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Earthquake Damage - General |
An earthquake is the motion or trembling of the ground produced by sudden
displacement of rock in the Earth's crust. Earthquakes result from crustal strain,
volcanism, landslides, and collapse of caverns. Earthquakes can affect hundreds of
thousands of square kilometers; cause damage to property measured in the tens of billions
of dollars; result in loss of life and injury to hundreds of thousands of persons;
and disrupt the social and economic functioning of the affected area. This set of
slides provides an overview and summary of effects caused by 11 earthquakes in eight
countries. The images show surface faulting, landslides, soil liquefaction, and structural
damage.Stress accumulates in response to tectonic forces until it exceeds the strength
of the rock. The rock then breaks along a preexisting or new fracture called a fault.
The rupture extends outward in all directions along the fault plane from its point
of origin (focus). The rupture travels in an irregular manner until the stress is
relatively equalized. If the rupture disturbs the surface, it produces a visible fault.
Earthquakes can affect hundreds of thousands of square kilometers; cause damage to
property measured in the tens of billions of dollars; result in loss of life and injury
to hundreds of thousands of persons; and disrupt the social and economic functioning
of the affected area. Although earthquakes in the United States occur most frequently
in states west of the Rocky Mountains, devastating earthquakes have also occurred
in the Midwest and East. All 50 states have some degree of risk from earthquakes.
Earthquakes can be measured in terms of either the amount of energy they release (magnitude)
or the degree of ground shaking they cause at a particular locality (intensity). Magnitude
is calculated from the record (wave amplitude) made by an earthquake on a calibrated
seismograph. The magnitude scale is logarithmic. An increase of one in magnitude represents
a tenfold increase in the recorded wave amplitude. However, the energy release associated
with an increase of one in magnitude is not tenfold, but about thirty fold. For example,
approximately 900 times more energy is released in an earthquake of magnitude 7 than
in an earthquake of magnitude 5. Intensity is determined from observations of the
earthquake's effect on people, structures, and the earth's surface at a given locality.
When a fault ruptures, seismic waves propagate outward in all directions and ground
shaking results. Generally the severity of ground shaking increases as magnitude increases
and decreases as distance from the fault rupture increases. The severity of the ground
shaking can be enhanced by certain soil and subsoil types. The intensity of the earthquake
is affected by the severity of the ground shaking, the duration of the shaking, the
response of structures in the affected area, etc. Hazards associated with earthquakes
include ground shaking, surface faulting, earthquake-induced ground failures, tectonic
uplift and subsidence, and tsunamis. Surface faulting, the offset of the earth's surface
by differential movement across a fault, shears and tears structures built on the
fault. Surface faulting is generally accompanied by horizontal or vertical distortion
of the earth's surface that can distort or tilt structures constructed near the fault.
Regional uplift and subsidence may accompany earthquakes caused by large displacements
on shallow faults. Such changes can damage harbor facilities, canals, roads, railroads,
and other structures. A tsunami is a water wave or a series of waves generated by
an impulsive vertical displacement of the surface of the ocean or other body of water
by an earthquake or other cause. These waves can extend the damaging effects of an
earthquake event thousands of kilometers from the earthquake focus. Ground failures
accompanying earthquakes include landslides, liquefaction, lateral spreads, differential
settlements, and ground cracks. Earthquake shaking often dislodges rock and debris
on steep slopes, triggering rock falls, avalanches, and landslides. These slides have
been known to bury entire towns and may be the most damaging aspect of the earthquake
event. Liquefaction occurs where ground water is near the surface in soils composed
of sands and silts. The soil temporarily loses strength and behaves as a viscous liquid.
Structures can settle or tip in the liquefied soil or be ripped apart as the ground
spreads laterally or flows. Flow failures can move over kilometers at speeds of tens
of kilometers per hour. They usually develop in loose, saturated sand on slopes greater
than five percent. When subsurface sand layers lose strength because of liquefaction,
lateral spreading can occur in overlying sediments allowing them to move down even
the gentlest slopes. Soils may lose shear strength allowing heavy structures to settle
or tip and lightweight, buried structures to rise buoyantly. Cracking may result from
movement along faults, differential compaction of the soil, or slides. Strong ground
shaking has compacted loose cohesionless materials and caused differential ground
settlements ranging from 5 cm to more than a meter. Many of these earthquake effects
are depicted in the slides included in this set.
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