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Earthquakes
Chapter 8
Created by Rachel Joseph
An Earthquake is…
the shaking and trembling
that results from the
movement of rock beneath
Earth's surface
The movement of Earth's
plates produces strong
forces that squeeze or pull
the rock in the crust
This is stress, a force that
acts on rock to change its
volume or shape
(deformation)
This stores energy in the
rock.
When the rock snaps from all
the stress, it releases
energy as seismic waves.
Where Do Earthquakes Occur?
• Most earthquakes take place near the edges
of tectonic plates. This figure shows the Earth’s
tectonic plates and the locations of recent
major earthquakes.
Stress
There are three
different types of
stress that occur on the
crust, shearing, tension,
and compression
These forces cause some
rocks to become fragile
and they snap
Some other rocks tend
to bend slowly like road
tar softened by the suns
heat
(shearing)
(compression)
(tension)
Faults
A fault is a break or crack in
the crust where slabs of
crust slip past each other.
The rocks on both sides of a
fault can move up or down or
sideways
When enough stress builds on
a rock, the rock shatters,
releasing energy, creating an
earthquake
Faults usually occur along
plate boundaries, where the
forces of plate motion
compress, pull, or shear the
crust too much so the crust
finally breaks.
Chapter 8
Section 1 What Are Earthquakes?
Elastic Deformation and Elastic Rebound
Click below to watch the Visual Concept.
Visual Concept
San Andreas Fault
• San Andreas fault in California. Notice it is not one
long fault, but many, somewhat connected smaller
ones. This line will parallel the transform boundary
between the North American Plate and the Pacific
Plate.
• Transform boundaries tend to have the most
earthquakes.
Hanging Wall? Foot Wall?
• See the block farthest to the
right that is shaped kind of
like a foot? That’s the foot
wall. Now look at the block
on the other side of the fault.
See how it’s resting or
hanging on top of the
footwall block? That’s the
hanging wall.
• Here’s another way to think
of it: the hanging wall block
is always above the fault
plane, while the foot wall
block is always below the
fault plane.
Strike-Slip Faults
Shearing creates
this fault
In this fault, rocks
on both sides of the
fault slide past each
other with a little up
and down motion
When a strike-slip
fault forms the
boundary between
two plates, it
becomes a transform
boundary
Normal Faults
Tension forces in Earth's
crust causes these types
of faults
Normal faults are at an
angle, so one piece of rock
is above the fault, while
the other is below the
fault
The above rock is called
the hanging wall, and the
one below is called the
footwall
Tension forces cause the
hanging wall to slip
downward
Normal faults occur along
the Rio Grande rift valley
in New Mexico, where two
pieces of Earth's crust
are diverging
Reverse Faults
Compression forces
produce this fault
This fault has the same
set up as a normal fault,
but reversed, which
explains it’s name
So the hanging wall gets
pushed up the footwall.
This fault produced part
of the Appalachian
Mountains in the eastern
United States
How Do Mountains Form?
The forces of plate
movement can build up
Earth's surface, so over
millions of years,
movement of faults can
change a perfectly flat
plain into a gigantic
mountain range
Sometimes, a normal
fault uplifts a block of
rock, so a fault-block
mountain forms
When a piece of rock
between two normal
faults slips down, a
valley is created
Mountains Formed by Folding
Sometimes, under
current conditions, plate
movement causes the
crust to fold
Folds are bends in rock
that form when
compression shortens and
thickens part of Earth's
crust
The colliding of two
plates can cause folding
and compression of crust
These plate collisions can
produce earthquakes
because rock folding can
fracture and lead to
faults
Anticlines and Synclines
Geologists use the
terms syncline and
anticline to describe
downward and upward
folds in rock
An anticline is a fold in a
rock that arcs upward
A syncline is a fold in a
rock that points
downward (sinks)
These folds in rocks are
found on many parts of
the earths surface
where compression
forces have folded the
crust
Plateaus
The forces that
elevate mountains
can also raise
plateaus, a large
area of flat land
elevated high above
sea level
Some form when a
vertical fault pushes
up a large flat piece
of rock
Like a lasagna, a
plateau consists of
many layers, so it is
wider than it is tall
Seismic Waves
Seismic Waves: vibrations
that travel through Earth
carrying the energy released
during an earthquake
an earthquake produces
vibrations called waves that
carry energy while they
travel out through solid
material
During an earthquake,
seismic waves go out in all
directions from the focus
They ripple like when you
through a stone into a lake or
pond
Epicenter: the point on the
surface directly above the
focus.
