Download Earthquakes

Faults &
Earthquakes
Deforming the Earth’s Crust
Deformation
• The process by which the shape of a rock
changes because of stress is called deformation.
• There are two basic types of deformation:
– Plastic deformation
– Elastic deformation
• When rock deforms in a plastic manner, it
folds like a piece of molded clay.
• With elastic deformation, the rock stretches like
a rubber band until it breaks.
• Elastic deformation can lead to earthquakes.
Stress
• Stress is a force that acts on rock to
change its shape or volume.
• Because stress if a force, it adds energy
to the rock.
• This energy is stored in the rock until
the rock either breaks, or changes
shape.
There are three types of stress
that occur in the Earth’s crust:
• Compression
• Tension
• Shearing
Compression
• The stress force
called compression
squeezes rock until
it folds, or breaks.
• Compression makes a mass of rock occupy a smaller
space.
• When compression occurs at a convergent boundary,
large mountain ranges can form.
Tension
• The stress force called tension pulls on the crust,
stretching rock so that it becomes thinner in the
middle.
• Tension occurs where two plates are moving apart,
such as mid-ocean ridges, or rift valleys.
Shearing
• Stress that pushes
a mass of rock in
two opposite,
horizontal directions
is called shearing.
• Shearing can cause rock to break and slip
apart.
• Shearing occurs at transform boundaries.
Folding
• The bending of rock
layers because of
stress in the Earth’s
crust is called
folding.
• Undisturbed rock
layers are horizontal,
so when we see a
fold we know that
deformation has
taken place.
Folds
• The two most common
types of folds are:
– Anticlines
– Synclines
• Anticlines are upward-arching folds.
• Synclines are downward, trough-like folds.
• Another type of fold is a monocline.
• In a monocline, both ends of the fold
are horizontal.
Faults
• When the stress on rocks causes them to break and
slip past each other, a fault is formed.
• The blocks of crust on each side of the fault are
called fault blocks.
• When faults are not
vertical one side of
the fault block will
be called a hanging
wall and the other
the footwall.
• The position of the fault block will determine which it is.
Faults
• There are 3 main
types of faults:
– Normal fault
– Reverse, or
thrust fault
– Strike-slip
Reverse Fault
Normal Faults
• Tension forces cause normal faults.
• The hanging
wall lies above
the fault and
the footwall
lies below the
fault.
• When movement occurs along the fault line,
the hanging wall slips downward.
• Normal faults are found at divergent plate
boundaries, where plates pull apart.
Reverse Faults
• Compression forces produce reverse faults.
• Reverse faults
have the same
basic structure as
a normal fault, but
the blocks move
in the opposite
direction.
• When movement occurs along the fault line, the
hanging wall slides up and over the footwall.
• Reverse faults are found at convergent plate
boundaries, where plates are pushed together.
Strike Slip Faults
• Shearing creates
strike-slip faults.
• The rocks on either side of the fault slip past each
other sideways with little up or down motion.
• A strike-slip fault that forms the boundary between
two plates is called a transform boundary.
Fault Block Mountain
• When the tension in
a normal fault uplifts
a block of rock, a
fault-block mountain
forms.
• The Grand Tetons in
Wyoming are an
example of a faultblock mountain
range.
Folded Mountains
• Folded mountains
form at convergent
boundaries where
continents have
collided.
• The Appalachian Mountains,
the Alps, and the Himalayas
are examples of folded
mountains.
Earthquakes
• An earthquake is the shaking and
trembling that results from the
movement of rock beneath Earth’s
surface.
• Not all Earthquakes occur at plate
boundaries. Sometimes they
happen in the middle of a tectonic
plate.
• Earthquakes can happen both near
the Earth’s surface or far below it.
• Earthquakes always
begin in rock below the
surface.
• Most earthquakes begin
in the lithosphere within
100 kilometers of the
surface.
• The focus is the point beneath Earth’s surface where
rock that is under stress breaks, triggering an
earthquake.
• The point on the surface directly above the focus is
called the epicenter.
Seismic Waves
• Seismic waves are vibrations that travel through Earth
carrying the energy released during an earthquake.
• Seismic waves that
travel through the Earth
are called body waves.
• Seismic waves that
travel along Earth’s
surface are called
surface waves.
• Each type of seismic waves travels through Earth’s
layers in a different way and at a different speed.
Body Waves
• There are two types
of body waves:
– P waves
– S waves
P-waves
• The first waves detected in an earthquake are
p waves, or pressure waves.
• P waves compress and expand the ground
like an accordion.
• P waves can travel through solids, liquids, or
gases.
S waves
• After P waves come secondary waves, or S waves.
• S waves are earthquake waves that vibrate from
side to side and thrust the ground up and down, or
back and forth.
• When S waves reach the surface, they shake
structures violently.
• S waves cannot move through liquids.
Surface Waves
• When P and S waves
reach the surface, some of
them are transformed into
surface waves.
• Surface waves move more
slowly than P and S waves,
but they produce the most
severe ground movements.
• They can actually make the
ground roll like ocean
waves.
• Other surface waves shake the ground from side to side.
All 3 waves
Detecting Seismic Waves
• Geologists use a seismograph to
record and measure the vibrations of
seismic waves.
Seismograph
• When the waves reach a seismograph,
the instruments creates a seismogram.
• A seismogram is a tracing of earthquake
motion.
• Until recently, scientists used
mechanical seismographs, like the one
in the picture.
• Today they use electronic seismographs
that convert ground movements into a
signal that can be recorded and printed.
Seismogram
Finding the Epicenter
• Scientists use seismograms to find the earthquakes epicenter.
• One method they use is called the S-P Time Method.
• They collect readings for the same earthquake from
seismographs stations at different locations.
• They then use this data to determine the distance each station
is from the earthquake.
• They can then triangulate
the results to find the
epicenter.
• It takes a minimum of 3
seismograph readings to
find the epicenter of and
earthquake.
Richter Scale
• There are many ways that scientists can measure an earthquake
• Magnitude is a measurement of earthquake strength based on
seismic waves and movement along faults.
• Charles Richter created the Richter magnitude
scale in the 1930s to compare earthquakes by
measuring ground motion and adjusting for
distance to find their strength.
• When magnitude increases by one unit the measured ground
motion becomes 10 times larger on the Richter scale.
• The Richter scale provides accurate measurements for small,
nearby earthquakes, but the scale does not work well for large,
or distant earthquakes.
Richter
Scale no.
No.of
earthquakes per
year
Typical effects of this magnitude
<¾
800,000
Detected only by seismometers
3.5 - 4.2
30,000
Just about noticeable indoors
4.3 - 4.8
4.800
Most people notice the, windows rattle.
4.9 – 5.4
1,400
Everyone notices the, dishes may break,
open doors swing.
5.5 – 6.1
500
Slight damage to buildings, plaster cracks,
bricks fall.
6.2 – 6.9
100
Much damage to buildings; chimneys fall,
houses move on foundations.
7.0 – 7.3
15
Serious damage; bridges twist, walls
fracture, buildings may collapse.
7.4 – 7.9
4
Great damage, most buildings collapse.
> 8/0
One every 5 to 10 Total damage, surface waves seen, objects
years
thrown in the air.
Mercalli Scale
• Seismologists can also
measure the intensity of
an earthquake.
• The Modified Mercalli
Intensity Scale is used to
measure the degree to
which an earthquake is
felt by people and the
amount of damage done
by it.
• The Mercalli scale uses Roman numbers from I to XII to
describe increasing earth quake intensity levels.