Download EARTHQUAKES

Section 21.2

WHAT IS AN
EARTHQUAKE?
An earthquake is a
movement of Earth’s
crust caused by a
release of energy
WHAT CAUSES AN
EARTHQUAKE?
 Eruption of a volcano
 Collapse of a cavern
or building
 Major cause – stress
between two plates

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Friction builds up between two plates
The stress causes the plates to deform or ‘stretch’
Eventually, the stress becomes great enough that
it overcomes friction and the plates move
The plates ‘snap’ back into their original shape
and begin building up friction again



Point on the fault (or plates) where movement
first occurs
Where an earthquake starts
3 types



Shallow (0-70km)
Intermediate (71-300km)
Deep (301km and higher)


Point on the Earth’s surface directly above
the focus
What is reported as the location of an
earthquake

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P-waves: primary
waves; travel
through solids,
liquids, and gases;
back and forth
motion
P waves are
______________
waves.

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S-waves: secondary
waves; travel
through solids
only; side to side
motion (snake)
S waves are
___________ waves

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L-waves: surface
waves
Combination of up
and down and
back and forth
motion
Cause the most
damage due to
their rolling action
and long
wavelengths

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Seismograph: an instrument that detects and
records earthquake waves
P-waves always arrive first, then S-waves, then
L-waves


P and S Waves Graph – pg.767 – Under
Graphing Skills
As distance increases, the travel time for
seismic waves increases also.

To locate the exact
location of an
epicenter, you must
have points from
three separate
seismograph
locations around
the world

Richter Scale
Developed by Charles Richter in the 1940’s
 Measures the amount of energy released by the
earthquake itself
 Scale from 1-10 (weakest - strongest)
 Each number is 32 times stronger than the next
lower number (Ex: ‘6’ is 32 times stronger than a
‘5’ and 1024 times stronger than a ‘4’)

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We cannot predict earthquakes other than
giving periods of time in which we think they
will occur.
Earthquakes are most common along sliding
plate boundaries, but they can occur in other
places.
http://earthquake.usgs.gov/learn/kids/