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Homework Help: Wave Motion of an earthquake

  1. Dec 12, 2009 #1
    1. The problem statement, all variables and given/known data

    Assume that P and S (Primary and Secondary) waves from an earthquake with a focus near the Earth's surface travel through the Earth at nearly constant but different average speeds. A monitoring station that is 1000 km from the epicenter detected the S wave to arrive at 42 seconds after the arrival of the P wave. If the P wave has an average speed of 8.0 km/s, what is the average speed of the S wave?

    2. Relevant equations
    v = Lambda/T
    v= Lambda * f

    3. The attempt at a solution

    The period of the wave should be 42 seconds which would make the frequency : 1/42 or .0238

    If the velocity is 8.0 km/s then to find Lambda you would have to:
    8 = lambda * .0238 which makes Lambda 336.13

    Where I get confused is when I multiply, 336.13 * .023, I get an answer of 8 again which would be the velocity of the primary wave. I assume that there is a new frequency that needs to be used in the equation, but I can't seem to figure it out. Any help would be great. Thanks.
  2. jcsd
  3. Dec 12, 2009 #2


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    Homework Helper

    This has nothing to do with frequencies and wavelengths and everything to do with speeds and distances. Replace the waves with cars and see what you get.
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