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• Wave Scales
• Seismic waves
Pg. 62
Pg. 63
Seismic Waves
Pg. 63
• Body waves-travel through the earth’s interior.
1. P waves(primary) -Fastest waves, first
to be detected.
–Move back and forth parallel to the motion of
the wave.
–Move through solids, liquids and gases.
• http://www.geo.mtu.edu/UPSeis/images/Pwave_animation.gif
2. S waves(secondary) - Second fastest
• Move perpendicular to motion of wave.
• Move only through solids
• http://www.geo.mtu.edu/UPSeis/images/Swave_animation.gif
• Surface waves-travel along the earth’s surface and
move the slowest.
Cause the most damage during earthquakes
1.Rayleigh wave(P) - Move rock up and down
2.Love wave(S) – Move rock side to side
Rayleigh Wave
• http://www.geo.mtu.edu/UPSeis/images/Rayl
eigh_animation.gif
Love wave
• http://www.geo.mtu.edu/UPSeis/images/Love
_animation.gif
Shadow zone – area on Earth’s surface where no
seismic waves from a particular earthquake can
be detected.
Seismology
• Seismology is the
study of
earthquakes.
• Seismographinstrument that
records seismic
waves.
Measuring Earthquakes
Magnitudestrength of an
earthquake.
• Richter scale
(measures
amplitude of
waves – gives an
absolute
measure of an
earthquake)
Intensity-how much damage
an earthquake causes.
• Mercalli scale
Not scientific – it is a relative
scale but helps people
interpret earthquake sizes.
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The Great Chilean Earthquake
The Earth’s crust is made up of “plates” that float on denser magma below, and move relative to one another.
Sometimes two continental plates will crash into one another, and sometimes an oceanic plate will move under a
continental one. This is happening off the west coast of South America, which is known as a subduction zone. On 22
May 1960, part of the oceanic plate near the coat of Chile moved under the South American continental plate. This
disturbance caused land along the Chilean coast to suddenly drop by about ten feet (three meters), and ground
further inland to rise by about twenty feet (six meters).
This abrupt shift caused the biggest earthquake ever recorded, 9.5 and a major tsunami. Many of the buildings in
the affected cities had been designed to withstand earthquakes, and a number of smaller quakes in the months
leading up to this disaster had given some warning. This lessened the impact of the earthquake itself, but it was the
tsunami that was responsible for most of the casualties and damage. The massive wave affected not only Chile, but
also caused deaths and damage as far away as Hawaii, Japan, and the Philippines.
Other Big Earthquakes in Modern Times
In terms of events that were studied, measured, and recorded on the Richter scale, the Great Chilean Earthquake
was far and away the biggest ever. Keeping in mind that the Richter scale runs along an exponential progression, the
next largest was at Prince William Sound, Alaska in 1964, which measured 9.2. The third biggest earthquake ever
recorded, was under the Indian Ocean in 2004, clocking in at 9.1 on the Richter scale and generating a formidable
tsunami that devastated much of the coastline of Southeast Asia, and killed nearly 228,000 people.
Historical Earthquakes
It is highly probable that some ancient earthquakes were of an even higher magnitude than the 1960 Chile disaster,
but because the Richter scale was not developed until the 1930s, it is difficult to quantitatively compare them.
Retrospective examination of seismograph records dating back to around 1900 allows estimates to be made, but for
earlier events, any assessment of the severity can only be based on eyewitness accounts, and the amount of damage
done. For example, the Lisbon earthquake of 1755 was by all accounts very severe, and it generated a massive
tsunami which magnified the death toll. The Shaanxi earthquake which occurred in China in the 1500s was the
deadliest known to history, killing almost one million people. This event may also have been the biggest known to
man, but there is simply no way to tell.
This is an isoseismal map showing the impact of the Magnitude 7.7 Alaska Earthquake
of July 9, 1958 in Modified Mercalli Scale units. Lituya Bay was in the area of XI
intensity. The isoseismal contours near the epicenter parallel the Fairweather Fault.
Map information from Seismicity of the United States, 1568-1989 (Revised), by Carl W.
Stover and Jerry L. Coffman, U.S. Geological Survey Professional Paper 1527, United
States Government Printing Office, Washington: 1993.
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