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ESS 8 - Earthquakes

ESS 8 - Earthquakes. Profs. Vidale & Creager. Bolt, 5-17. What is a wave?. A wave is a disturbance that travels far through a medium while particles of the medium move a small amount back and forth and do not experience a net translation.

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ESS 8 - Earthquakes

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  1. ESS 8 - Earthquakes Profs. Vidale & Creager Bolt, 5-17

  2. What is a wave? • A wave is a disturbance that travels far through a medium while particles of the medium move a small amount back and forth and do not experience a net translation. • Examples: ripples on a pond, the ”wave” at sports events

  3. Seismic wave radiation • Radiation - waves that travel outward and carry energy • Examples • Light energy from space heater • Travels too fast to see go, 300,000 km/sec • Water waves from a splash, few m/sec • Sound waves from a speaker, 300 m/sec • Seismic waves (motions) are just vibrations of the ground, like sound waves are vibrations of the air

  4. Doppler shift 1 2 3 3 2 1 Higher pitch Lower pitch For example, approaching siren has a higher pitch than a receding siren

  5. zebu.uoregon.edu/~js/space/ lectures/lec05.html Moving wave sources Doppler shift Sonic boom

  6. Breakingsound barrier http://www.fly.faa.gov/carf/

  7. Shuttle sonic boom

  8. The “Thrust SST”, 1997

  9. Breaking sound barrieron land! 763 mph, 1997

  10. Old speed record - Mach 6.7 4520 miles per hour

  11. X-15 world records • 4520 miles per hour - Mach 6.7 • 2 km/sec, no longer champ • 354,000 ft off the ground • 100 km high • Flew for just 2 minutes at a time • Intended to prototype moonships • NASA vs Air Force turf war, Air Force lost

  12. X-43A- new record in 2004 • Mach 9.6 (7000 mph), off California coast • Unpiloted, 12-foot long vehicle • Ramjet powered (air breathing) • Burned for just 10 seconds • $250M research project

  13. Types of Seismic Waves Walt on Disneyland train • P waves • S waves • Surface wave train • Love • Rayleigh body waves

  14. Amplitude

  15. Wavelength

  16. Period

  17. Frequency Frequency: How many waves pass a point in a given amount of time For sound: frequency=pitch

  18. Types of Seismic Waves Earthquake Station • Body waves • Surface waves * Earthquake Station *

  19. Raypathsandwavefronts

  20. P waves • Longitudinal - material moves back and forth (vibrates) in same direction that wave travels, produces compression/dilatation cycle • Fastest type of wave, so arrives first • termed Primary wave • Typical velocities in crust: 5 - 7 km/sec • Travels through solids, fluids, or gas

  21. P waves Sun movie

  22. Another viewof P wave motion P movie

  23. Strike-slip P-wave radiation pattern This is left lateral strike-slip faulting in map view N L L N N Strong in some directions: Lobes Weak in other directions: Nodes N L L

  24. Different “First Motions” in different directions Down Up Vertical ground motion Down Up

  25. Up Down Up Down First direction of motion:beach ball diagrams

  26. Raypaths bend as seismic waves travel

  27. First Motions and Rupture modes Map views 3-D view

  28. Three primary focal mechanisms Deformation Map view of first motions Strike-slip Normal Thrust or reverse

  29. S waves • Shearing - material moves back and forth perpendicular to the direction the wave travels in a twisting motion. • Slower than P wave, arrives second • termed Secondary wave • Typical velocities in crust: 3-5 km/sec • P waves travel 5-7 km/s • Travels through solids, but not fluids • because there is no restoring force for the perpendicular motions

  30. S waves

  31. S movie S wave in a solid

  32. Surface Waves • Travel on surface of Earth • Two types • Love waves • Rayleigh waves • Travel a bit slower than S waves • Are the largest amplitude waves • so the P wave can serve as a warning to take cover or shut down critical facilities • warning ranges from a few to 100 seconds • Can get 1 s of warning for each 10 km in distance

  33. More surface waves • Need a surface to travel along, which is the rock-air interface at the Earth’s surface. • Motion is strongest near the surface • Most strongly generated by earthquakes near the surface

  34. Rayleigh waves Love waves

  35. Review of Waves P S Love Rayleigh Bolt, 1-9

  36. Where is energy? • In waves, energy has two forms • Strain or deformation - like the energy stored by deforming a spring - 1/2 kx2 • Motion or vibration - kinetic energy in physics - 1/2 mv2 • Vibration is the most damaging, but either kind of energy can cause damage

  37. Bouncing ball on a spring At rest stretched moving compressed

  38. Water waves are different • Energy again has two parts • One part is kinetic energy • The other part is gravitational • There is no twisting energy, not much is strain

  39. Amplitude of seismic waves • Amplitude is strength of shaking • Depends on magnitude • Determines amount of damage • Amplitude decreases with distance from the earthquake • energy spreading out over larger area • P wave smallest • S waves larger • Surface waves largest

  40. Because the waves travel at different velocities • As waves radiate outward from the earthquake, through the Earth, they separate into a predictable pattern with • P waves arriving first • then S waves • then surface waves S P surface Time

  41. P waves, then S waves, then surface waves At 80°, as drawn 10 m 20 m 30-50 m Body waves Mike’s movie

  42. Motion has 3 components Love Transverse Rayleigh Radial S Vertical P Tromp movie

  43. Complications for Seismic waves • Reflection • Refraction • Conversion

  44. Reflection on mirror - simple

  45. Refraction - Object in water (bending of rays)

  46. Refraction of light by water Because speed of light waves is slower in water than in air Seismic waves refract too And can also switch between P and S Refraction (water in fishbowl) Press, 19-1

  47. Reflection -complicated Includes refraction through curved glass www.nashobawinery.com

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