Measuring Earthquakes

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Measuring Earthquakes
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• Most earthquakes take place along faults in the
  upper 25 miles of the earth's surface
• sudden motion causes shock waves (seismic waves)
  to radiate from their point of origin called the
  focus
• detected by sensitive scientific instruments
  called seismographs

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• A seismic wave is simply a means of transferring
  energy from one spot to another within the earth
• P (primary) waves
• similar to sound waves,
• move back and forth
• can travel through
• solids and liquids
• travel between
• 6 and 13 km/sec
• S (secondary) waves
• a kind of shear wave,
• moves perpendicularly to the
• direction the wave is traveling
• "transverse" waves
• only travel through solids
• travel between 3.5 and 7.5 km/sec
• can be much more destructive

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• A highly simplified simulated recording of
  earthquake waves (a seismogram)

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• seismograms are made by a seismograph (a pen or
  needle suspended over a slowly progressing drum
  covered with paper)

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• severity of an earthquake can be expressed in
  several ways
• magnitude measure of the amplitude of the
  seismic waves (usually expressed by the Richter
  Scale)
• intensity a subjective measure that describes
  how strong a shock was felt at a particular
  location (expressed by the Modified Mercalli
  Scale - values ranging from I to XII )

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The Richter Scale

• developed in 1935 by Charles F. Richter of the
  California Institute of Technology
• each whole number increase in magnitude
  represents a tenfold increase in measured
  amplitude
• terms of energy, each whole number increase
  corresponds to an increase of about 31 times the
  amount of energy released

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The Modified Mercalli Scale

• measures intensity
• a way of measuring or rating the effects of an
  earthquake at different sites
• Intensity ratings are expressed as Roman numerals
  between I at the low end and XII at the high end

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This map plots the Mercalli Intensity ratings of
localities near the Oct. 17, 1989 Loma Prieta
(World Series) earthquake It is called an
isoseismal map one draws contour lines to
enclose locations having higher intensities
intensities typically increase close to an
earthquake's epicenter
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Offset fence Point Reyes
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The San Andreas Fault
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• Strike-slip Fault Example

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Kobe, Japan 1995
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Surface Faulting - Landers, CA 1992
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October 17, 1989 Cypress viaduct of Interstate
Highway 880 in Oakland
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Damage to the Intercontinental Hotel during
Mexico City's 1985 earthquake was severe even
though the building was relatively new
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• estimate the magnitude of the tiny earthquake we
  can make on a lab table
• just kidding

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• you will now conduct a Virtual Earthquake
• http//www.lalc.k12.ca.us/target/technology/second
  ary/earthquakes/index.html
• follow the directions on the website and then
  answer the following
• Student Discussion Questions
• What do we call the place in the earth's crust
  where an earthquake originates?
• What are the 2 main types of earthquake waves?
  How do they differ?
• What two factors affect the speed at which
  earthquake waves travel?
• Why is the difference in arrival time between S
  and P-type waves important to seismologists?
• Explain why reports from 3 seismic stations are
  needed to determine an earthquakes epicenter. Why
  are 2 not enough?