Earthquakes and Earths Structure

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Earthquakes and Earths Structure

• Objectives
• i. Describe the causes of an earthquake
• ii. Describe seismic waves
• iii. Discuss how earthquakes are measured
• iv. Explain the relationship between plate
  boundary types and earthquakes
  
• v. Discuss the prediction of earthquakes and the
  mitigation of earthquake hazards

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• A man walks his bike past a leaning building
  destroyed by the 1995 earthquake in Kobe, Japan.

Fig. 7-CO, p.146
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• Damage after a 7.9 magnitude earthquake struck
  Ahmedabad, India, 2001.

Fig. 7-1, p.147
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Anatomy of an earthquake

• When you apply enough stress to a rock, it can
  deform in one of three ways elastically,
  fracture and plastic deformation.
  
• At first the rock deforms elastically. If the
  stress is removed, the rock springs back to its
  original size and shape (rubber band example),
  releasing the stored elastic energy.

Fig. 7-4, p.149
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fracture

• Every rock has a limit beyond which is cannot
  deform elastically. It may suddenly fracture,
  releasing the elastic energy and springing back
  to its original shape. The resultant rapid
  motion creates vibrations that travel through the
  Earth and are felt as an earthquake.

Fig. 7-2, p.148
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• Under other conditions rock can exceed its
  elastic limit and continue to deform like putty.
  This is called plastic deformation, and the rock
  deformed this way will keep its new shape when
  the stress is released (so does not store the
  energy used to deform it, and earthquakes do not
  occur).

Fig. 7-3, p.148
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• A road is built across an old fault (the San
  Andreas Fault).
  
• Plates move 1-16cm/yr. Friction prevents
  continual slippage (next slide).

Fig. 7-4a, p.149
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• So rock stretches or compresses elastically and
  potential energy builds up. When the stress is
  so great, rock snaps loose or fractures the
  ground rises and falls and undulates back and
  forth (earthquake).

Fig. 7-4b, p.149
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• An earthquake is a sudden motion or trembling of
  the Earth caused by the abrupt release of energy
  stored in the rocks.

Fig. 7-4c, p.149
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• Now a weakness in rock has formed, so other
  earthquakes commonly occur along the same faults.
  To right is a portion of the San Andreas Fault
  it is part of the boundary between the Pacific
  plate (left) and North-American plate.

Fig. 7-5, p.149
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• Earthquake Waves
• Seismology is the study of EQs and the nature of
  the Earths interior on evidence of seismic waves
  (waves that travel through rock, produced by EQs
  and explosions).
• Focus initial rupture point
• Epicenter point on Earths surface directly
  above the focus.

Fig. 7-6, p.150
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• Earthquakes produce several types of seismic
  waves
  
• Body Waves travel through the Earths interior
  and carry some of the energy from the focus to
  the surface
  
• Surface Waves radiate from epicenter along the
  Earths surface (like when you throw a rock in a
  calm lake).

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• Body Waves two main types are P and S
  waves.
  
• Model of a P (primary and compressional) wave.
  The spring moves parallel to the direction of
  wave propagation, causing compression and
  expansion of rock. Move fast (4-7 km/sec in
  crust) and travel through air, liquid and solid
  material.

Fig. 7-7, p.150
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• S (shear or secondary) waves only travel
  through solid material, at 3-4 km/sec in the
  crust arrive after P waves. Example to right
  illustrates that S waves move parallel to rope,
  but individual particles move at right angles to
  rope length.

Fig. 7-8, p.151
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• Surface waves, which radiate from the epicenter
  along Earths surface, travel slower than body
  waves. They produce an up and down and rolling
  motion, and a side to side vibration (so Earths
  surface rolls like ocean waves and writhes from
  side to side).

Fig. 7-9, p.151
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• Seismograph a device that records seismic waves
  it creates a seismogram, which is a record of
  Earths vibration.

Fig. 7-10, p.151
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• Seismogram that recorded north-south ground
  movements during the 1989 Loma Prieta earthquake.