P, S, and Surface Waves
How Earthquakes Form
Everyday, about 8,000
earthquakes hit Earth, but
most of them are too little
to feel
Earthquakes will always begin
when the stress builds up the
rock in a fault.
Friction keeps the rocks
from moving, so the stress
builds up.
Eventually the energy is so
great the friction is
overcome and the rock
breaks or moves.
This release of energy and
movement of rock is an
earthquake
Focus: the point beneath
Earth's surface where rock
that is under stress breaks
Aftershocks
An earthquake
that occurs
after a larger
earthquake in
the same area
May strike
hours, days, or
even months
later
Seismic Waves Ctd.
There are three
different types of
seismic waves: P waves,
S waves, and surface
waves
An earthquake sends out
two of those waves, P
and S waves (called
body waves as they
travel through the
interior of the Earth).
When they reach the
top of the epicenter,
surface waves form
Primary Waves
Also known as P Waves
The first waves to come
are these waves
(fastest)
P waves are earthquake
waves that compress
and expand the ground
like an accordion
(compressional)
P waves cause buildings
to expand and contract
Travel through solids
and liquids.
Secondary Waves
Also known as S Waves
(shear)
After P waves, S waves
come next (slower)
S waves are earthquake
waves that vibrate from
one side to the other as
well as down and up
(transverse)
They shake the ground
back and forth
When S waves reach the
surface, they shake
buildings violently
Unlike P waves, which
travel through both
liquids and solids, S
waves cannot move
through any liquids
Surface Waves (Love/Rayleigh Waves)
When S waves and P
waves reach the top,
some of them are
turned into surface
waves (L-waves) S+P
Surface waves move
slower than P waves and
S waves, but they can
produce violent ground
movements
Some of them make the
ground roll like ocean
waves
Other surface waves
move buildings from side
to side
Chapter 8
Section 1 What Are Earthquakes?
Seismic Waves: Surface Waves
Click below to watch the Visual Concept.
Visual Concept
Detecting Seismic Waves
Geologists use
instruments called
seismographs to
measure the
vibrations of seismic
waves
Seismographs
records the ground
movements caused
by seismic waves as
they move through
the Earth
Mechanical Seismographs
Until just recently, scientists
have used a mechanical
seismograph
a mechanical seismograph
consists of a heavy weight
connected to a frame by a
wire or spring
When the drum is not moving,
the pen draws a straight line
on paper wrapped around the
drum
Seismic waves cause the drum
to vibrate during an
earthquake
the pen stays in place and
records the drum's vibrations
The higher the jagged lines,
the more severe earthquake
The weight on the
seismograph never
moves. The machine
moves back and
forth.
Locating the Epicenter
• Since the P waves
travel faster than
the S waves,
scientists can use
the difference in
arrival times to see
how far away the
earthquake
occurred.
• It does not tell the
direction however.
Determining Direction
• One station can only
learn how far away
the quake occurred.
• They would draw a
circle at that radius.
• Two stations can
isolate the epicenter
to two possible
locations.
• If three stations
combine their data,
the quake occurred
where the three
circles overlap.
Chapter 8
Section 2 Earthquake Measurement
S and P Time Method: Finding an Epicenter
Click below to watch the Visual Concept.
Visual Concept
Measuring Earthquakes
There are many ways to
measure an earthquake
There are at least 20
different types of scales.
3 of them are the Mercalli
scale, Richter scale, and
the Moment Magnitude
scale
Magnitude is a
measurement of
earthquake strength
(energy) based on seismic
waves and movement along
faults
The Mercalli Scale
Developed in the twentieth
century to rate
earthquakes according to
their intensity.
The intensity of an
earthquake is the strength
felt by people and the
damage caused.
Is not a precise
measurement, the rating is
based on the damage done.
But, the 12 steps explain
the damage given to people,
land surface, and buildings
The same earthquake could
have different Mercalli
ratings because of the
different amount of
damage in different spots
•The Mercalli scale uses Roman numerals
to rank earthquakes by how much
damage they cause
The Richter Scale
The Richter scale is a
rating of the size of
seismic waves as measured
by a particular type of
mechanical seismograph
Developed in the 1930’s
All over the world,
geologists used this for
about 50 years
Electric seismographs
eventually replaced the
mechanical ones used in
this scale
Provides accurate
measurements for small,
nearby earthquakes
Does not work for big, far
away ones
Richter Magnitude v. Energy
The Moment Magnitude Scale
Geologists use this scale
today
It’s a rating system that
estimates the total energy
released by an earthquake
(basically the Richter scale
modified for distance)
Can be used for any kind of
earthquakes, near or far
Some news reports may
mention the Richter scale,
but the magnitude number
they quote is almost always
the moment magnitude for
that earthquake
We are in a place of moderate danger of earthquakes
happening
Chapter 8
Section 3 Earthquakes and Society
Earthquake-Hazard Level
Click below to watch the Visual Concept.
