The symbol for the moment magnitude scale is , with the subscript " w " meaning mechanical work accomplished . The moment magnitude is a dimensionless value defined by Hiroo Kanamori as where is the seismic moment in dyne ⋅ cm ( 10 − 7 N ⋅ m ) . The constant values in the equation are chosen to achieve consistency with the magnitude values produced by earlier scales , such as the Local Magnitude and the Surface Wave magnitude . Thus , a magnitude zero microearthquake has a seismic moment of approximately , while the Great Chilean earthquake of 1960 , with an estimated moment magnitude of 9.4 – 9.6 , had a seismic moment between and . < ! -- Seismic moment is not a direct measure of energy changes during an earthquake . The relations between seismic moment and the energies involved in an earthquake depend on parameters that have large uncertainties and that may vary between earthquakes . Potential energy is stored in the crust in the form of elastic energy due to built-up stress and gravitational energy . During an earthquake , a portion \ Delta W of this stored energy is transformed into The potential energy drop caused by an earthquake is related approximately to its seismic moment by where \ overline \ sigma is the average of the '' absolute '' shear stresses on the fault before and after the earthquake ( e.g. , equation 3 of ) and \ mu is the average of the shear moduli of the rocks that constitute the fault . Currently , there is no technology to measure absolute stresses at all depths of interest , nor method to estimate it accurately , and \ overline \ sigma is thus poorly known . It could vary highly from one earthquake to another . Two earthquakes with identical M _ 0 but different \ overline \ sigma would have released different \ Delta W
Great Chilean earthquake
1960 Valdivia earthquake
Moment magnitude scale