For very large earthquakes (Magnitude greater than 7), the Richter scale saturates, so that the S waves from an M 7 earthquake are not appreciably different from those of an M 7.8 earthquake.

For these larger events, seismologists use a slightly different measure of the earthquake size called the moment magnitude. The moment magnitude is directly related to the size of the earthquake rupture area or fault plane, and does not saturate for large events. For smaller events the Richter and moment magnitudes are similar.