loss estimation study, although it may
be possible to make good use of pre-existing data to supplement
geological information. Approaches developed for California to iden-
tify liquefaction susceptible areas (Youd and Perkins, 1978) can be
used, with proper judgment and knowledge of local conditions, to
guide preparation of approximate maps for areas in other parts of
the country.
ATC-13 contains relations giving estimated fractions of lique-
faction susceptible areas experiencing different damage states (i.e.,
degrees of liquefaction) as a function of MMI. These were prepared
using expert opinion, and are generally applicable throughout the
United States. A very recent paper by Youd and Perkins (1988)
presents a method for mapping liquefaction severity index (LSI),
which is related to the extent and severity of liquefaction phenomena
within liquefaction susceptible soils. The specific relations suggested
in this paper are derived from experiences in California, and cannot
be extrapolated directly to other parts of the country.
The same ATC-13 report also gives damage ratios for the losses
to buildings affected by liquefaction. These clearly are engineering
judgments, but they do provide some guidance. Combining this
damage information with maps of liquefaction susceptible areas and
estimations of extent and severity of liquefaction provides property
loss estimates from these collateral hazards. It is essential to keep
in mind that different manifestations of liquefaction (e.g., lateral
spreading and sand boils) have quite different implications concerning
damage.
This method is untried and must be used with caution and judg-
ment and tailored to local situations, but it does permit systematic
evaluation of potentially important losses.
Landslides
Here landslides is used to cover all permanent earth movements
other than those involving saturated cohesionIess soils.
In a very comprehensive review of earthquake-induced landslides
to date, Keefer (1984) reviewed 40 earthquakes worldwide and ranked
the abundance of different types of landslides. He listed disrupted
soil slides as very abundant; soil stumps, soil lateral spreads, soil
block slides, and soil avalanches as abundant; and rapid soil flows
as moderately common. Of these types of landslides involving soils,
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only soil lateral spreads and rapid soil flows most likely involve
liquefaction or failure of sensitive clays. The most notable example
of failure of sensitive clays in this country occurred In Anchorage,
Alaska in 1964 (Seed, 1973~. Technical methods for assessing specific
sites are available, but generally are beyond the means of a broad
area loss estimate study. Since a combination of very sensitive clay
and very strong shaking must be present if such failures are to occur,
local geotechnical and geological experts should be able to decide
whether or not this particular problem is present.
Soil falls, disrupted soil slides, and soil avalanches can occur
as failures of dry, cohesive or cohesionIess soils (Keefer, 1984) and
were assessed as particularly common to abundant in the 1811-
1812 New Madrid (Missouri) and 1906 San Erancisco earthquakes.
Likewise, landslides in cohesive soils are not restricted to sensitive
clays, as shown by Keefer's historic review. Soil slumps, for exa~nple,
have been at least moderately common in many earthquakes where
sensitive clays do not exist.
Landslides in rock (e.g., rockfaDs and debris flows) are expected
only in steep terrain, in rock formations experiencing a loss of
strength during ground shaking. It is very difficult to determine,
even with the most careful testing and evaluation, whether or not
a slide is likely, In relation to ground-shaking intensity, at a given
rock site. Thus it is necessary to rely very heavily on past experience
and judgment. AT~13 contains results from expert opinion con-
cerning the probability of such events. The historical review of 40
worldwide earthquakes by Keefer (1984) formed the basis for develop-
ing methods to identify seismically induced landslide susceptibility.
These methods have subsequently been applied to several California
metropolitan areas, including San Mateo County (Wieczorek et al.,
1988) and the Los Angeles Region (Wilson and Keefer, 1985~. Of the
40 earthquakes in the review, only 10 were from California. At least
a half dozen were from other regions in the United States, including
Alaska, Hawaii, Missouri, Montana, South Carolina, and Washing-
ton, with the rest of the historical earthquakes from other parts of
the world having a variety of geologic ~d climatic conditions.
ATC-13 also provides estimates for the likelihood of darnage to
facilities, given that landsliding occurs again from expert opinion.
The warning in the last paragraph under liquefaction applies even
more to landslides in rock. Locally steep rock slopes, particularly in
closely jointed and weakly cemented rocks, are highly susceptible to
seismically induced rockfalIs and rockslides as noted by Keefer (1984~.
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These slopes need not be particularly high; a steep slope 10-20 m
high could generate a damaging rockfall. Many metropolitan areas
are naturally hilly and in combination with highway cuts, hillside
excavations for building foundations create sufficiently hazardous
conditions for rockfall and rockslides during earthquakes. Within
the United States, Los Angeles, Memphis, Portland, Salt Lake City,
San Diego, San Francisco, and Seattle are a few examples of large
metropolitan areas with recognized potential for seismically induced
landslides involving rock.
TSUNAMIS
Tsunami inundation areas for the West Coast of the United
States, including Alaska and Hawaii, are available in the form of
FEMA flood maps (FIRM or Flood Insurance Rate Maps). For the
East Coast, the potential hurricane storm surge exceeds possible
tsunami heights, and tsunami hazard zones are thus of less impor-
tance and are not plotted on FIRMs. Other maps by USGS or the
U.S. Army Corps of Engineers are also available and show the extent
of expected tsunami run-up for the West Coast. A mean recurrence
interval for the tsunami of 100 or 500 years, or both
used as a basis for developing these maps.
. · . .
is commonly
Even though tsunami hazard maps are available, there are two
basic difficulties to overcome in estimating losses. First, as with
ground failures, one must assess the degree of damage to various
structures that might be caused by a range of water heights and
velocities. All structures located within the shaded tsunami run-up
zones on a map are not expected to be subjected to the same effects.
Second, the tsunami map obtained will generally be predicated on a
variety of causative earthquakes that may be located thousands of
miles away, and thus tsunami losses may be unrelated to the losses
associated with a local earthquake scenario. Generally, the major
tsunami risk in California, Oregon, Washington, and Hawaii is posed
by distant earthquakes, such as large Alaskan or Chilean events.
In southern Alaska, a local earthquake could cause a tsunami, and
the earthquake and tsunami losses might thus be combined in a
particular scenario.
SElCHES
Here seiches refers to waves induced in lakes, reservoirs, and so
on as a result of ground shaking (or perhaps because of permanent
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tectonic movement of the underlying earth). If these waves are of
sufficient amplitude, facilities along the waterfront may be damaged
and there might be overtopping and thus damage to any earth dam
· · —
retaping a reservoir.
A specialized study Is needed to determine whether or not there
might be a problem and if so the extent of it. Fortunately, seiches
are usually not a problem.
i.;
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Electrical equipment that is not anchored securely to the foundation can move
and fall over, as happened to this unit during the 1971 San Fernando earth-
quake (M 6.63. This earthquake did extensive damage to electric power facilities,
freeway bridges, and water and gas distribution systems. Photo courtesy of G.
Hou~ncr.
:—~~
. ~ ~~ t
..
Collapse of an airport control tower in Anchorage, Alaska during the 1964
earthquake (M 8.3-8.6~. One man was killed in the collapse and one was
injured. All air traffic control and advisory services were disrupted in the
Anchorage area due to damage to telephone, interphone, and teletypewriter
communication lines. Photo courtesy of G. Mourner.
Representative terms from entire chapter:
sensitive clays
