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Earthquakes at Transform Plate Boundaries

There are large, shallow focus earthquakes at transform plate boundaries; the San Andreas Fault Zone is the best known.

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Shake Maps

Shake Maps

Credit: USGS
Source: http://commons.wikimedia.org/wiki/File:1906_San_Francisco_earthquake_seismograph.gif
License: CC BY-NC 3.0

There were only five seismographs around the Bay Area when the 1906 San Francisco earthquake struck. So seismologists had to be pretty creative when they tried to figure out the earthquake’s intensity.

Why It Matters

  • A damage and shaking report compiled by Lawson came out in 1908. The report compiled earthquake intensities from more than 600 sites.
  • Jack Boatwright, a USGS geologist, used this and other information to create a ShakeMap for the 1906 earthquake.
  • Credit: U.S. Geological Survey
    Source: http://en.wikipedia.org/wiki/File:1906_Boatwright_intensity.jpg
    License: CC BY-NC 3.0

    Boatwright intensity map of 1906 San Francisco Earthquake created from Lawson's report [Figure2]

  • A ShakeMap of the Loma Prieta 1989 earthquake was made from seismic recordings.
  • The magnitude 7.8 – 1906 earthquake can be compared with the magnitude 6.9 Loma Prieta earthquake in 1989.

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With the links below, learn more about determining the extent of shaking. Then answer the following questions.

  1. What type of seismic information is in the 1908 Lawson report?
  2. How did later researchers assign numbers to these observations?
  3. What information did Jack Boatwright seek out decades after the quake to add more data to the ShakeMap?
  4. Compare the 1989 and 1906 ShakeMaps. What differences do you see? What is the cause of those differences?

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