Visual Concept
Earth’s Liquid Core
• This was one of the
proofs that the Earth
has a liquid core.
• You have an Swave shadow
where the S waves
do not pass through
liquids.
How Earthquakes Cause Damage
The severe shaking
provided by seismic
waves can damage or
destroy buildings and
bridges, topple utility
poles, and damage gas
and water mains
With their side to side,
up and down movement,
S waves can damage or
destroy buildings,
bridges, and fracture
gas mains.
Earthquake Hazard
• Earthquake hazard is a measurement of how likely an
area is to have damaging earthquakes in the future.
• An area’s earthquake-hazard level is determined by
past and present seismic activity.
• The greater the seismic activity, the higher the
earthquake-hazard level.
Earthquake Forecasting
• Forecasting when and
where earthquakes will
occur and their
strength is difficult.
• By studying areas of
seismic activity,
seismologists have
discovered some
patterns in earthquakes
that allow them to
make some general
predictions.
Earthquake Forecasting, continued
• Strength and Frequency Earthquakes vary in
strength. The strength of earthquakes is
related to how often they occur.
• This table shows more detail about the
relationship.
Earthquake Forecasting, continued
• Another method of forecasting an earthquake’s strength,
location, and frequency is the gap hypothesis.
• The gap hypothesis is based on the idea that a major
earthquake is more likely to occur along the part of an active
fault where no earthquakes have occurred for a certain
period of time.
Earthquake Forecasting, continued
• An area along a
fault where
relatively few
earth-quakes
have occurred
recently but
where strong
earthquakes
have occurred in
the past is called
a seismic gap.
Earthquake Forecasting, continued
• Using the Gap
Hypothesis Not all
seismologists believe the
gap hypothesis is an
accurate method of
forecasting
earthquakes.
• But some seismologists
think the gap hypothesis
helped forecast the
approximate location
and strength of the 1989
Loma Prieta earthquake
in California.
Chapter 8
Section 3 Earthquakes and Society
Gap Hypothesis and Seismic Gaps
Click below to watch the Visual Concept.
Visual Concept
Earthquakes and Buildings
• Earthquakes can easily
topple buildings and
destroy homes. Today,
older structures in
seismically active
places, such as
California, are being
made more
earthquake resistant.
• Retrofitting is the name
given to the process of
making older structure
more earthquake
resistant.
Earthquakes and Buildings, continued
• A common way of
retrofitting an older
home is to securely
fasten it to its
foundation.
• Steel is often used
to strengthen
buildings and
homes made of
brick.
Earthquakes and Buildings, continued
• Earthquake-Resistant
Buildings A lot has been
learned from building
failure during
earthquakes.
•
With this knowledge,
architects and engineers
use new technology to
design and construct
buildings and bridges to
better withstand
earthquakes.
•
The next slide shows some
of the technology used to
make earthquakeresistant buildings.
Earthquakes and Buildings, continued
Are You Prepared for an Earthquake?
• Before the Shaking Starts
The first thing should do
safeguard your home
against earthquakes.
•
Place heavier objects on
lower shelves so they do
not fall during an
earthquake.
•
Find safe places within
each room of your home
and outside of your home.
•
Make a plan with others
to meet in a safe place
after the earthquake is
over.
These students are
participating in an
earthquake drill.
Earthquake Preparations, continued
• When the Shaking Starts
If you are indoors,
crouch or lie face down
under a table or desk.
•
If you are outside, cover
your head with your
hands and lie face
down away from
buildings, power lines, or
trees.
•
If you are in a car on an
open road, you should
stop the car and remain
inside.
Earthquake Preparations, continued
• After the Shaking Stops Try
to calm down and get
your bearings.
•
Remove yourself from
immediate danger, such
as downed power lines,
broken glass, and fire
hazards.
•
Do not enter any
damaged buildings unless
you are told it is safe by
someone in authority.
•
Beware that aftershocks
may cause more damage.
San Francisco: after 1906 quake