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Earthquakes
• An Earthquake is any
vibrating, shaking or rapid
motion of the Earth’s crust.
• Most Earthquakes occur at
zones of weakness or a break
in bedrock known as a fault.
• When the stress built up in the
rock is greater than what the
rock can resist, the crust shifts.
• A sudden release of energy
radiates in all directions as
vibrations.
• The focus is the place
underground where the actual
break occurs.
• The epicenter is the surface
directly above the focus.
• After the earthquake waves move
away from the epicenter.
• When the waves reach a location
the earth seems to shake.
• The further a location is from the
epicenter the weaker the waves are
when they reach that location.
San Andreas Fault Video(45MIN)
https://www.youtube.com/watch?v=b5f09mh-gIM
San Andreas Fault Video (4.5 min)
https://www.youtube.com/watch?v=_YLjIvJXhpg
P (primary waves)- travel
through liquids and solids.
P waves move faster than S
waves.
P waves cause particles to
move in the direction of the
wave.
S (secondary waves)- travel
only through solids.
S waves are slower than P
waves.
S waves cause particles to
move at right angles to the
direction of the wave.
L (Land) Waves are caused
when P Waves and S Waves
Combine to cause a circular
motion of the rock particles.
L waves cause the most
destruction during an
Earthquake
• The characteristics waves have
in different conditions allows
geologists to infer properties of
the Earth’s interior.
• http://sunshine.chpc.utah.edu/labs/s
eismic/index.htm
Measuring Earthquakes
There are two different ways to
measure earthquakes.
• Magnitude scales- based on
seismometer readings.
• Intensity scales- based on
reports of damage.
Magnitude Scales
• Seismometer- an instrument designed
to detect and measure seismic waves.
A pen is attached to a heavy mass on
a spring. When the ground shakes, the
mass remains stationary and the paper
moves under it.
• Seismograph- a seismometer that
makes a recording of seismic waves.
• Each magnitude is 10 times greater
than the previous number.
• A magnitude 5 earthquake is 10 times
stronger than a magnitude 4
earthquake. A magnitude 6 is 100
times stronger than a magnitude 4.
Intensity scale• The Mercalli scale is based on the
damage to structures and accounts of
witnesses.
• 1-2 - may not be felt
• 5-6 - some dishes and windows break
• 7-8 - hard to stand, collapse of some
structures.
• 10-11 - Visible waves on ground, total
Since we
know how fast
P-waves and
S-waves
travel, we can
use this
information to
calculate the
distance to an
epicenter
Damage from Earthquakes
• Shaking- buildings may be damaged
or collapse.
• Movement of crust- damages roads
and railroads.
• Tsunamis- earthquakes in oceans
cause large waves which may swamp
coastal areas thousands of miles
away.
Human Dangers
• Fires- broken gas pipes often ignite.
• Broken water mains make it hard to
fight fires.
• Landslides often occur in soft ground.
Areas of high risk have stricter building
codes to help keep structures from
falling apart during quakes.
Locations of Earthquakes and
Volcanoes
• Most all earthquakes and volcanoes
occur in a belts of crustal activity.
• The largest belt surrounds the Pacific
ocean which is an old ocean plate and
is subducting all around its borders. It
is called the “Ring of Fire”.
Damage from Volcanoes
• Ash- may cover large areas, very
difficult to deal with. Machines may
not work, clogs air filters.
• Lava- smaller areas covered but
nearly unstoppable.
• Toxic gasses- hot, fast-moving gasses
may suffocate people.