The seismic moment is a measure of the size of an earthquake based on the area of fault rupture, the average amount of slip, and the force that was required to overcome the friction sticking the rocks together that were offset by faulting. Seismic moment can also be calculated from the amplitude spectra of seismic waves.
Moment = μ A D
μ = shear modulus = 32 GPa in crust, 75 GPa in mantle
A = LW = area
D = average displacement during rupture