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The Earth is formed of several layers that have very different physical and
chemical properties. The outer layer, which averages about 70 kilometers 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.
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Figure 1: Cross section of the
Earth's Plate Tectonic Structure
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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 kilometers of the surface).
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 characterized by deep-ocean trenches, shallow to deep
earthquakes, and mountain ranges containing active volcanoes. 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.
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