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Primary and Secondary Waves. (“The Slinky”). An Overview of Seismic Waves. (“The Rope”). The Outline. The Types of Seismic Waves Seismic Wave Velocities P and S Wave Shadow Zones Seismographs P and S Waves Help Locate Earthquakes The S Wave and The Richter Scale Nuclear Testing

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Presentation Transcript
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Primary and Secondary Waves

(“The Slinky”)

An Overview of Seismic Waves

(“The Rope”)


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The Outline

  • The Types of Seismic Waves

  • Seismic Wave Velocities

  • P and S Wave Shadow Zones

  • Seismographs

  • P and S Waves Help Locate Earthquakes

  • The S Wave and The Richter Scale

  • Nuclear Testing

  • Conclusion


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Types of Seismic Waves

  • Body waves: move through the earth

    - P and S waves

  • Surface waves: travel near the earth’s surface

  • - Rayleigh and Love waves


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Types of Seismic Waves

  • Surface waves: trapped near the earth’s surface

Love wave: horizontal motion that is  to wave direction

Rayleigh wave: causes the ground to shake in elliptical motion, with no  motion


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Types of Seismic Waves

  • Body waves: move through interior of the earth

P wave: shakes the ground back and forth in same and opposite direction as the wave direction

S wave: shakes the ground back and forth  to wave direction


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Wave Velocity

  • P waves: travel at 4-7 km/s

  • S waves: travel at 2-5 km/s

Blue primary waves followed by red secondary waves move outward in concentric circles from the epicenter of an earthquake off British Columbia and Washington State


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Wave Velocity

  • Differences in velocity allow mapping of the Earth’s interior


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P Wave Shadow Zone

  • P waves refract as they pass through the mantle-core boundary

  • Allows for better re-construction of Earth’s interior


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S Wave Shadow Zone

  • Leave a larger shadow zone

  • Allows for better re-construction of Earth’s interior


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What’s a Seismograph?

  • Records P and S waves

  • Base and graph move with the ground

  • Mass and pen stay rigid

  • Record is called a seismogram


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The 1st Seismograph?

  • Invented by Choko (136 C.E.)

  • Cu vessel with 8 dragon heads attached to it

  • Each holding balls in their mouths and 8 open-mouthed frogs directly below each one


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Today’s Seismograph

  • Called broadband because can sense ground motion over a wide range of frequencies

  • Measure amount of electrical energy needed to keep the mass centered


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P & S Waves Locate Earthquakes

  • Measure the arrival time between P and S waves

  • At least 3 seismograms needed

Blue P waves followed by red S waves move outward in concentric circles from the epicenter of an earthquake off British Columbia and Washington State


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The S Wave and The Richter Scale

  • S wave amplitude determines the Richter Magnitude

  • Developed to make a quantitative measure of the relative sizes of earthquakes in S. Calif.


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P & S Waves and Nuclear Testing

  • Underground nuclear explosions create P and S waves

  • Challenge: detect very small explosions


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Conclusion

  • P and S waves are a tool in locating quakes

  • “Harmless” compared to surface waves

    - think “The Slinky” and “The Rope”

  • Better understanding of these seismic waves have resulted in more accurate maps of Earth’s interior


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