A:
The effects from earthquakes are caused by ground shaking, surface faulting,
ground failure, and less commonly,
tsunamis.
Ground
Shaking
Ground shaking is a term used to describe
the vibration of the ground during an
earthquake. Ground shaking is caused by
body and surface seismic waves. As a
generalization, the severity of ground
shaking increases as magnitude increases
and decreases as distance from the
causative fault increases. Although the
physics of seismic waves is complex,
ground shaking can be explained in terms
of body waves, compressional, or P, and
shear, or S, and surface waves, Rayleigh
and Love. P waves propagate through the
Earth with a speed of about 15,000 miles
per hour and are the first waves to cause
vibration of a building. S waves arrive
next and cause a structure to vibrate from
side to side. They are the most damaging
waves, because buildings are more easily
damaged from horizontal motion than from
vertical motion. The P and S waves mainly
cause high-frequency vibrations; whereas,
Rayleigh and Love waves, which arrive
last, mainly cause low-frequency
vibrations. Body and surface waves cause
the ground, and consequently a building,
to vibrate in a complex manner. The
objective of earthquake-resistant design
is to construct a building so that it can
withstand the ground shaking caused by
body and surface waves.
In land-use
zoning and earthquake-resistant design,
knowledge of the amplitude, frequency
composition, and the time duration of
ground shaking is needed. These quantities
can be determined from empirical data
correlating them with the magnitude and
the distribution of Modified Mercalli
intensity of the earthquake, distance of
the building from the causative fault, and
the physical properties of the soil and
rock underlying the building. The
subjective numerical value of the Modified
Mercalli Intensity Scale indicates the
effects of ground shaking on man,
buildings, and the surface of the Earth.
When a fault
ruptures, seismic waves are propagated in
all directions, causing the ground to
vibrate at frequencies ranging from about
0.1 to 30 Hertz. Buildings vibrate as a
consequence of the ground shaking; damage
takes place if the building cannot
withstand these vibrations. Compressional
and shear waves mainly cause
high-frequency (greater than 1 Hertz)
vibrations which are more efficient than
low-frequency waves in causing low
buildings to vibrate. Rayleigh and Love
waves mainly cause low-frequency
vibrations which are more efficient than
high-frequency waves in causing tall
buildings to vibrate. Because amplitudes
of low-frequency vibrations decay less
rapidly than high-frequency vibrations as
distance from the fault increases, tall
buildings located at relatively great
distances (60 miles) from a fault are
sometimes damaged.
Taken from:
Hays, W.W., ed., 1981, Facing Geologic and
Hydrologic Hazards -- Earth Science
Considerations: U.S. Geological Survey
Professional Paper 1240B, 108 p.
Surface
Faulting
Surface faulting -- the differential
movement of the two sides of a fracture at
the Earth's surface-- is of three general
types: strike-slip, normal, and reverse.
Combinations of the strike-slip type and
the other two types of faulting can be
found. Although displacements of these
kinds can result from landslides and other
shallow processes, surface faulting, as
the term is used here, applies to
differential movements caused by
deep-seated forces in the Earth, the slow
movement of sedimentary deposits toward
the Gulf of Mexico, and faulting associated with salt domes.
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Photograpch
credit: unknown |
Death and
injuries from surface faulting are very
unlikely, but casualties can occur
indirectly through fault damage to
structures. Surface faulting, in the case
of a strike-slip fault, generally affects
a long narrow zone whose total area is
small compared with the total area
affected by ground shaking. Nevertheless,
the damage to structures located in the
fault zone can be very high, especially
where the land use is intensive. A variety
of structures have been damaged by surface
faulting, including houses, apartments,
commercial buildings, nursing homes,
railroads, highways, tunnels, bridges,
canals, storm drains, water wells, and
water, gas, and sewer lines. Damage to
these types of structures has ranged from
minor to very severe. An example of severe
damage occurred in 1952 when three
railroad tunnels were so badly damaged by
faulting that traffic on a major rail
linking northern and southern California
was stopped for 25 days despite an
around-the-clock repair schedule.