Fig. 7-11, p.152
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Measurement of Earthquake Strength

• 1. Mercalli Scale measures intensity of an EQ
  based on structural damage the more buildings
  destroyed, the more intense the EQ. This scales
  does not measure the energy released during an
  EQ.
• 2. Richter Scale (Charles Richter, 1935)
  expresses the amount of energy released during an
  EQ it measure the largest EQ body wave recorded
  on a seismograph. What is a drawback of this
  method?
• Today, seismologists calculate moment
  magnitude measure of amount of movement and
  surface area of a fault that moved during an EQ.
  This better reflects the total amount of energy
  released during an EQ.

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Richter Magnitude Measurement
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EQ strength (cont)

• A moment magnitude of 6.5 has the energy about
  equal to the atomic bomb dropped on Hiroshima at
  the end of World War II.
  
• On both the moment magnitude and Richter scales
  the energy of the quake increases by a factor of
  about 30 for each increment on the scale.
  
• The strength of rock determines the largest
  possible EQ (strong rock can store more elastic
  energy). The largest ever recorded on the moment
  magnitude scale were 8.5 and 8.7, about 900 times
  greater then energy released by the Hiroshima
  bomb.

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• Locating the source of an EQ.

Fig. 7-12, p.153
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• Time-travel curve.

Fig. 7-13, p.153
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• Locating the EQ epicenter by triangulation from
  three seismic stations that recorded the quake.

Fig. 7-14, p.154
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• Most EQ occur along plate boundaries, where
  plates diverge, converge or slip past one
  another.

Fig. 7-15, p.154
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• Earthquake hazard map of the San Andreas Fault
  zone in California (numbers indicate the
  probability of an EQ with magnitude 5 in the
  next 30 years).

Fig. 7-16, p.155
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• EQ at a transform plate boundary the San Andreas
  Fault Zone. This is a transform plate boundary
  between the Pacific and North American plates.
  The fault is vertical, the rocks move
  horizontally on opposite sides of the fault this
  is called a strike-slip fault. The SAF is a
  right-lateral strike-slip fault.

Fig. 7-17, p.155
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• The 1906 EQ and fire destroyed most of San
  Francisco.

Fig. 7-18, p.155
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• The 1994 magnitude 6.6 EQ that struck Northridge
  near LA caused much building damage. About
  10,000 EQ occur every year along the SAF and its
  associated faults (SAFZ).

Fig. 7-19, p.156
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• Earthquakes at a convergent plate boundary, occur
  along the upper part of the sinking plate where
  it scrapes past the opposing plate in a
  subduction zone. Called the Benioff Zone.
• What about a continental-continental convergent
  boundary? Oceanic-oceanic convergent boundary?

Fig. 7-20, p.156
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EQ at divergent plate boundaries

• EQ occur frequently at the Mid-Oceanic Ridge
  system as a result of faults that form as the two
  plates separate. Would the EQ be deep or shallow
  along the ridge?

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Fig. 7-21, p.157
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Fig. 7-22, p.159
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Fig. 7-23, p.160
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• A tsunami can develop from an earthquake.

Fig. 7-24, p.161
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• The sea floor drops, sea level falls with it.

Fig. 7-24a, p.161
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• Water rushes into the low spot and
  overcompensates, creating a bulge.

Fig. 7-24b, p.161
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• A tsunami develops when part of the sea floor
  drops during an EQ. Water rushes to fill the low
  spot, but the intertia of the rushing water
  forces too much water into the area, creating a
  bulge in the water surface. The long, shallow
  waves can build up into destructive giants when
  they reach shore. May travel at 750 km/hr with
  wave crests 100-150 km apart.

Fig. 7-24c, p.161
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Fig. 7-25, p.161
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Fig. 7-26, p.162
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Fig. 7-27, p.163
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• Velocities of P waves in the crust and upper
  mantle. Generally, P wave velocity increases
  with depth. What is the low velocity zone?

Fig. 7-28, p.163
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• Cross section of Earth showing paths of seismic
  waves. They bend gradually because of increasing
  pressure with depth. They bend sharply where
  they cross major layer boundaries in the Earths
  interior. Note that S waves do not travel
  through the liquid outer core, so direct S
  waves are only observed within an arc of 105
  degrees of the epicenter.
• P waves are refracted sharply at the
  core-mantle boundary, so there is a shadow zone
  of no direct P waves from 105-140 degrees from
  the epicenter.

Fig. 7-29, p.164
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• Earths magnetic field. The magnetic north pole
  is offset 11.5 degrees from the geographic pole.

Fig. 7-30, p.165
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p.167