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.

A "slip" is the relative displacement of formerly adjacent points on opposite sides of a fault, measured on the fault surface.

The "spectrum" is a curve showing amplitude and phase as a function of frequency or period, or how much of each type of shaking there is from an earthquake.