The
displacements, lengths, and widths of
surface fault ruptures show a wide range.
Fault displacements in the United States
have ranged from a fraction of an inch to
more than 20 feet of differential
movement. As expected, the severity of
potential damage increases as the size of
the displacement increases. The lengths of
the surface fault ruptures on land have
ranged from less than 1 mile to more than
200 miles. Most fault displacement is
confined to a narrow zone ranging from 6
to 1,000 feet in width, but separate
subsidiary fault ruptures may occur 2 to 3
miles from the main fault. The area
subject to disruption by surface faulting
varies with the length and width of the
rupture zone.
Taken from:
Hays, W.W., ed., 1981, Facing Geologic and
Hydrologic Hazards -Earth Science
Considerations: U.S. Geological Survey
Professional Paper 1240B, 108 p.
Ground
Failure
Liquefaction
Induced
Liquefaction is not a type of ground
failure; it is a physical process that
takes place during some earthquakes that
may lead to ground failure. As a
consequence of liquefaction, clay-free
soil deposits, primarily sands and silts,
temporarily lose strength and behave as
viscous fluids rather than as solids.
Liquefaction takes place when seismic
shear waves pass through a saturated
granular soil layer, distort its granular
structure, and cause some of the void
spaces to collapse. Disruptions to the
soil generated by these collapses cause
transfer of the ground-shaking load from
grain-to-grain contacts in the soil layer
to the pore water. This transfer of load
increases pressure in the pore water,
either causing drainage to occur or, if
drainage is restricted, a sudden buildup
of pore-water pressure. When the
pore-water pressure rises to about the
pressure caused by the weight of the
column of soil, the granular soil layer
behaves like a fluid rather than like a
solid for a short period. In this
condition, deformations can occur easily.
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Photograph
credit: Loma Prieta Collection,
Earthquake Engineering Research
Center, University of California,
Berkeley |
Liquefaction is
restricted to certain geologic and
hydrologic environments, mainly areas
where sands and silts were deposited in
the last 10,000 years and where ground
water is within 30 feet of the surface.
Generally, the younger and looser the
sediment and the higher the water table,
the more susceptible a soil is to
liquefaction.
Liquefaction
causes three types of ground failure:
lateral spreads, flow failures, and loss
of bearing strength. In addition,
liquefaction enhances ground settlement
and sometimes generates sand boils
(fountains of water and sediment emanating
from the pressurized liquefied zone). Sand
boils can cause local flooding and the
deposition or accumulation of silt.
Lateral Spreads
- Lateral spreads involve the lateral
movement of large blocks of soil as a
result of liquefaction in a subsurface
layer. Movement takes place in response to
the ground shaking generated by an
earthquake. Lateral spreads generally
develop on gentle slopes, most commonly on
those between 0.3 and 3 degrees.
Horizontal movements on lateral spreads
commonly are as much as 10 to 15 feet,
but, where slopes are particularly
favorable and the duration of ground
shaking is long, lateral movement may be
as much as 100 to 150 feet. Lateral
spreads usually break up internally,
forming numerous fissures and scarps.
Damage caused by
lateral spreads is seldom catastrophic,
but it is usually disruptive. For example,
during the 1964 Prince William Sound,
Alaska, earthquake, more than 200 bridges
were damaged or destroyed by lateral
spreading of flood-plain deposits toward
river channels. These spreading deposits
compressed bridges over the channels,
buckled decks, thrust sedimentary beds
over abutments, and shifted and tilted
abutments and piers.
Lateral spreads
are destructive particularly to pipelines.
In 1906, a number of major pipeline breaks
occurred in the city of San Francisco
during the earthquake because of lateral
spreading. Breaks of water mains hampered
efforts to fight the fire that ignited
during the earthquake. Thus, rather
inconspicuous ground-failure displacements
of less than 7 feet were largely
responsible for the devastation to San
Francisco in 1906.
