Primary and Secondary Waves

1
Primary and Secondary Waves
(The Slinky)
(The Rope)

• An Overview of Seismic Waves

2
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

3
Types of Seismic Waves

• Body waves move through the earth
• - P and S waves

• Surface waves travel near the earths surface
• - Rayleigh and Love waves

4
Types of Seismic Waves

• Surface waves trapped near the earths surface

Love wave horizontal motion that is ? to wave
direction
Rayleigh wave causes the ground to shake in
elliptical motion, with no ? motion
5
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
6
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
7
Wave Velocity

• Differences in velocity allow mapping of the
  Earths interior

8
P Wave Shadow Zone

• P waves refract as they pass through the
  mantle-core boundary
• Allows for better re-construction of Earths
  interior

9
S Wave Shadow Zone

• Leave a larger shadow zone
• Allows for better re-construction of Earths
  interior

10
Whats a Seismograph?

• Records P and S waves
• Base and graph move with the ground
• Mass and pen stay rigid
• Record is called a seismogram

11
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

12
Todays 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

13
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
14
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.

15
P S Waves and Nuclear Testing

• Underground nuclear explosions create P and S
  waves
• Challenge detect very small explosions

16
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 Earths interior