Primary P Waves

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Primary (P) Waves

• Fastest moving waves
• First to be recorded by seismographs
• P waves can travel through both solids and
  liquids through the earth
• Have a back and forth motion
• Compression waves - cause rock particles to move
  together and apart along the direction of the
  waves.

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P-wave motion
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Secondary (S) Waves

• Travel slower
• Can only travel through solids
• Shear waves
• They cause rock particles to move at right angles
  to the direction in which the waves are traveling

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S-Wave Motion
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Surface (L) Waves

• P and S waves that reach the earths surface
• Slowest waves
• Travel like ocean waves along the earths surface
• Surface waves cause the outer layer of the Earth
  to rise and fall
• Surface waves cause the most damage

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Surface Waves
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What is an earthquake?

• An earthquake is the vibration of Earth produced
  by the rapid release of energy
• Usually occur when rocks under stress suddenly
  shifts along a fault
• Energy released radiates in all directions from
  its source, or the focus
• Energy is in the form of waves
• Sensitive instruments (seismographs) around the
  world record the event

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Recording Earthquakes

• Seismic waves can be detected and recorded by
  using an instrument called a seismograph.
• Seismograph consists of 3 separate sensing
  devices.
• One device records the vertical movement of the
  ground. The other two record horizontal motion.
• It records motion by tracing wave-shaped lines on
  paper or translating the motion into electronic
  signals.

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How do we know what we know about the inside of
the Earth?
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Question

• Have any earthquakes happened in the last week?
• Today?
• Last hour?
• http//earthquake.usgs.gov/eqcenter/recenteqsww/

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Earthquakes Chapter 6
1755 Cape Ann (Boston) earthquake, estimated at
Richter magnitude 6.0-6.3
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Earthquake destruction
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Surface expression of Faults
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Elastic Rebound

• Strain is slow and constant
• The rocks on each side of a fault are moving
  slowly.
• Fault doesnt move due to friction (locked)
• stress in the rocks increases
• Stress overcomes friction
• The rocks then rebound back to their original
  shape but in a new position
• Releases energy as seismic waves

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Foreshocks, main shock, aftershocks
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Fault zones
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Which depth causes the greatest damage?
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Seismic Gaps
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Earthquake Preparation

• How can you prepare for an earthquake?
• What should you do during an earthquake?
• What should you do after an earthquake?

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Magnitude

• Magnitude is a measure of the energy released by
  an earthquake.
• Seismologists express the magnitude using a
  magnitude scale, such as the Richter scale
• Increase in 1 unit 10 times more shaking
• Increase in 1 unit 30 times more energy released

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Frequency of Earthquakes
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Mercalli Scale

• The Mercalli scale expresses the intensity of an
  earthquake, or the amount of damage it causes
  (results)
• It uses roman numerals and a verbal description
  of the associated damage (I to XII)

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USA seismic hazard
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Seismic hazard and risk

• Seismic hazard Likelihood of an earthquake of a
  certain magnitude in a certain amount of time
• Seismic risk hazard X human vulnerability
• 1989 Macquarie Ridge, New Zealand
• M 8.3, No deaths
• 1960 Agadir, Morocco
• M 5.5, 12,000 deaths
• Level of hazard? risk?
• Aleutian Islands, AK
• Boston, MA

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MA seismic hazard
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What earthquake is this (the M7.5 one)?
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Tsunami

• A Tsunami is a giant ocean wave
• Faulting may cause a sudden drop or rise in the
  ocean floor. The entire water column will also
  drop or rise with the ocean floor.
• The water adjusts to the change in sea level,
  resulting in a series of long, low waves that
  develop into tsunamis.

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Tsunami Animations

• http//www.pbs.org/wnet/savageearth/animations/tsu
  nami/main.html
• 29.12.04 Interactive. Indian Ocean tsunami, wave
  animation Special reports Guardian Unlimited