Prepared by Mark R. Noll

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• Prepared by Mark R. Noll
• SUNY College at Brockport

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Earthquakes

• Earthquakes are caused by a sudden release of
  energy
• The amount of energy released determines the
  magnitude of the earthquake
• Seismic waves carry the energy away from its
  origin

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Elastic Rebound Theory

• Rocks will deform elastically
• Elastic energy stored
• If blocks of crust move smoothly little elastic
  energy is built-up
• If crustal blocks are locked, elastic energy is
  stored

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Elastic Rebound Theory

• Rocks are brittle break
• Ruptures occur when elastic limit is exceeded
• Locked faults eventually reach the limit and
  suddenly break
• Movement occurs and elastic energy is released

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Fig. 18.1 Origin of Earthquakes by elastic
rebound
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Seismic Waves

• Produced by released elastic energy divided
  into 2 types of waves
• Body waves move by elastically deforming rock
• Elastic waves are divided into P- and S- waves
• Similar to the way that light and sound waves
  move

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Seismic Waves

• P-waves
• Primary waves, arrive first
• Alternating pulses of compression and dilation
  (expansion) parallel to wave path
• P waves may pass through solids, liquids, and
  gases
• Compression produces temporary changes in volume
  density of material

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Seismic Waves

• S-waves
• Secondary waves, arrive second
• S waves cause a shearing effect
• Waves are perpendicular to the direction of
  travel
• Elastically change the shape of materials
• Liquids and gases do not behave elastically

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Seismic Waves

• Surface waves
• Restricted to traveling along the Earths surface
• Travel more slowly than P or S waves
• Similar to ocean waves travelling through rock
• Orbital motion

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Fig. 18.3. Motion of seismic waves
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Earthquake Locations

• Global distribution of earthquakes confirms their
  association with plate boundaries
• Most foci are within 100 km of the surface
• Rocks become less brittle, more ductile with
  depth

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Earthquake Locations

• May be grouped into 1 of 3 categories
• Shallow focus surface to 70 km deep
• Intermediate focus 70 to 300 km deep
• Deep focus 300 to 700 km deep
• Earthquakes are not likely to occur deeper than
  700 km

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Earthquake Locations

• Locating earthquakes
• Earthquakes may be located by analyzing the
  arrival time of P and S waves
• Distance from the recording station is determined
  by the difference in arrival time of the P and S
  waves
• A minimum of 3 seismograph stations is needed to
  determine the epicenter

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Earthquake Locations

• The arrival time method determines the location
  on the map below which the earthquake occurred
• Epicenter
• The exact point is at some depth below the
  surface
• Focus

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Fig. 18.2. Earthquake epicenter and focus
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Earthquake Intensity

• Intensity is an evaluation of the severity of
  ground motion
• Based on
• Total energy released
• Distance from epicenter
• Rock type

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Earthquake Magnitude

• Magnitude is a measure of the energy released
• Magnitude measurements are based on
• Measurement of seismic waves
• Measurement of the amount of energy released
• Evaluation of damage caused

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Earthquake Magnitude

• The Richter scale measures the amplitude of
  seismic waves
• The Richter scale is logarithmic - Each unit on
  the scale relates a 10 fold increase in the
  amplitude of the seismic wave
• Amplitude may be related to the energy released
  1 Richter unit 30x the energy

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Earthquake Magnitude

• Moment magnitude scale measures the amount of
  energy released
• Designed to differentiate large earthquakes
• May be used to calculate energy of old events by
  slip along fault

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Earthquake Magnitude

• The modified Mercali scale relates what people
  feel and the damage done by an earthquake
• Less precise method
• Typically uses surveys of people living working
  in the area
• Events are reasonably well correlated to Richter
  magnitudes

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Earthquake Hazards

• Earthquakes pose a significant factor killing
  thousands each year
• Primary effect is ground motion
• Ground motion may cause additional indirect
  damage by
• Liquefaction of soil
• Landslides
• Fires from broken gas pipes

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Fig. 18.7. Examples of earthquake damage
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Earthquake Prediction

• Prediction is proving a difficult goal to achieve
• Prediction is based on several methods
• Behavior of animals
• Identification of seismic gaps
• Measurement of small scale movement

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Earthquake Prediction

• Seismic Risk
• Use historic data, occurrence, intensity
• Evaluate related risk factors, e.g. landslide
  potential
• Risk maps used to develop regional planning and
  building codes

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Fig. 18.6. Seismic risk map of the U.S.
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Earthquakes Tectonics

• Earthquake frequency correlates with plate
  boundaries
• Divergent boundaries narrow zone of shallow
  focus, low intensity quakes
• Convergent boundaries
• Subduction zones shallow to deep quakes of
  varying intensity
• Collision zones wide zone of shallow to
  moderate depth quakes of varying intensity

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Earthquakes Tectonics

• Transform boundaries shallow focus quakes that
  follow the pattern of faults of varying intensity
• Intraplate zones of infrequent seismic activity
  that are associated with incomplete rifting
  events or paleo plate margins

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Fig. 18.14. Earths seismicity
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The Earths Interior

• Seismic waves pass through the Earth
• Waves are reflected and refracted
• Seismic rays follow curved paths due to
  variations in seismic velocities
• The Earth has a layered structure with respect to
  seismic velocities

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The Earths Interior

• P and S waves are reflected and refracted as they
  pass through the Earths different layers
• The paths that they take are not straight lines
• P waves (compressional) pass through solids,
  liquids, or gases
• S waves (shear) only travel through solids
• The structure of the core has been determined
  using seismic waves

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The Earths Interior

• An S wave shadow exists beyond 103O from the
  epicenter of the earthquake
• The S wave shadow is caused by the liquid core
• S waves cannot pass through a liquid
• The radius of the outer core has been determined
  by the size of the S wave shadow

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Fig. 18.17. S wave shadow zone
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The Earths Interior

• P waves are strongly reflected and refracted by
  the liquid outer core
• A P wave shadow zone is found between 103O and
  143O from the epicenter
• Weak P waves may be detected in the shadow zone
• Evidence for a solid inner core
• P wave velocities give us good estimates of the
  density of the crust and mantle

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Fig. 18.18. P wave shadow zone
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Seismic Structure of the Earth

• Seismic wave velocities vary with depth
• Variation with depth is not regular
• Discontinuities exist at certain depths
• Represent discrete changes in the layered Earth
  structure

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Seismic Structure of the Earth

• Mohorovicic Discontinuity (Moho)
• First discovered by Andrija Mohorovicic
• Occurs between 5 and 70 km deep
• Represents the base of the crust
• Compositional change from feldspar rich to
  olivine rich causes change in seismic velocities

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Seismic Structure of the Earth

• Low-velocity zone
• Layer from 100 to 250 km deep
• Seismic velocities usually increase with depth
• Decrease by 6 in low velocity zone
• Caused by partially molten mantle that slows
  seismic waves

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Seismic Structure of the Earth

• Other discontinuities exist
• A sharp rise in velocities occurs at 400 km
  deep
• Likely transition of olivine to magnesium spinel
• Increase in density results in higher seismic
  velocities

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Fig. 18.20. Internal structure of the Earth
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Convection in the Earth

• In the core
• Seismic waves show composition structure of
  core
• 3-D models may show flowing molten iron
• In the mantle
• Investigations show a complex convection system
  occurring in the entire mantle system

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Fig. 18.24. Convection in the Earth
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End of Chapter 18