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Homework Help: Calculation of Seismic Moment

  1. Nov 13, 2015 #1


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    Gold Member

    1. The problem statement, all variables and given/known data
    The great Sumatra earthquake of late 2004 had a main shock with an M_w magnitude of 9.1 and a large aftershock in early 2005 had an M_w of 8.6.
    Calculate the seismic moment, M_o (energy release) of each earthquake.

    2. Relevant equations

    M_w = 2/3*log(M_o)-6.0
    Where log = log base 10.

    3. The attempt at a solution

    This is really an easy calculation. Let's analyze the second earthquake.

    M_o = 7.943*10^21 N*m

    Now, if we convert it to dyne-cm, we times it by 10*7. So we get 7.943*10^28 dyne-cm.

    However, this report here: http://ceeserver.cee.cornell.edu/pll-group/doc/March_28_2005_Tsunami.pdf [Broken]
    States the answer should have been 1.1*10^29 dyne-cm

    Did I do something wrong in my calculation?
    Also note that there is serious issues with the equation. I have found many different values used instead of "6.0" and I have no idea which one is right. I am using the 'relevant' equation that I found in the notes/book.
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Nov 13, 2015 #2


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    Staff: Mentor

    These are approximate things, so I would not put too much weight to the discrepancy.
  4. Nov 13, 2015 #3


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    Gold Member

    Excellent. Thank you for the help.

    I also found that there are two different versions for the appropriate units.
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