Transcript What Are Earthquakes?

What Are Earthquakes?
Key Concept:
Sudden motions along breaks in
Earth’s crust can release energy in the
form of seismic waves.
Where Earthquakes Happen?
• The majority of
earthquakes happen
near the boundaries
of tectonic plates.
• Large earthquakes
have occurred in the
interior of tectonic
plates.
• Earthquakes happen
along faults.
Faults at Tectonic Plate Boundaries
Divergent Boundaries
• Plates pull away from each other.
• Tension causes plates to break up into fault blocks.
• Some of the fault blocks drop down relative to the others.
• Earthquakes happen as the fault block moves.
Faults at Tectonic Plate Boundaries
• An example of a
divergent boundary is a
mid-ocean ridge.
• There the lithosphere is
thin.
• Earthquakes that happen
at the mid-ocean ridge
are shallow.
• At divergent boundaries,
earthquakes occur at
depths less than 20 km
below the ocean floor.
Earthquakes at convergent
Boundaries
• When two plates collide
with each other.
• Two things may happen,
1) Two plates crumple up
and make a mountain.
2) One plate subducts
under the other.
• Compression occurs as
the two plates collide.
Earthquakes at convergent
Boundaries
• Compression causes the
lithosphere to break up
into fault blocks.
• These blocks thrust over
one another as the plates
move.
• Two types of earthquakes
may occur.
• Between the two plates &
• Inside the down going
plate.
Earthquakes at Transform
Boundaries
• As two plates move past
each other, stress causes
rocks to shear.
• Earthquakes happen
along these strike-slip
faults as the plates move.
• Most transform
boundaries occur
between oceanic
lithosphere.
• Earthquakes occur at
shallow depths.
Fault Zones
• An area along a plate
boundary, where
there are many
connected faults is
called a fault zone.
• At a fault zone can
have different depths,
length and cut
through the
lithosphere in different
directions.
Why Earthquakes Happen
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1)
As plates move, stress is placed on the edges.
In response to stress, rocks deform.
Rocks deform in two ways:
Plastic deformation: like a piece of clay being
molded.
2) Elastic deformation: stretching like a rubber
band.
• Like a rubber band, plates will break if stretched
too far.
• The break releases energy as the rocks return
to their original form.
Elastic Rebound
• The sudden return of
deformed rock to its
original shape is
called elastic rebound
• Elastic rebound
occurs when the
stress on a rock
becomes so great,
that it breaks.
Elastic Rebound
• This causes rocks on both
sides of the fault to slide
past each other.
• In this sudden motion, stress
is released in the form of
energy waves.
• Energy waves traveling
through rock are called
seismic waves.
• The strength of an
earthquake is determined by
amount of energy released
during elastic rebound.
Earthquake Waves
•
Earthquakes are the
result of energy
movement as seismic
waves.
• There are 3 types of
seismic waves:
1) P wave
2) S wave
3) Surface wave
• Each wave travels thru
Earth differently.
P Waves
• Also called pressure waves.
• Are the fastest waves
• Also called primary waves, because they are the first to
be detected.
• P waves can move thru liquids, solids and gasses.
S Waves
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Also called shear waves.
Second fastest wave.
Move side to side.
Can not travel through liquids.
Called secondary waves because always arrives after P
waves.
Surface waves
•
Only move along the
Earth’s crust.
• Produce motions along
the top of the crust.
• Since these waves focus
on the surface, they
cause the most damage.
• Two type of surface
waves:
1) Move up and down.
2) Move back and forth.