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EARTHQUAKES
Section 19.1Forces within Earth
• Most earthquakes happen because the
Earth’s crust moves along plate
boundaries
• Usually the movement is very slight
because the rocks resist the movement
• This causes stress to build up
Stress
• Is the total force acting on crustal rocks
per unit of area
• Movement occurs when stress overcomes
the strength of the rocks
• There are three kinds of stress
– Compression is the stress that decreases the
volume of a material
– Tension is the stress that pulls a material
apart
– Shear is the stress that twists a material
Strain
• The deformation of a material in response
to the stress
• Rocks can take bending, twisting and
stretching to a point. That critical point is
when the rocks break releasing energy
(earthquake)
Elastic Deformation
• Caused when a material is compressed,
bent or stretched
• Generally is temporary
• Once stress is reduced to zero, materials
return to normal
Plastic Deformation
• Caused when stress builds up past a
critical point
• Rocks are permanently deformed
• Most materials will display both elastic and
plastic deformation behavior—the amount
of stress and strain determine which
Faults
• Any fracture or system of fractures along
which the Earth moves.
• The surface along which the earth moves
is called the fault plane.
• There are several types of fault: reverse,
normal and strike slip
San Andreas Fault Line
Reverse Faults
• Form due to horizontal and vertical
compression
• Squeezes the rock and causes a
shortening of the crust
• Rock on one side pushes up relative to the
other side
Normal Faults
• Movement is partly horizontal and partly
vertical
• Horizontal movement pulls rocks apart and
stretches the crust
• Vertical movement occurs as stretching
causes rock on one side to move down
relative to other side
Strike Slip Fault
• Caused by horizontal shear
• Movement is mainly horizontal and in
opposite directions
Earthquake Waves
• Seismic waves are the vibrations of the
ground that occurs during an earthquake
• Three main types of seismic waves
– Primary waves
– Secondary waves
– Surface waves
Primary Waves
• Also called P waves
• Squeeze and push rocks in the direction
along which the waves are traveling
• Movement is similar to that of a coiled
wire—the movement is back and forth
movement
Secondary Waves
• Also called S waves
• Slower than P waves so they are felt
second hence the name
• Their motion cause rocks to move a right
angles in direction of waves
Surface Waves
• Third type and slowest of waves
• Travel only along Earth’s surface
• Can cause ground to move sideways and
up and down like ocean waves
• Cause the most destruction because they
take the longest time to pass and cause
the most movement of the ground
Focus and Epicenter
• The focus is the where the waves
originate; usually several km below Earth’s
surface
• The epicenter is the point on Earth’s
surface above the focus
• Surface waves originate from the
epicenter and spread out
Learning Targets
• Define stress and strain as they apply to
rocks.
• Distinguish among the three types of
movement of faults.
• Contrast the three types of seismic waves.
19.2 Seismic Waves and Earth’s
Interior
• Most vibrations of Earth can’t be felt far
away from the epicenter
• But a seismometer can detect even slight
movement
• A seismometer is a sensitive instrument
that measures Earth motion
• Produces a seismogram a paper or
computer graph of the movement
Clues to Earth’s Interior
• Seismic waves change speed and
direction at the boundaries between
different materials
• P waves and S waves initially travel
through the mantle following direct paths
• P waves move through mantle and are
refracted when they strike the core (they
bend)
• S waves will not travel through liquid and
do not travel through the core
Learning Targets
• Describe how a seismometer works.
• Explain how seismic waves have been
used to determine the structure and
composition of Earth.
19.3 Measuring and Locating
Earthquakes
• Richter scale
• Moment magnitude scale
• Modified Mercalli scale
Learning Targets
• Compare and contrast earthquake
magnitude and intensity and the scales to
measure each.