Forces Inside Earth
 Objectives:
 Explain how earthquakes result from the
buildup of three different stresses in Earth’s
crust
 Describe normal, reverse, and strike-slip
faults
Causes of Earthquakes
 Passing the Elastic Limit Causes Faulting
 Applied stresses can cause rocks to bend and
stretch
 Eventually rocks will break away from one
another
 Area in which the rocks break and move is
called a fault
 Vibrations and shaking of the Earth’s crust
is called an earthquake
 Earth’s crust movement causes the stresses
applied
Types of Faults
 Normal Fault from Tension/Pulling Apart
 Earth’s plates move apart
 Movement of the plates causes tension
 Rocks above the fault surface move
downward in relation to the rocks below the
fault surface
Types of Faults
 Reverse Faults from Compression/Squeezing
 Compression forces are applied because
Earth’s plates are coming together
 Causes rocks to bend and break
 Rocks above the fault surface are forced up
and over the rocks below the fault surface
Types of Faults
 Strike-slip Fault from Shearing/ sliding or
slipping past
 Earth’s plates move sideways to one another
 Creates a shear force
 Rocks on either side of the fault surface are
moving past each other without much upward
or downward movement
Causes of the Stress Forces
 Volcanoes
 Collapsing of caverns
 Plate Tectonics
 Convergent boundary
 Divergent boundary
 Strike Slip boundary
Earthquake Information
 Objectives
 Compare and contrast primary,
secondary, and surface waves
 Explain how an earthquake epicenter
is located using seismic wave
information
 Describe how seismic wave studies
indicate the structure of Earth’s
interior
Types of Seismic Waves
 Focus – the point in Earth’s interior where the
energy waves are produced
 Epicenter -The point on Earth’s surface
directly above the earthquake focus
 Seismic wave - the energy waves that move
outward from the earthquake focus and make
the ground quake
Types of Seismic Waves
 Seismic Waves
 Primary waves like a slinky
 Energy waves that cause rocks to move
back and forth in the same direction
 Compression and stretching forces are
created
 Secondary waves whipping a rope
 Energy waves that cause rocks to move at
right angles to the wave
Types of Seismic Waves
 Surface waves like ocean waves
 Energy waves that reach the Earth’s
surface
 Energy waves that move rocks in an
elliptical motion
Locating an Epicenter
 Seismic waves do not travel through
earth’s surface at the same speed
 Primary waves are the fastest
 Surface waves are the slowest
 Seismograph stations
 Record the information from the earthquake
by recording the different types of waves as
they reach the station
Locating and Epicenter
 Epicenter Location
 If information is received from the earthquake
at three stations, then the epicenter can be
located
 A circle is drawn around each station on a
map
 The radius is equal to the distance from the
station to the epicenter
 The point in which all three circles intersect is
the earthquake epicenter
Using Seismic Waves to Map
Earth’s Interior
 The speed of the waves changes as the
depth changes
 The speed changes in accordance to the
density of the layers of Earth
Using Seismic Waves to Map
Earth’s Interior
 Structure of Earth
 Inner core
 Solid innermost layer and very dense
 Composed of mostly nickel and iron
 Outer core
 Liquid layer that surrounds the inner core
 Composed of nickel and iron
Using Seismic Waves to Map
Earth’s Interior
 Mantle
 Largest layer located above the inner core
 Composed of silicon, oxygen, magnesium,
and iron
 Crust
 Outermost layer
 Solid and similar in composition to the
mantle, mainly silicon and oxygen
Using Seismic Waves to Map
Earth’s Interior
 Moho Discontinuity
 Moho’s Boundary between the crust and the
mantle
 Seismic waves speed up as they reach this
area
 Seismic waves speed up as they reach more
dense areas and slow down when the reach less
dense areas
 The more solid the layer the more dense
Using Seismic Waves to Map
Earth’s Interior
 Shadow zone
 The area between 105-140 degrees from the
epicenter
 Area in which seismic waves can’t be
detected
 What is the cause?
 Secondary waves can’t be transmitted
through liquid layers
 Primary waves are slowed and deflected by
the outer core and speed up again when
the reach the solid inner core
Destruction by Earthquakes
 Objectives
 Define magnitude and Richter scale
 List ways to make your classroom and home
more earthquake safe
Measuring Earthquakes
 Seismology
 Seismologists - People who study
earthquakes and seismic waves
 Use instruments called seismographs
 Record seismic waves
 A drum with a sheet of paper vibrates and a
stationary pen marks the vibrations on the
paper
 The height of the lines are used to measure
the energy released from the earthquake
called the magnitude
Measuring Earthquakes
 Earthquake Magnitude
 Usually determines the strength of the break
 Doesn’t determine the duration or the size
 For each increase of 1is 10 times stronger
 An earthquake of 4 is 10 times stronger than an
earthquake of 3
Tsunamis
 Caused by the movement of the ocean floor
 Causes a disruption in the water
 Some are so wide that a large ship can travel
over the wave without knowing
 Recent earthquake in the Indian Ocean
created a tsunamis that was 100 feet high and
moving at 500 mph
Earthquake Safety
 Quake proofing your home
 Place heavy objects near the floor
 Fix gas appliances in place
9.4 Living on a Fault
 Objectives
 Recognize that most loss of life in an
earthquake is caused by the destruction of
human-made structures
 Consider who should pay for making
structures seismic safe
Who should pay for earthquake
preparation?
 Seismic safe structures
 Structures that are resistant to the vibrations
of Earth’s crust
 Structures are made with moorings
 Made of steel and are filled with alternating
layers of rubber and steel
 These structures absorb the energy
produced from the earthquake
What is an Earthquake Short
Video
 http://www.sciencekids.co.nz/videos/earth/
whatisanearthquake.html
 Earthquake Long Video
http://video.pbs.org/video/1690329036/