EarthQuakes
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One of the most frightening and destructive phenomena of nature is a severe earthquake and its terrible
aftereffects. An earthquake is a sudden movement of the Earth, caused by the abrupt release of strain that has
accumulated over a long time. For hundreds of millions of years, the forces of plate tectonics have shaped the
Earth as the huge plates that form the Earth's surface slowly move over, under, and past each other. Sometimes
the movement is gradual. At other times, the plates are locked together, unable to release the accumulating
energy. When the accumulated energy grows strong enough, the plates break free. If the earthquake occurs in a
populated area, it may cause many deaths and injuries and extensive property damage.

Scientists have begun to estimate the locations and likelihoods of future damaging earthquakes. Sites of
greatest hazard are being identified, and definite progress is being made in designing structures that will
withstand the effects of earthquakes.

Where Earthquakes Occur
The Earth is formed of several layers that have very different physical and chemical properties. The outer layer,
which averages about 70 kilometres in thickness, consists of about a dozen large, irregularly shaped plates that
slide over, under and past each other on top of the partly molten inner layer. Most earthquakes occur at the
boundaries where the plates meet. In fact, the locations of earthquakes and the kinds of ruptures they produce
help scientists define the plate boundaries.

There are three types of plate boundaries: spreading zones, transform faults, and subduction zones. At spreading
zones, molten rock rises, pushing two plates apart and adding new material at their edges. Most spreading zones
are found in oceans; for example, the North American and Eurasian plates are spreading apart along the mid-
Atlantic ridge. Spreading zones usually have earthquakes at shallow depths (within 30 kilometres of the surface).

Illustration of Plate Boundary Types
















Transform faults are found where plates slide past one another. An example of a transform-fault plate boundary is
the San Andreas fault, along the coast of California and northwestern Mexico. Earthquakes at transform faults
tend to occur at shallow depths and form fairly straight linear patterns.

Subduction zones are found where one plate overrides, or subducts, another, pushing it downward into the
mantle where it melts. An example of a subduction-zone plate boundary is found along the northwest coast of
the United States, western Canada, and southern Alaska and the Aleutian Islands. Subduction zones are
characterised by deep-ocean trenches, shallow to deep earthquakes, and mountain ranges containing active
volcanoes.

Map of the Tectonic Plates












Earthquakes can also occur within plates, although plate-boundary earthquakes are much more common. Less
than 10 percent of all earthquakes occur within plate interiors. As plates continue to move and plate boundaries
change over geologic time, weakened boundary regions become part of the interiors of the plates. These zones
of weakness within the continents can cause earthquakes in response to stresses that originate at the edges of
the plate or in the deeper crust. The New Madrid earthquakes of 1811-1812 and the 1886 Charleston earthquake
occurred within the North American plate.

How Earthquakes Happen
Measuring Earthquakes
Volcanoes and Earthquakes
Predicting Earthquakes
Thanks & References,  Further Reading, Teaching Materials & Lesson Plans, Kids Stuff, Links & Resources     Printables     Colouring Pages     FEMA     USGS Earthquake     Earthquakes    kipedia
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