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Chapter 15
EARTHQUAKES!
BFRB Pages 215-226
Earthquake causes
• An earthquake is the shaking of
the Earth’s crust caused by a
release of energy
• The movement of the Earth’s
plates causes most earthquakes
Elastic rebound theory
• As Earth’s plates move past one another, friction
causes the plates to get “stuck”
• Stress and pressure builds up and causes the plate to
become deformed (bend)
– (the plates are solid, but elastic - they can change shape)
• Eventually, the pressure overcomes the friction and
the plates slip past one another
• This sudden plate movement is the earthquake. The
greater the plate movement, the larger the
earthquake.
• The plates are in a new location, but they go back to
their original shape (elastic rebound).
Earthquake Depth
• Earthquake depth depends on the type of
plate boundary where the movement occurs
• Diverging boundaries (mid-Atlantic Ridge),
and transform (sliding) boundaries (San
Andreas Fault) tend to have earthquakes
which are closer to the surface (shallow focus)
• Subduction boundaries tend to have very deep
earthquakes as the subducting plate plunges
deeper into the mantle (deep focus)
Shallow focus
Intermediate focus
Subducting plate
Deep focus
• The point where the actual movement of
the plates takes place, and where the
energy is released from is called the focus
• The point on the Earth’s surface that is
directly above the focus is called the
epicenter
• When an earthquake occurs, energy
waves are released and move outward
from the focus
Types of Faults
(We’ll discuss these more when we do Geologic History)
• NORMAL - one side of a fault slips down relative
to another
• REVERSE (& Thrust) - one side of a fault is
driven up and over the other
• STRIKE-SLIP – occur where plates meet evenly
and slip past each other horizontally. (The angle
at which a fault cuts through the earth is referred
to as the strike, so a strike-slip fault happens
when plates slip along the strike).
– most common fault type in California
FAULT ANIMATIONS
Seismometers/
Seismographs
• Earthquakes generate
seismic waves which
can be detected with
a sensitive instrument
called a seismograph
aka seismometer
• The actual readout of
a seismometer is
called a seismogram
3 Types of Waves
P-WAVE
S-WAVE
L-WAVE
P-Waves
• COMPRESSIONAL WAVES (push)
• cause the rocks in the crust to be
squeezed and stretched
• the first waves received by a
seismograph station because they
move the fastest
• These waves can move through solids,
liquids, and gases
S-Waves
• SHEAR WAVES
• Cause the rocks to move side to
side, causing a ripping (shearing)
motion
• Slower than p-waves
• Can only move through solids
****S waves = Side to Side, Slow, Solids
P & S Wave Animations
L-Waves
• LONGITUDINAL WAVES
• aka SURFACE WAVES
• when P & S-waves reach the surface,
they cause wave-like ripples on the
surface of Earth like when you
throw a rock in a pond
Wave Speed
• Determined by the characteristics of
the material that the earthquake’s
wave is moving through!
• The more solid the material, the
faster the waves can move through
it (but remember, P-waves will
always move faster than S-waves)
Remember when we were studying
the layers of the Earth and you asked
me, “ If we’ve never been there, how
do we know that these layers exist?
Well, here’s how!!!!
Movement of Waves Inside the Earth
• The velocity of the P-waves and S-waves
increase as they travel deeper into the
Earth
• The boundary between the crust and the
mantle is a thin layer of denser rocks.
Because of this increase in density, the P
& S-waves suddenly increase in velocity.
• This thin, dense layer is called the
Mohorovicic discontinuity or Moho
…Wave Movement…
• When the waves reach the outer core, the P-waves
slow down and the S-waves stop.
• S-waves can’t travel through liquids, and P-waves
move fastest through solids.
• **The change in velocity of the P-Waves and
ceasing of the S-waves is evidence that the outer
core is liquid!!!**
• After passing through the outer core, the P-waves
increase in velocity as they pass through the solid
inner core, then decrease as they go through the
other side of the liquid outer core, then increase in
velocity again when they go through the rest of the
solid Earth.
