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Types of Faults and
seismic waves
What is a fault?
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A fault is a break in the rocks that
make up the Earth’s crust, along
which rocks on either side have
moved past each other.
The direction of movement along the
fault plane determines the fault type.
GEOLOGICAL CONCEPTS
TYPES OF FAULT
Footwall/Hanging wall from Mining geology
Miners put their lamps on the hanging wall and
walk on the foot wall.
Three major Faults
Normal
Reverse
Strike-slip
Normal Fault
The hanging wall moves down relative to the foot wall.
This fault type is caused by tensional stress.
Normal Fault
Normal Fault
Normal Fault
Reverse Fault
The hanging wall
moves up relative to
the foot wall.
This fault is caused by
Compressional stress.
Reverse Fault
Reverse Fault
Reverse Fault
Strike-Slip Fault
The fault exists
between two pieces
of crust and the
Movement occurs
horizontally where
the sides slide past
each other.
This fault type is caused by shear stress.
Strike-Slip Fault
Strike Slip Fault
Strike-Slip Fault
Fault Movement = Earthquakes
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An earthquake is caused by a sudden slip
on a fault. The tectonic plates are always
slowly moving, but they get stuck at their
edges due to friction. When the stress on
the edge overcomes the friction, there is
an earthquake that releases energy in
waves that travel through the earth's
crust and cause the shaking that we feel.
Focus- where the slip happens below ground
Epicenter-where the shaking is first felt above
ground directly above the focus.
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Slippage along fault initiates seismic
waves in all directions radiating from
the focus.
Seismic waves are vibrations that
travel through the Earth carrying the
energy released during earthquakes
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There are two categories of seismic
waves:
Body Waves: Can travel through
Earth’s inner layers (core, mantle,
crust)
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Primary and Secondary waves
Surface Waves: Can only travel
through the surface layers (crust)
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Love and Rayleigh
Primary Waves
P waves- are the first waves to arrive
at a seismograph station.
-fastest form of wave, sometimes
called compression waves.
- Can move through both liquids and
solids.
- These waves cause rock particles to
move back and forth in the same
direction as the wave is traveling
(push-pull).
Secondary Waves
S-Waves- arrive after the primary
waves at the seismograph station.
-Can travel through solids only.
-Cause particles to move back and
forth at right angles to the line of
wave movement
SEISMOLGY
EARTHQUAKE LOCATION
Travel time versus distance plots
Can construct distance versus
travel time for earthquakes
knowing properties of
earth’s interior
Note that with increasing
distance the arrival times
for the various waves
separates.
The 3 min 45 sec difference
between the arrival time of
the P and S waves
corresponds to a distance of
2000 km.
SEISMOLOGY
LOCATING EARTHQUAKES
-Need three different
Triangulation
seismic stations to locate
an earthquake.
-Measure the P-S arrival
times and convert these
to distance.
-Triangulate using radius
given by P-S travel
times.
-Triangulation for position of
an earthquake near New
Madrid using stations in
Columbus, St Louis and
Memphis
Surface Waves
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Rayleigh wave-surface wave causing
the ground to shake in an elliptical
motion
Because of its motion, it moves the
ground up and down, and side-toside in the same direction that the
wave is moving
Most of EQ shaking is due to this
type of wave
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Love wave- produce entirely
horizontal motion
Moves the ground side-to-side
Named after A.E.H. Love a British
mathematician
http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm#P
article_Motion
Measuring Earthquakes
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Magnitude- the amount of energy
released during an EQ
Richter Scale- based on the largest
seismic waves generated by the EQ
on a factor of 10.
Moment Magnitude Scale- considers
the size of the fault rupture, the
amount of movement, and rock’s
stiffness
Modified Mercalli Scale
Measures the intensity of the EQ using
a rating system of the damage
 Scientists compile information from
various observers within different zip
codes to get a better understanding
of the devastation caused by an EQ
to determine its intensity.