Sudden movements of the ground, earthquakes
may be induced by movement along faults, volcanic
eruptions, landslides, bomb blasts, and other triggering mechanisms.
Earthquakes can be extremely devastating and costly
events, in some cases killing tens or even hundreds of thousands
of people and leveling entire cities in a matter of a few
tens of seconds. A single earthquake may release energy
equivalent to hundreds or thousands of nuclear blasts, and
may cost billions of dollars in damage, not to mention the
toll in human suffering. Earthquakes are also associated with
secondary hazards, such as tsunami, landslides, fire, famine,
and disease that also exert their toll on humans.
The lithosphere (or outer rigid shell) of the Earth is broken
into 7 large tectonic plates and many other smaller
plates, each moving relative to the others. Most of the earthquakes
in the world happen where two of these plates meet
and are moving past each other, such as in southern California.
Most really big earthquakes occur at boundaries where
the plates are moving toward each other (as in Alaska), or
sliding past one another (as in southern California). Smaller
earthquakes occur where the plates are moving apart, such as
along mid-oceanic ridges where new magma rises and forms
ocean spreading centers.
The area that gets the most earthquakes in the conterminous
United States is southern California along the San
Andreas Fault, where the Pacific plate is sliding north relative
to the North American plate. In this area, the motion is characterized
as a “stick-slip” type of sliding, where the two
plates stick to each other along the plate boundary as the two
plates slowly move past each other, and stresses rise over tens
or hundreds of years. Eventually the stresses along the
boundary rise so high that the strength of the rocks is exceeded,
and the rocks suddenly break, causing the two plates to
dramatically move (slip) up to a few meters in a few seconds.
This sudden motion of previously stuck segments along a
fault plane is an earthquake. The severity of the earthquake is
determined by how large an area breaks in the earthquake,
how far it moves, how deep within the Earth the break
occurs, and the length of time that the broken or slipped area
along the fault takes to move. The elastic rebound theory
states that recoverable (also known as elastic) stresses build
up in a material until a specific level or breaking point is
reached. When the breaking point or level is attained, the
material suddenly breaks, and the stresses are released in an
earthquake. In the case of earthquakes, rows of fruit trees,
fences, roads, and railroad lines that became gradually bent
across an active fault line as the stresses built up are typically
noticeably offset across faults that have experienced an earthquake.
When the earthquake occurs, the rocks snap along the
fault, and the bent rows of trees, fences, or roads/rail-line
become straight again, but displaced across the fault.
Some areas away from active plate boundaries are also
occasionally prone to earthquakes. Even though earthquakes
in these areas are uncommon, they can be very destructive.
Boston, Massachusetts; Charleston, South Carolina; and New
Madrid, Missouri (near St. Louis), have been sites of particularly
severe earthquakes. For instance, in 1811 and 1812
three large earthquakes with magnitudes of 7.3, 7.5, and 7.8
were centered in New Madrid and shook nearly the entire
United States, causing widespread destruction. Most buildings
near the origin of the earthquake were toppled, and several
deaths were reported (the region had a population of
only 1,000 at the time but is now densely populated). Damage
to buildings was reported from as far away as Boston and
Canada, where chimneys toppled, plaster cracked, and
church bells were set to ringing by the shaking of the ground.
Many earthquakes in the past have been incredibly
destructive, killing hundreds of thousands of people, like the
ones in Armenia, Iran, and Mexico City in recent years. Some
earthquakes have killed nearly a million people.
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