Earthquakes and seismic waves (Chapter 11)

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Earthquakes and seismic waves(Chapter 11)

• Earthquakes are natural hazards, destructive to
  life and property, caused by seismic waves
• Seismic waves move through the Earths interior
  and can be used like X-rays to understand the
  Earth s interior

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How do earthquakes occur?

• Stresses build up in the crust, usually due to
  lithospheric plate motions
• Rock deform (strain) as the result of stress.
  The strain is energy stored in the rocks.

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How do earthquakes occur?

• When the rocks reach their elastic limit, they
  break, and energy is released in the form of
  seismic waves, radiating out from the earthquake
  focus
• The rocks return to their original shape, with a
  displacement (slip) along the fault

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Earthquake focus center of rupture or slip,
seismic waves radiate out from the
focusEarthquake epicenter the point on the
Earths surface over the focus
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Kind of seismic waves

• P-waves most rapid (8 km/sec)
• S-waves slower (5 km/sec), cannot move through
  liquids
• Surface waves even slower, move only on
  surface, most destructive

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Surface waves
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Detecting and measuring seismic waves

• Seismometers
• The paper roll moves with the ground
• The pen remains stationary, because of the
  spring, hinge and weight

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Seismogram

• Tells you
• How far away the earthquake occurred, based on
  the time difference between p and s wave
  arrivals
• Magnitude of ground motion, based on the
  amplitude of the surface waves

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The time interval between p and s-wave arrivals
tells you how far away the epicenter is
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Triangulate from 3 stations to locate the
epicenter
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Earthquake magnitude is related to the amount of
energy released by the earthquake. The Richter
magnitude ML is measured using the amplitude of
the seismic waves. Another measure is called the
moment magnitude Mw
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Some Notable Earthquakes
Indonesia (12/04)
China (5/08) Pakistan (10/05)
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Earthquake hazards

• Ground vibration and motion
• Liquifaction of clay rich sediments
• Landslides
• Tsunami

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• Earthquake hazards
• Liquifaction of clay rich sediments

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Earthquake hazards

• Liquifaction of clay rich sediments
• Landslides

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Earthquake hazards Tsunamis seismic sea waves

• very long wavelength, deep wavebase
• speeds up to 800 km/hour, 15- 30 meters high

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Tsunami Northern Sumatra December 2004
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http//www.tsunami.noaa.gov/
Many tsunamis originate at circum-Pacific
convergent margins, therefore the Pacific Tsunami
Warning system was established
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Preparing for earthquakes

• Assess hazards (how will an earthquake affect an
  area)
• Assess likelihood (probability, based on location
  of faults and prior events)
• Modify zoning regulations and building codes to
  withstand hazard

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• Forecasting Earthquakes
• Unlike, volcanic eruptions, short-term warnings
  (precursors) are not well known
• Real-time warning (after earthquake has occurred)
  can be effective radio waves fiber optic
  transmissions to shut down reactors, close gas
  lines, etc. within seconds or minutes
• Long term earthquake probability is estimated
  using the Seismic Gap Method

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http//www.iris.edu/seismon