Download Earthquakes

Earthquakes
Nelson’s Class
2008
Earthquakes
• An earthquake is the shaking and vibrating
of the earth caused by large and sudden
releases of energy that accompany
movement, or displacement, of the
outermost layer of the earth, called the
crust.
• Energy can be released by stress in the
Earth’s crust.
Large earthquakes can occur at
depths of 650 to 700 km
• Prentice Hall defines earthquakes as the
shaking and trembling that results from the
movement of rock beneath Earth’s
surface.
• Most earthquakes are too small to notice,
but a large earthquake can cause changes
in the Earth’s surface.
What are some forces that are
found in Earth’s Crust?
• When Earth’s plates move, they create a great
force that squeeze or pull the rock found in the
crust.
• These forces are called stress.
• Stress is a force that acts on rock to change its
shape or volume.
• Volume is the amount of space that the rock
takes up.
• Stress is a force and it adds energy to the rock.
Types of stress:
•
•
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There are three types of stress:
Tension
Compression
Shearing
• These forces cause rocks to become
brittle and snap.
Tension
• Tension is a push or pull on the crust.
• It stretches the crust (stretches the rock).
• The crust becomes thin in the center when
this happens.
• It occurs when two plates are moving
apart.
• Plates can push apart or get pushed apart
by melted rock from the core.
Compression
• Compression squeezes rock until it folds
or breaks.
• One plate pushing against another can
compress rock.
• When plates collide, the edges sometimes
crumple up into mountain ranges.
• One plate can also slide under the other.
• Plates can get stuck and energy builds
until it is released in an earthquake.
Shearing
• Shearing occurs when stress pushes a
mass of rock in two opposite directions.
• Shearing can cause rock to break and slip
apart or change its shape.
• During shearing two plates are trying to
slide by each other.
• They may rub against each other, or stick
and grind and cause energy to build up
time and time again.
Faults
• There are three main
kinds of faults:
normal, reverse, and
strike-slip faults.
• A fault is a fracture, or
break, in Earth’s
lithosphere. Most
faults are located
along tectonic plate
boundaries.
• Faults are classified
by how they move.
Normal Fault
• In a normal fault, the
hanging wall slips down
relative to the footwall.
• Stress that pulls rock
apart causes a normal
fault.
• Earthquakes along
normal faults are
common near boundaries
where tectonic plates are
moving apart, such as in
the Great Rift Valley of
Africa.
Reverse Fault
• Along a reverse fault,
the block of rock
moves up.
• Stress that presses
rocks together causes
reverse faults.
• These faults can
occur near collision
zone boundaries
between plates.
Strike-Slip Fault
• Blocks of rock move
sideways on either
side of the fault plane.
• Stresses that push
blocks of rock
horizontally cause
earthquakes along
strike-slip faults.
• These faults occur
where plates scrape
past each other.
Over time:
• Movement of rocks along normal and
reverse faults can push up mountains and
form deep valleys.
• As rocks move along strike-slip faults,
rocks that were once in continuous layers
can become separated by hundreds of
kilometers.
Earthquake Vocabulary:
• Focus: area beneath Earth’s surface
where rock that is under stress breaks,
triggering an earthquake.
• Epicenter: the point on the surface directly
above the focus.
• Seismic waves: vibrations that travel
through Earth carrying the energy
released during an earthquake.
Main Categories of Seismic Waves
• An earthquake sends out two types of
waves from its focus: S-waves, and
P-waves.
• When S-waves and P-waves reach Earth’s
surface at the epicenter, surface waves
develop.
P-waves
• Primary Waves or P-waves are the fastest waves.
• They are the first to reach any location after an
earthquake.
• They travel on an average of 5 kilometers per second
(3 mi/s).
• They can travel through solids, liquids and gases.
• As they pass through materials they pull and push
materials apart.
• Buildings experience this push and pull as primary
waves pass through the ground where they are built.
S-Waves
• S-waves are the second waves to arrive at any
location after an earthquake. They start at the
same time as the P-waves but travel at about
half the speed as primary waves.
• As they travel through a material, the materials
particles move up and down, or from side to
side.
• S-waves rock small buildings from side to side
during an earthquake.
• S-waves can travel through rock but they can
not travel through liquids or gases.
Surface waves
• Surface waves are seismic waves that
move along the Earth’s surface, not
through its interior.
• They make the ground roll iup and down
or shake from side to side.
• Surface waves cause the largest ground
movement and the most damage.
• Surface waves travel more slowly than
• P-waves or S-waves.
How are seismic waves measured?
• Seismic waves are measured with a
seismograph.
• Seismograph: an instrument that
constantly records ground movements.
• Separate seismographs are used to
measure up- and- down movements and
side- to -side movements.
• The recording produced by a seismograph
is called a seismogram.
Locating an Earthquake
• In order to locate the epicenter of an earthquake,
scientists must have seismograms from at least three
seismic locations.
• Scientist find the difference between the arrival times of
the P-waves and S-waves of the three stations.
• The time difference is used to determine the distance of
the epicenter from each station. The greater the
difference in time, the farther away the epicenter.
• A circle is drawn around each station, with a radius
corresponding to the epicenter’s distance from the
station. The point where the three circles meet is the
epicenter.
Seismograms
• Scientist can use seismograms to help
them locate the focus of an earthquake.
• The seismogram records the time when
the first primary waves arrives.
• This wave travels by a direct path.
Magnitude
• Charles Richter and Beno Gutenberg invented
the Richter Scale during the 1930’s.
• The Richter scale measures an earthquake
according to how fast the ground moves at a
seismic station.
• Today most scientists prefer the moment
magnitude scale, which is more accurate for
larger earthquakes.
• The moment magnitude scale is based on the
total amounts of energy released by an
earthquake.
• Both scales have top values of 10.
Damage from earthquake
• The Mercalli Scale was developed to rate
an earthquake according to the amount of
damage in a given location.
• The Mercalli Scale uses Roman numerals
to rank earthquakes by how much damage
they caused.
• Uses a rating of 1 to 12.
Safety
• The best way to protect yourself is to drop,
cover and hold.
• Stay away from large objects.
Damages caused by earthquakes:
• Damage caused by earthquakes includes
shaking, liquefaction, aftershocks, and
tsunamis.
Summary
• Study your notes.
• Read sections assigned in the text.