…Wave Movement…
• Seismograph stations on the opposite side of the
earth from the epicenter only receive P-waves (the
liquid outer core block the S-waves).
• Some seismograph stations do not receive any
waves – neither P nor S.
• This is because the P-waves refract (bend) as they
travel through the liquid outer core, and the Swaves don’t make it through at all.
• This area where no waves reach is called the
SHADOW ZONE
• The location of the shadow zone, of course,
depends on the location of the epicenter
Locating an Epicenter
• The location of an earthquake epicenter can be
found by using the measurement of seismic
3
waves from seismic stations and a P-wave/ Swave Time Travel Graph (ESRT’s page 11)
• Epicenters are found by drawing circles around
each seismograph station that are the exact
distance from the epicenter that was determined
using the Time Travel Graph.
• Where all three circles intersect is the
location of the epicenter.
Things to Remember
• P-waves always arrive before S-waves
• The farther a seismograph is from the epicenter of
the earthquake, the greater the time difference
between the arrival of the P-waves and S-waves
• Knowing the distance from the epicenter of one or
two seismographs will only give you a rough area of
where the earthquake occurred. To find the exact
position of the epicenter, you must have three
seismograph station recordings
• Count ‘em…THREE
• Did I mention that the number was 3 – no more,
no less….
Earthquake Strength and Damage
• Seismometers can also be used to
determine the magnitude (strength)
of an earthquake
• The Richter Scale is used to measure
the amount of energy released by an
earthquake
Richter Scale
• Rating scale from 1 to 10
• Each increase on the Richter Scale is
an increase of 10 times in the strength
of the earthquake
–Ex. an earthquake measuring a 5 is
10 times stronger than a 4, and 100
times stronger than a 3
• Each increase is also 32 times greater
in energy
Damage & Prediction
• Essentially, the seismic waves that cause the
ground to shake cause earthquake damage
– The shaking, depending on severity, can cause
building foundations to fail, gas and water lines
to rupture, electrical disruptions, avalanches, etc
• Earthquake prediction is important to
determine the areas where damage is most
likely to occur
Partial collapse of the 'Million Dollar Bridge' into the Copper
River in Alaska following the 1964 earthquake. (9.2 magnitude)
LIQUIFACTION –
The ground literally flows like a liquid due to the pressure that
the earthquake waves put on the water that surrounds the
particles. The water exerts enough pressure on the particles that
they no longer are held together by the friction between them –
the water in between the particles enables them to flow easily
and no longer support the structure that is built on it.
This was a communications building in Spitak during the
1988 Armenian earthquake.
Elementary School torn apart by a landslide that occurred during the 1964
Alaska earthquake. Notice the large crack in the foreground that also was
caused by the earthquake. (9.2 magnitude)
Land Subsidence of 2 meters was caused by the Edgecumbe,
New Zealand earthquake. (6.3 magnitude)
This fault is now known as the Edgecumbe Fault
Epicenter of the Manhattan earthquake on
January 17, 2001 is plotted on an aerial photograph
Emergency Preparedness
– Scientists use EQ data to identify places
where earthquakes are likely to occur
– Government authorities then use this
information to write emergency
preparedness plans.
– These plans are for both before AND after
an earthquake strikes.
– From you’ve seen in the previous slides and
what you’ve learned about earthquakes,
complete the information on the following
slides.
Emergency Preparedness
– What can authorities do BEFORE an EQ strikes to
mitigate (reduce) the damage and deaths that an EQ
may cause?
Emergency Preparedness
– What can citizens do BEFORE an EQ strikes to
mitigate (reduce) the damage and deaths that an EQ
may cause?
Emergency Preparedness
– What can authorities plan for AFTER an EQ strikes
to mitigate (reduce) the damage and deaths that an
EQ may cause?
Emergency Preparedness
– What can citizens plan for AFTER an EQ strikes to
mitigate (reduce) the damage and deaths that an EQ
may cause?