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EARTHQUAKES
“Whole lotta shakin’ goin’ on” – Jerry Lee Lewis
WHAT IS AN EARTHQUAKE?
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An earthquake is a vibration of the Earth produced by
the rapid release of energy.
This energy release is the result of slippage along a
fault.
TYPES OF FAULTS
SEISMIC WAVES
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With the energy release of an earthquake, waves of seismic
energy radiate omnidirectionally and are recorded as lines on
a seismograph.
Waves that travel along the surface of the Earth are called
surface waves.
Waves that travel through the Earth’s interior are called body
waves.
Body waves are divided into primary (P) waves, which occur
as “push-pull”, and secondary (S) waves, which occur in a
shaking manner.
THE “RING OF FIRE”
SAN ANDREAS FAULT
Runs the length
of California
and occasionally
slips, generating
frequent
earthquakes.
 People often
mistake the
motion of the
fault, as if
California will
slip into the
Pacific Ocean.
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FINDING THE EPICENTER
The epicenter is the
location on the Earth’s
surface directly above
the earthquake focus.
 The difference in the
velocity of the P and S
waves is used to
calculate the location.
 The key is the time
differences between the
waves and several
locations are used to
triangulate the
epicenter.
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MEASURING THE MAGNITUDE
The Mercalli Intensity Scale was developed in
1902 to measure the intensity of an earthquake.
 Earthquake destructiveness can vary widely
based on the depth of the focus, the nature of the
soil and sub-soil, and distance from the epicenter.
 A second scale, the Richter Scale, was adopted in
1935 to better assess the magnitude.
 This scale uses the amplitude of the largest wave
on a seismograph.
 The largest earthquakes ever recorded measured
approximately 8.6 on the Richter scale.
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THE RICHTER SCALE
MAJOR EARTHQUAKES THROUGH HISTORY
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San Francisco, CA – 1906 – estimated 7.0, $9.5 billion in
damage (2012 dollars)
Messina, Italy – 1928 – estimated 7.1, 123,000 dead, shortly
after the earthquake, a 40 foot tsunami decimated the
coastline.
Anchorage, AK – 1964 – 9.2, 143 deaths.
Tangshan, China – 1976 – 7.0, nearly 250,000 dead.
San Francisco, CA – 1989 – 7.1, 62 dead, $6 billion in damage.
Northridge, CA – 1994 – 6.7, 61 dead, $15 billion in damage.
Kobe, Japan – 1995 – 6.9, 5,000 dead, damage exceeds $100
billion.
Sumatra, Indonesia – 2004 – 9.2, nearly 300,000 dead from
ensuing tsunami.
Tohoku, Japan – 2011 – 9.0, 15,000 dead, tsunami caused
nuclear disaster, over $50 billion in damage.
USING EARTHQUAKES TO STUDY THE
INNER EARTH
Scientists noted that P waves were detected
nearly all over the Earth after an earthquake.
 The time difference between P waves in different
locations lead scientists to infer that the Earth’s
interior contained layers of differing densities.
 S waves were less commonly detected.
 Since S waves were known to be unable to be
propagated through a liquid, it was surmised
(correctly) that a portion of the Earth’s interior is
liquid.
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WAVES THROUGH THE EARTH
THE ASTHENOSPHERE
Located just beneath the Lithosphere, in the
Earth’s crust, the Asthenosphere consists of hot,
weak rock, that exhibits plasticity.
 It is this characteristic that allows the monstrous
tectonic plates to “float” and move about.
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CAN WE PREDICT EARTHQUAKES?
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Some changes in the land (uplift or subsidence) has been observed
prior to some earthquakes.
Animals have been observed to behave in an erratic fashion prior to
an earthquake.
To date, no one has been able to predict the onset of an earthquake
with any degree of accuracy.
Actuarial scientists have determined the likelihood of future
earthquakes along known faults, but this is expressed as a percent.