The energy release can also be roughly estimated by converting the moment magnitude, Mw, to energy using the equation
log E = 5.24 + 1.44Mw,
where Mw is the moment magnitude.
Moment Magnitude
(MW) = 2/3 log10(MO) - 9.1
Moment (MO) = rigidity x area x slip
where rigidity is the strength of the rock along the fault, area is the area of the fault that slipped, and slip is the distance the fault moved. Thus, stronger rock material, or a larger area, or more movement in an earthquake will all contribute to produce a larger magnitude.