# Homework Help: Calculation of Seismic Moment

1. Nov 13, 2015

### RJLiberator

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.

3/2(8.6+6.0)=log(M_o)
21.9=ln(M_o)/ln(10)
e^(ln(10)21.9)=M_o
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. Nov 13, 2015

### Staff: Mentor

These are approximate things, so I would not put too much weight to the discrepancy.

3. Nov 13, 2015

### RJLiberator

Excellent. Thank you for the help.

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