Chapter 8
Chapter 8
Section 1
 When you bend a stick, you notice that is changes shape
while you bend it
 The stick will spring back if you stop applying force.
 But if you don’t stop bending the stick
 It changes permanently.
 If its elastic limit is passed, the stick may break
 As it breaks you can feel vibrations in the stick.
 Rocks are like other solid materials
 If enough force pulls or pushes on them, they will
change shape.
 They may even break
 After breaking, the ends of the broken pieces may
snap back.
 This snapping back is called elastic rebound.
 Inside Earth, pushing and pulling forces cause rocks to
change shape slowly over time.
 As they are strained, potential energy builds up in
them.
 This energy is released suddenly when the rocks finally
break or move.
 The breaking and the movement that follows causes
vibrations that move through rock.
 If they are strong enough, the vibrations are felts as
earthquakes
 An earthquake is a movement of the ground that
occurs when rocks inside Earth pass their elastic limit,
break suddenly, and experience elastic rebound.
 When part of a rock breaks, rocks on either side move
as a result of elastic rebound.
 The surface where rocks break and move is called a
fault.
 Rocks can break in different ways, depending on the
forces that cause the break.
Three types
of faults.
1. Normal
Fault
2. Reverse Fault
3. Strike-Slip
Fault
 Normal faults form
where tension forces
pull rocks apart
 The rock above the
fault moves down.
 Reverse faults are caused
by compression
 Rocks pushed together
or compressed
 When the two rocks
push together, rock
above the fault is pushed
up.
 Sections of rock move
past one another in
opposite directions along
Earth’s surface.
 Also called shearing.
 Strike-slip faults are
caused by shear forces.
 Earthquakes release
energy causing
vibrations
 When this energy is
released, it moves away
from the fault in the
form on seismic waves.
 The point deep inside
the Earth where energy is
released causing an
earthquake is a focus
 Some of the energy from the earthquake travels
straight up to Earth’s surface where it can be felt.
 The epicenter is the point on Earth’s surface directly
above the earthquake focus.
 When seismic waves leave the focus of an earthquake,
some travel through Earth’s interior, and other travel
along the surface.
 Three Types of Waves
 Primary Waves
 Secondary Waves
 Surface Waves
 Seismic waves that travel fastest through rock material
are primary waves or P-waves.
 Primary waves cause the material to move from side to
side, in the same direction that the wave is moving.
 Other seismic waves that travel through Earth’s
interior are called secondary waves.
 Secondary waves, or S-waves, do not move as fast as Pwaves.
 As they move through rock material, they cause the
material to vibrate at right angles to the direction of
the wave.
 Seismic waves that travel along Earth’s surface are
called surface waves.
 They are the largest and slowest type of seismic wave.
 They cause more damage than other types of waves.
 Surface Waves move in different ways.
 They may move rock and soil in a backward rolling
motion.
 Like waves of water
 Some shake or sway the rock and soil from side to side.
 Scientists who study
earthquakes are called
seismologist
 They use instruments
called seismographs to
record seismic waves.
 One type of seismograph
has a drum that holds a
roll of paper on a frame
 When seismic waves
reach the station, the
drum vibrates.
 The pen on the
pendulum traces a record
of the vibration
 The height of the lines
traced on the paper
measures the magnitude
of the earthquake.
 Magnitude is the
measure of energy
released by an
earthquake.
 The epicenter of an earthquake is the point on the
surface of Earth directly above the focus
 Far away from the epicenter, the P-waves and S-waves
arrive at different times.
 But close to the epicenter, the waves arrive at almost
the same time.
 Once scientists know the P-wave and S-wave arrival
times for at least three seismograph stations, they can
figure out the location of an earthquakes epicenter.
 They draw circles on a map.
 Each circle shows the distance from the seismograph
station to the earthquake.
 The point where three or more circles intersect is the
location of the epicenter.
 Some earthquakes are not felt on the surface of Earth.
 People do not even know these small earthquakes are
happening.
 Larger earthquakes, on the other hand, can cause
major damage.
 Richter magnitude is based on the measurements of
heights of seismic waves as they are recorded on
seismographs.
 Scientists use this information to determine the
Richter magnitude of an earthquake.
 Richter magnitude describes how much energy an
earthquake releases.
 Very weak earthquakes have low magnitudes like 1.0
 Strong earthquakes have high magnitudes in the range
of 6 to 7
 For every increase of 1.0 on the Richter scale, an
earthquake actually releases 32 times more energy.
 This means that an earthquake with a magnitude of
7.5 releases 32 times more energy than an earthquake
of 6.5
 Another way to measure earthquakes is by the
modified Mercalli intensity scale.
 This scale measures the intensity of an earthquake.
 Intensity is a measure of the amount of damage to
structures and to rocks and soil in a specific area.
 The amount of damage depends on
 How strong the earthquake is
 Kinds of structures in an area
 Distance from epicenter
 Nature of the surface material
 The Mercalli scale uses Roman numerals I through XII
 An earthquake with an intensity of I would be felt by
few people
 An intensity – VI earthquake would be felt by everyone
 An intensity – XII would cause major damage to
Earth’s surface and to human-built structures.
 When an earthquake occurs on the ocean floor,
powerful waves are produced
 These waves travel outward from the earthquake in all
directions.
 A powerful seismic sea wave is called a tsunami

 Tsunamis traveling in open ocean water are low and
fast moving.
 But tsunamis change as they approach land.
 The speed of the tsunami slows and the height of the
wave increases.
 Huge tsunami waves can be up to 30 meters high.
Giant Sea Waves
"In 1992 a mild earthquake, barely noticed,
hit San Juan del Sur in Nicaragua.
Minutes later the peaceful harbor was
drained dry as if someone had pulled a giant
bath plug and let the water out. Amazed at
the sight, curious people flocked to the
harbor to look. As they stared, a giant
tsunami rushed in and swept people and
buildings far out to sea. This three-part
illustration is an example of how the water is
drained in a harbor, then builds up height
before rushing back to the shore."
- Dr. Eldridge M. Moores