Flow Failures
- Flow failures, consisting of liquefied
soil or blocks of intact material riding
on a layer of liquefied soil, are the most
catastrophic type of ground failure caused
by liquefaction. These failures commonly
move several tens of feet and, if
geometric conditions permit, several tens
of miles. Flows travel at velocities as
great as many tens of miles per hour. Flow
failures usually form in loose saturated
sands or silts on slopes greater than 3
degrees.
Flow failures
can originate either underwater or on
land. Many of the largest and most
damaging flow failures have taken place
underwater in coastal areas. For example,
submarine flow failures carried away large
sections of port facilities at Seward,
Whittier, and
Valdez, Alaska, during the 1964
Prince William Sound earthquake. These
flow failures, in turn, generated large
sea waves that overran parts of the
coastal area, causing additional damage
and casualties. Flow failures on land have
been catastrophic, especially in other
countries. For example, the 1920 Kansu,
China, earthquake induced several flow
failures as much as 1 mile in length and
breadth, killing an estimated 200,000
people.
Loss of Bearing
Strength
- When the soil supporting a building or
some other structure liquefies and loses
strength, large deformations can occur
within the soil, allowing the structure to
settle and tip. The most spectacular
example of bearing-strength failures took
place during the 1964 Niigata, Japan,
earthquake. During that event, several
four-story buildings of the Kwangishicho
apartment complex tipped as much as 60
degrees. Most of the buildings were later
jacked back into an upright position,
underpinned with piles, and reused.
Soils that
liquefied at Niigata typify the general
subsurface geometry required for
liquefaction-caused bearing failures: a
layer of saturated, cohesionless soil
(sand or silt) extending from near the
ground surface to a depth of about the
width of the building.
Taken from:
Hays, W.W., ed., 1981, Facing Geologic and
Hydrologic Hazards -- Earth Science
Considerations: U.S. Geological Survey
Professional Paper 1240B, 108 p.
Landslides
Past experience has shown that several
types of landslides take place in
conjunction with earthquakes. The most
abundant types of earthquake induced
landslides are rock falls and slides of
rock fragments that form on steep slopes.
Shallow debris slides forming on steep
slopes and soil and rock slumps and block
slides forming on moderate to steep slopes
also take place, but they are less
abundant. Reactivation of dormant slumps
or block slides by earthquakes is rare.
Large
earthquake-induced rock avalanches, soil
avalanches, and underwater landslides can
be very destructive. Rock avalanches
originate on over-steepened slopes in weak
rocks. One of the most spectacular
examples occurred during the 1970 Peruvian
earthquake when a single rock avalanche
killed more than 18,000 people; a similar,
but less spectacular, failure in the 1959
Hebgen Lake, Montana, earthquake resulted
in 26 deaths. Soil avalanches occur in
some weakly cemented fine-grained
materials, such as loess, that form steep
stable slopes under nonseismic conditions.
Many loess slopes failed during the New
Madrid, Missouri, earthquakes of 1811-12.
Underwater landslides commonly involve the
margins of deltas where many port
facilities are located. The failures at
Seward, Alaska, during the 1964 earthquake
are an example.
The size of the
area affected by earthquake-induced
landslides depends on the magnitude of the
earthquake, its focal depth, the
topography and geologic conditions near
the causative fault, and the amplitude,
frequency composition, and duration of
ground shaking. In past earthquakes,
landslides have been abundant in some
areas having intensities of ground shaking
as low as VI on the Modified Mercalli
Intensity Scale.
Taken from:
Hays, W.W., ed., 1981, Facing Geologic and
Hydrologic Hazards -- Earth Science
Considerations: U.S. Geological Survey
Professional Paper 1240B, 108 p.
Tsunamis
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Photograph
credit: NOAA/EDIS |
Tsunamis are
water waves that are caused by sudden
vertical movement of a large area of the
sea floor during an undersea earthquake.
