What is an earthquake?
• An earthquake is a sudden, rapid shaking of the Earth
caused by the breaking and shifting of rock beneath the
Earth’s surface.
• For hundreds of millions of years, the forces of plate
tectonics have shaped the Earth as the huge plates that
form the Earth’s surface move slowly over, under and
past each other. Sometimes the movement is gradual.
At other times, the plates are locked together, unable to
release the accumulating energy. (This should be review)
• When the energy grows strong enough, the plates break
free causing the ground to shake.
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Where do earthquakes appear?
Most earthquakes take place near the edges of tectonic
plates. Earthquakes can also occur at faults, which are
cracks in the Earth’s crust.
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What causes earthquakes?
• Elastic deformation leads to earthquakes.
Rocks deform in response to stress in the
Earth’s crust. When the rock can no longer
bend under stress it will break
• Elastic rebound is the return of the deformed
rock to its original shape. During this process
energy is released as seismic waves and these
waves result in earthquakes.
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What Is Seismology?
• Seismology is the study of earthquakes and seismic
waves that move through and around the earth.
• A seismologist is a scientist who studies earthquakes
and seismic waves.
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How do earthquake waves travel?
• Seismic waves are the waves of energy caused by the
sudden breaking of rock within the earth or an explosion.
They are the energy that travels through the earth and is
recorded on seismographs.
• There are several different kinds of seismic waves, and
they all move in different ways. The two main types of
waves are body waves and surface waves.
• Body waves can travel through the Earth's inner layers,
but surface waves can only move along the surface of
the planet like ripples on water. Earthquakes radiate
seismic energy as both body and surface waves.
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• Seismic waves traveling through the Earth
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BODY WAVES
• Traveling through the interior of the earth,
body waves arrive before the surface
waves emitted by an earthquake. These
waves are of a higher frequency than
surface waves.
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P-waves
• Aka pressure waves or primary waves
• They are the first waves to be detected.
• They can travel through solids, liquids and
gases.
• P wave animation
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S-waves
• Aka shear waves or secondary waves
• They are the second waves to be
detected.
• They cannot travel through liquids.
• S wave animations
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SURFACE WAVES
• Traveling only through the crust, surface waves
are of a lower frequency than body waves, and
are easily distinguished on a seismogram as a
result.
• Though they arrive after body waves, it is
surface waves that are almost entirely
responsible for the damage and destruction
associated with earthquakes
• This damage and the strength of the surface
waves are reduced in deeper earthquakes.
• Surface wave animations
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Locating Earthquakes
• Seismographs-instruments located at or near
the surface of the Earth that record seismic
waves
• Seismogram-a tracing of earthquake motion
created by a seismograph
• S-P time method- allows one to locate the
epicenter of an earthquake; a method of
comparing numerous seismograms by lining up
data for p and s waves on a distance-time graph
• How a seismograph works
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Locating Earthquakes
• Epicenter-the point on the Earth’s surface
directly above an earthquake’s starting
point
• Focus-the point inside the Earth where an
earthquake begins
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How strong was the earthquake?
• The Richter Magnitude Scale
• Created in 1930 by Charles Richter
• Measures the strength of earthquakes
(aka magnitude)
• It is a logarithmic scale-each time the
magnitude increases by one unit, the
measured ground motion becomes 10x
larger
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