by the end of this unit, you should be able to: discuss stress and strain and their roles in...
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Learning Targets
By the end of this unit, you should be able to: Discuss stress and strain and their roles in
earthquakes Know the differences between elastic and
plastic deformation Compare and contrast the three types of
faults and three different seismic waves Explain the process of locating an earthquakes
epicenter Discuss the relationship between plate
tectonics, stresses and earthquakes
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What are Earthquakes?
The shaking or trembling caused by the sudden release of energy
Usually associated with faulting or breaking of rocks
Continuing adjustment of position results in aftershocks
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What is the Elastic Rebound Theory?
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
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Stress and Strain Stress occurs when there is a
force on the rocks. Strain is the response to stress
1. Compression-squeeze together
2. Tension-pull apart3. Shear-distortion
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Stress Strain Curve Elastic deformation- low stress,
material bends and stretches (pulling of rubber band-goes back into shape
Plastic deformation- stress builds past elastic point, causes permanent deformation Failure occurs when there is a rupture
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Stress Strain Curve
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Faults
Crack in the earth where plates moves Types of Faults
ReverseHorizontal and vertical pressure that squeezes the rock or land together
Seen at convergent boundaries
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Normal Fault Movement is vertical and horizontal
Caused by tension Divergent boundary
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Strike Slip Fault Also known as a transform fault Caused by horizontal sheering Example
San Andreas Fault in California
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Reading
Chapter 19.1 Pg. 528-533
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What are Seismic Waves?
Energy that is released by an earthquake Energy travels in the form of waves Two types:
Body waves P and S
Surface waves
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Body Waves: P and S waves
Body waves P or primary waves
fastest waves travel through solids, liquids, or gases compression wave, material movement is in
the same direction as wave movement
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S or secondary waves slower than P waves travel through solids only shear waves - move material perpendicular
to wave movement
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Surface Waves: R and L waves
Surface Waves Travel just below or along the ground’s
surface Slower than body waves; rolling and side-
to-side movement Especially damaging to buildings
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Clues to Earth’s Interior
Earth’s internal structure Waves change speed and direction
depending on the material they go through S Waves do not go through the outer
core When the waves change scientists can
gain information about the consistency and density of our earth’s layers
Shadow zone is created where no P or S waves travel
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The Focus and Epicenter of an Earthquake
Focus The point within Earth where faulting begins is
the focus---below the surface Epicenter
The point directly above the focus on the surface
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How is an Earthquake’s Epicenter Located?
Seismic wave behavior P waves arrive first, then S waves, then L
and R Average speeds for all these waves is
known After an earthquake, the difference in
arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.
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How is an Earthquake’s Epicenter Located?
Three seismograph stations are needed to locate the epicenter of an earthquake
A circle where the radius equals the distance to the epicenter is drawn
The intersection of the circles locates the epicenter
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How are the Size and Strength of an Earthquake Measured?
Magnitude Richter scale measures total amount of energy
released by an earthquake; independent of intensity
Amplitude tells us the size of the seismic wave The greater the amplitude the stronger the
earthquake Intensity
Modified Mercalli Scale subjective measure of the kind of damage done
and people’s reactions to it isoseismal lines identify areas of equal intensity
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Where Do Earthquakes Occur and How Often?
~80% of all earthquakes occur in the circum-Pacific belt most of these result from convergent margin
activity ~15% occur in the Mediterranean-Asiatic
belt remaining 5% occur in the interiors of
plates and on spreading ridge centers more than 150,000 quakes strong enough to
be felt are recorded each year
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The Economics and Societal Impacts of EQs
Damage in Oakland, CA, 1989• Building collapse
• Fire• Tsunami• Ground failure