Tsunamis are often called tidal waves, but
this term is a misnomer. Unlike regular
ocean tides, tsunamis are not caused by
the tidal action of the Moon and Sun. The
height of a tsunami in the deep ocean is
typically about 1 foot, but the distance
between wave crests can be very long, more
than 60 miles. The speed at which the
tsunami travels decreases as water depth
decreases. In the mid-Pacific, where the
water depths reach 3 miles, tsunami speeds
can be more than 430 miles per hour. As
tsunamis reach shallow water around
islands or on a continental shelf; the
height of the waves increases many times,
sometimes reaching as much as 80 feet. The
great distance between wave crests
prevents tsunamis from dissipating energy
as a breaking surf; instead, tsunamis
cause water levels to rise rapidly along
coast lines.
Tsunamis and
earthquake ground shaking differ in their
destructive characteristics. Ground
shaking causes destruction mainly in the
vicinity of the causative fault, but
tsunamis cause destruction both locally
and at very distant locations from the
area of tsunami generation.
Where Have
Tsunamis Occurred Historically?
East Coast
Historically, no tsunamis have been
generated on the east coast, a consequence
of the low level of seismic activity and
the lack of vertical fault displacement.
No tsunami occurred during the Charleston,
South Carolina, earthquake of 1886, one of
the largest earthquakes in the United
States. In addition, none of the tsunamis
occurring in the Atlantic Ocean region has significantly affected the east coast of the
United States.
The only tsunami known to have been
recorded on the Atlantic Coast of the
United States was generated by an
earthquake off the Burin Peninsula of
Newfoundland on November 18, 1929; it
caused a wave height of 1 foot.
West Coast
Tsunamis generated by earthquakes in South
America and the Aleutian-Alaskan region
have posed a greater hazard to the west
coast of the United States than locally
generated tsunamis. For example, the 1946
Aleutian tsunami produced waves heights of
12 to 16 feet at Half Moon Bay, Muir
Beach, Arena Cove, and Santa Cruz,
California. The 1960 Chilean tsunami
produced wave heights of 12 feet at
Crescent City, California. The 1964
Alaskan tsunami generated waves of more
than 20 feet at Crescent City, California,
where it caused $7.5 million in damage and
11 deaths. It also produced waves ranging
from 10 to 16 feet along parts of the
California, Oregon, and Washington coasts.
In contrast, for example, the 1906 San
Francisco, California, earthquake produced
local tsunami waves of only about 2
inches. The largest known locally
generated tsunami on the west coast was
caused by the 1927 Point Arguello,
California, earthquake that produced waves
of about 7 feet in the nearby coastal
area.
Alaska
The combination of seismic activity in the
Aleutian-Alaskan trench where the Pacific
and North American tectonic plates collide
and the vertical displacements of faults
make this region of Alaska a source of
tsunamis. The earliest recorded tsunami in
this region was in 1788. Four major
tsunamis were generated in 1946, 1957,
1964, and 1965; the 1964 Alaskan tsunami
caused over $80 million in damage and
killed 107 people.
Hawaii
The Hawaiian Islands have experienced many
destructive tsunamis because of their
location in the Pacific Ocean where about
90 percent of all recorded tsunamis take
place. Since 1819, more than 100 locally
and distantly generated tsunamis have been
recorded in the Hawaiian Islands with 16
of them causing significant damage. More
than one-half of all tsunamis recorded in
the Hawaiian Islands were generated in the
Kuril-Kamchatka-Aleutian regions of the
northern and northwestern Pacific.
Tsunamis generated in that area produce
the greatest waves on the northern side of
the islands. About one-fourth of the
historic tsunamis affecting Hawaii were
generated along the western coast of South
America. Tsunamis generated in the island
areas of the Philippines, Indonesia, the
New Hebrides, and Tonga-Kermadec have been
recorded in the Hawaiian Islands, but they
have not been damaging. The worst locally
generated tsunamis were generated in 1869
and 1975 on the southeastern coast of the
big island of Hawaii; they caused
destructive waves of as much as 59 feet.
Taken from:
Hays, W.W., ed., 1981, Facing Geologic and
Hydrologic Hazards -- Earth Science
Considerations: U.S. Geological Survey
Professional Paper 1240B, 108 p.
