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Earthquakes
What are Earthquakes?

Earthquakes are
What are Earthquakes?


As the plates move they
put forces on
themselves and each
other.
What are Earthquakes

When the break occurs, the stress is released as energy
which moves through the Earth
______________________________________,
which we feel and call an earthquake.
What causes the stress?


There are three main forces that drive deformation
within the Earth and create the stress that causes
earthquakes.
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Which of the following things are elastic?
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Pasta
Rubber bands
Super balls
Rocks
Plastic
Metal
Dirt
Elasticity

Elasticity is a physical property of materials which
return to their original shape after the stress that
caused their deformation is no longer applied

What does that mean?
Elasticity

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We can use the elastic properties of rock to explain
how energy is stored and released within the Earth’s
crust.
Elastic Rebound

Explains how energy is stored
in rocks

Rocks bend until the strength
of the rock is exceeded

Rupture occurs and the
rocks quickly rebound to an
undeformed shape

Energy is released in waves
that radiate outward from the
fault
More Stuff About Earthquakes
Faults

Any stress on the plates can cause an earthquake if the
elastic limit is reached.

Each type of stress, however, results in a specific type of
fault.
Normal Fault
=
Reverse Fault
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AKA Thrust Fault
=
Strike Slip
=
•
No change in elevation, rock slides past one another.
Focus and Epicenter
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Focus:
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Epicenter:
Types of Earthquakes
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- Foci are less than 70 km depth.
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- Foci are between 70 and 300 km
depth.
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- Foci are greater than 300 km.
Measuring Earthquakes
How do we measure an earthquake?

There are two main scales with which we can measure an
earthquake.
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Mercalli
Richter
The Mercalli Scale

In 1902 Guiseppe Mercalli devised a scale for measuring
the intensity of an earthquake by

In 1931 seismologists modified his scale to adapt to
modern American building standards.

We can use Mercalli’s modified scale to find the epicenter
of an earthquake.
I
Barely felt
II
Felt by a few sensitive people, some suspended objects may swing
III
Slightly felt indoors as though a large truck were passing
IV
Felt indoors by many people, most suspended objects swing, windows and dishes
rattle, standing autos rock
V
Felt by almost everyone, sleeping people are awakened, dishes and windows break
VI
Felt by everyone, some are frightened and run outside, some chimneys break, some
furniture moves, slight damage
VII
Considerable damage in poorly built structures, felt by people driving, most are
frightened and run outside
VIII
Slight damage to well built structures, poorly built structures are heavily damaged,
walls, chimneys, monuments fall
IX
Underground pipes break, foundations of buildings are damaged and buildings shift
off foundations, considerable damage to well built structures
X
Few structures survive, most foundations destroyed, water moved out of banks of
rivers and lakes, avalanches and rockslides, railroads are bent
XI
Few structures remain standing, total panic, large cracks in the ground
XII
Total destruction, objects thrown into the air, the land appears to be liquid and is
visibly rolling like waves
The Richter Scale

Prior to 1935 earthquakes were measured by using the
Mercalli Scale.
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Charles Richter (1935) created a numerical method
based on the
__________________________________________
recorded on a seismogram and the distance from the
epicenter.
Richter Scale

Richter magnitudes are based on a logarithmic scale, base
10
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This means that for each whole number you go up on the
Richter scale the strength of the earthquake goes up ten
times.
Magnitude Examples
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A magnitude of 3 is ____ times ____________
than a magnitude of 2
A magnitude of 5 is ____ times ____________
than a magnitude of 7
A magnitude of 6 is ____ times ____________ than a
magnitude of 2
M1
M2
M3
So, what does it mean?

Think of it in terms of energy released by explosives:

A magnitude 1 earthquake releases as much energy as blowing
up 6 ounces of TNT

A magnitude 8 earthquake releases as much energy as blowing
up 6 million tons of TNT!
That is roughly 6 nuclear bombs!
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Measuring Via Richter Scale
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What do we need to determine the size of the
earthquake?
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Seismograph
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Determine the ________________________________________
Determine ___________________________________________
Determine ___________________________________________
Nomogram
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Find the S-P interval
Find the amplitude
Draw a line that connects the above points to find magnitude
Example
Measuring Earthquakes
Waves Edition
Seismic Waves
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There are three types of waves that are created when
stress is released as energy in earthquakes
P-waves
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Primary waves are the fastest of the three waves and the
first detected by seismographs.
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Move at about 3.5 miles/second
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They are able to move through both liquid and solid rock.
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P waves, like sound waves, are longitudinal waves, which
means that they compress and expand matter as they
move through it.
S-waves
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Secondary waves are the waves directly following the P
waves.
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Move at about 2.1 miles/second
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S waves move through solids only
Serpentine movement
Body Waves
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Both P and S waves are called body waves because they
move within the Earth's interior.
Surface Waves
Travel just below or along the ground’s surface
 Most damaging waves!
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Move at about 1.2 miles/second
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Two Types:
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L – Waves
R - Waves
Surface Waves

L – waves, or Love Waves, move parallel to the Earth's
surface and perpendicular to the direction of wave
propagation.

R – waves, or Rayleigh waves, move in an elliptical motion,
producing both a vertical and horizontal component of
motion in the direction of wave propagation.