I'm just curious... why do you want to know how much energy is released?

There are a number of articles on the net about the measurement - I did a search on : energy released earthquake and got 37,900 matches.

Princeton seems to have an interesting site:
http://lasker.princeton.edu/ScienceProjects/curr/eqmag/eqmag.htm

Maybe you just need the Ms value?
To calculate the amount of energy released from an earthquake use the formula:
log E = 1.44Ms + 5.24 (where E is the energy released by the earthquake in Joules), or,
E=1.74 x 10 (5+1.44Ms) - the part in parenthesis is 10 to the 5th plus 1.44Ms

Here's another good one from:
http://www.imsa.edu/edu/geophysics/geosphere/tectonics/eqenergy.html
The equation for determining the relationship between the magnitude and the energy released is
M = 2/3 log10E - 9.9
where the seismic energy, E, is given in ergs (note that 10,000,000 ergs = 1 joule).

And yet another good link below with a table comparing a quake to TNT.

Canie

• Magnitude Comparison to TNT