What is an earthquake?
What causes earthquakes and where do they happen?
The earth has four major layers: the inner core, outer core, mantle
and crust. (figure 2) The crust and the top of the mantle make up a
thin skin on the surface of our planet. But this skin is not all in one piece
– it is made up of many pieces like a puzzle covering the surface of
the earth. (figure 3) Not only that, but these puzzle pieces keep slowly
moving around, sliding past one another and bumping into each other. We call
these puzzle pieces tectonic plates, and the edges of the plates are
called the plate boundaries. The plate boundaries are made up of
many faults, and most of the earthquakes around the world occur on these
faults. Since the edges of the plates are rough, they get stuck while the
rest of the plate keeps moving. Finally, when the plate has moved far enough,
the edges unstick on one of the faults and there is an earthquake.Why does the earth shake when there is an earthquake?
While the edges of faults are stuck together, and the rest of the block is
moving, the energy that would normally cause the blocks to slide past one
another is being stored up. When the force of the moving blocks finally
overcomes the friction of the jagged edges of the fault and it
unsticks, all that stored up energy is released. The energy radiates outward
from the fault in all directions in the form of seismic waves like
ripples on a pond. The seismic waves shake the earth as they move through it,
and when the waves reach the earth’s surface, they shake the ground and
anything on it, like our houses and us! (see P&S Wave inset)ALL INFORMATION TAKEN FROM: USGS - EARTHQUAKES
EARTHQUAKE IN THE CLASSROOM
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