19.2

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19.2 Seismic Waves Earths Interior
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Measuring Seismic Waves

• Seismometer sensitive instruments that detect
  seismic waves
• Rotating drum covered with paper
• Pendulum with writing instrument attached
• Mass on pendulum and pen are at rest due to
  inertia
• When ground shakes, the pen registers it on paper
• Seismogram record produced by seismometer

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Seismometer
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Travel-Time Curve

• Seismic waves are recorded at various facilities
• Seismologists construct graph showing arrival of
  waves

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http//chuma.cas.usf.edu/juster/A3/travel20time
20curve.jpg
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Distance from epicenter

• Measure arrival of primary (P), secondary (S),
  and surface waves
• As distance increases, separation between waves
  increase
• Information is used to locate epicenter

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Clues to Earths Interior

• Seismic waves help give information about Earths
  interior
• Seismic waves change speed and direction at
  boundaries between different materials
• Through mantle P and S waves follow direct paths
• When P waves strike Earths core they refract
  (bend)

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• By creating Travel-time curves and path of each
  wave, seismologist learn about difference in
  density and composition
• S waves do not travel through liquid, which means
  they do not travel through core
• Means Earths core is at least partially liquid

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Earths composition
http//geology.com/rocks/pictures/peridotite.jpg

• Waves can also change speed
• Speed tells scientists thickness and composition
  of Earths different regions
• Upper mantle peridotite
• Outer core mostly liquid iron and nickel
• Inner core mostly solid iron and nickel

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http//images.encarta.msn.com/xrefmedia/aencmed/ta
rgets/illus/ilt/T629167A.gif
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In-Class Assignment/Homework