Seismic Moment

Seismic moment is a quantity that combines the area of the rupture and the amount of fault offset with a measure of the strength of the rocks - the shear modulus.

Seismic Moment = (Shear Modulus) x (Rupture Area) x (Fault Offset)

Usually we measure the moment directly from seismograms, since the size of the very long-period waves generated by an earthquake is proportional to the seismic moment. The physical units of seismic moment are force x distance, such as newton-meter or dyne-centimeter. These are the same units as energy, but we use the explicit N-m or dyne-cm forms to distinguish the physical character of the quantity as a moment.

For scientific studies, the moment is the preferred measure we use to compare earthquake size since it has fewer limitations than the magnitudes, which often reach a maximum value (we call that magnitude saturation).

To compare seismic moment with magnitude we use a formula constructed by Hiroo Kanamori of the California Institute of Technology:

Mw = log10(Seismic Moment)/1.5 - 10.7

where the units of the moment are in dyne-cm. We call Mw the moment magnitude. Note that moment is often reported in N-m (newton-meters), to convert to dyne-cm, multiply the N-m value by 107.

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.