Earthquakes
Chapter 5
An Earthquake is…
• The movement of
Earth's plates
produces strong
forces that squeeze
or pull the rock in the
crust. We feel this as
shaking and trembling.
• Stress is a force that
acts on rock to change
its volume or shape
3 Types of 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
Faults
A fault is a break 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, creating
faults
Faults usually occur along
plate boundaries, where
the forces of plate
motion compress, pull, or
shear the crust too much
so the crust smashes
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
When movement affects along
a normal fault, the hanging wall
slips 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
setup as a normal fault,
but reversed, which
explains it’s name
Just like the normal fault,
one side of the reverse
fault is at an angle of the
other
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 crashing 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 arcs
downward
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
How Earthquakes Form
Everyday, about 8,000
earthquakes hit Earth, but
most of them are too little
to feel
Earthquakes will always
begin in a rock beneath
the surface
A lot of earthquakes begin
in the lithosphere within
100 km of Earth's surface
Focus: the point beneath
Earth's surface where
rock that is under stress
breaks
The focus triggers an
earthquake
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 to the
focus
They ripple like when
you through a stone into
a lake or pond
Seismic Waves Ctd.
There are three
different types of
seismic waves:
Primary waves,
Secondary waves,
and surface waves
Primary Waves
Also known as P Waves
The first waves to
come are these waves
P waves are
earthquake waves that
compress and expand
the ground like an
accordion
P waves cause
buildings to expand
and contract
Secondary Waves
Also known as S Waves
After p waves, S waves
come
S waves are earthquake
waves that vibrate from
one side to the other as
well as down and up
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
When S waves and P
waves reach the top,
some of them are
turned into surface
waves
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
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
Measuring Earthquakes
There are many things to
know about the measures
of an earthquake
There are at least 20
different types of
measures
3 of them are the Mercalli
scale, Richter scale, and
the Moment Magnitude
scale
Magnitude is a
measurement of
earthquake strength
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
of ground motion in a given
place
Is not a precise
measurement
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
ones
The Moment Magnitude Scale
Geologists use this scale
today
It’s a rating system that
estimates the total
energy released by an
earthquake
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
Locating the Epicenter
• Sine 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.
• If three stations
combine their data,
the quake occurred
where the three
circles overlap.
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.