Transcript Earthquakes - section 12.1

The Changing Earth
Chapter Twelve: Earthquakes
• 12.1 Earthquakes
• 12.2 Volcanoes
Investigation 12A
Earthquakes
• What conditions affect the timing, duration,
and intensity of an earthquake?
12.1 What is an earthquake like?
• Transform fault plate
boundaries are
especially likely to
cause earthquakes.
• The energy of an
earthquake is spread
through Earth’s crust
by seismic waves.
12.1 What is an earthquake like?
• Foreshocks are small
bursts of shaking that
may precede a large
earthquake.
• Aftershocks are small
tremors that follow an
earthquake, lasting for
hours or even days after
the earthquake.
12.1 What causes earthquake?
• An earthquake is the
movement of Earth’s
crust resulting from
the release of builtup potential energy
between two stuck
lithospheric plates.
12.1 What causes earthquakes
•
The point below the surface where the
rock breaks and causes an earthquake is
called the earthquake focus.
12.1 What causes earthquakes?
•
As soon as the rock breaks, there is
movement along the broken surface
causing a split in the surface called a
fault.
12.1 What causes earthquakes?
• The seismic waves from an earthquake are usually
strongest at the epicenter, the location on the Earth’s
surface directly above the area where the rock breaks
in the crust.
12.1 Stick-slip
motion
An earthquake is a
form of stick-slip motion.
The plates can get stuck
together due to friction
until the potential
energy is stronger than
the rock. The potential
energy is then released
as an earthquake.
12.1 Stick-slip motion
•
Three conditions are needed for stick-slip
motion:
1. Two objects that are touching each other
where at least one of the objects can move.
2. A force, or forces, that will cause the
movement.
3. Friction strong enough to temporarily keep
the movement from starting.
Use the stick-slip door model to identify these conditions.
12.1 Friction
• Friction is a resistance to slip that occurs
when two objects rub against each other.
12.1 Lithospheric plates have
many sections
• A moving line of grocery
carts is a better example
of a moving lithospheric
plate.
• Although a plate may be
moving as a single
unit, its boundaries act
like they were made of
many small sections
like the line of carts.
12.1 Lithospheric plates have
many sections
• A lithospheric plate
may be thousands of
km. across.
• It takes a long time
for movement on one
end of the plate to
affect a section
further away.
12.1 When do earthquakes happen?
• The release of built-up potential energy causes
earthquakes.
• An earthquake is a stress reliever for a lithospheric
plate.
• Once a quake occurs, potential energy builds up
again.
12.1 Seismic waves
• Seismic waves are recorded and measured by
an instrument called a seismograph.
• P-waves and S-waves are called body waves.
• Surface waves are on the surface of Earth and
are the slowest, but most destructive waves.
12.1 Seismic waves
• The first seismic
waves recorded
after an earthquake
will be P-waves
because they are
the fastest waves.
• S-waves are
recorded next,
followed by the
surface waves.
12.1 Seismic waves
• In a quarter-mile race, the track
is so short that fast and slow
cars are often just fractions of a
second apart. In a long race,
like the Indianapolis 500, the
cars might be minutes apart.
• This analogy explains why the
difference between the arrival
of P-waves and S-waves is
greater the further from the
epicenter a seismic station is.
• The time difference
between slow and fast
cars is related to the
length of the race track.
12.1 Seismic waves
• Seismic waves radiate from
the focus after the
earthquake.
• At least three seismic
stations are required to
accurately determine the
epicenter of an earthquake.
• The larger the difference in
arrival time, the farther the
epicenter is from the station.
12.1 Seismic mystery
• Garrett Euler was puzzled
because earthquake-like
signals were arriving at all of
his 32 seismic stations at
the same time.
• Euler was able to show that
each burst of seismic
signals matched the time
each Cameroonian soccer
goal occurred!
12.1 Measuring seismic waves
• The Richter scale rates earthquakes according
to their magnitude (size) of the seismic waves
recorded on a seismograph.
12.1 Measuring earthquake damage
• The Moment
Magnitude scale rates
the total energy
released by an
earthquake.
• The Modified Mercalli
scale has 12
descriptive categories
that rate the damage
suffered by buildings,
the ground, and people.
12.1 Earthquakes and plate
boundaries
• Earthquakes commonly
occur at the boundaries
of lithospheric plates.
This is because plate
boundaries tend to be
zones of seismic activity.