Seismology The study of seismic waves'

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• Seismology The study of seismic waves.
• Seismograph Instrument that records seismic
  waves.
• Seismogram The record of ground motion that is
  produced by a seismograph.

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How does a seismograph work?
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To measure horizontal ground motion
A heavy mass is decoupled from the Earth by means
of a pendulum.
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When the ground moves, the mass tends to remain
stationary because of its inertia, but the
support (frame) moves with the Earth.
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The movement of the Earth relative to the
stationary mass is recorded on a rotating drum.
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To measure vertical motion The principle is the
same, but the mass is suspended on a spring.
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With a modern seismograph, the ground motion is
also recorded on a computer.
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AS1 Seismograph
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The Great Sumatra Earthquake of 2004
Seismograms Recorded at Boston College
Magnitude 9.0 December 26, 2004
Magnitude 8.7 March 28, 2005
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AS1 Seismograms
Same Distance, Different Magnitudes
Three Sumatra Earthquakes Recorded at Weston
Observatory Boston College
?133
Seismograms are shown on the same scale.
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Magnitude 6.7 Gulf of California January 4, 2006
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Same Magnitude, Different Distances
Seismograms are shown on the same scale.
El Salvador Magnitude 7.7, January 13, 2001
?33
India Magnitude 7.7, January 26, 2001
?106
AS1 Seismograms
El Salvador and India Earthquakes Recorded at
Devlin Hall Boston College
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Same Earthquake, Different Distances
Seismograms are shown on the same scale.
Phoenix Country Day School, Paradise Valley,
AZ ?7
AS1 Seismograms
Weston High School,Weston, MA ?38
Parkfield, CA Earthquake Magnitude 6.0, 09/28/04
Time (sec/102)
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