A magnitude scale has been developed for South Australian earthquakes which is simple and consistent. It is based on the maximum trace amplitude among the S waves recorded on vertical component Benioff seismographs, and applies to distances up to 3.6 degrees. The extension of the Richter local magnitude scale to Benioff and other highly sensitive instruments used to study small magnitude earthquakes and with no consideration of differences in crustal structure has drawbacks and, in the present case, could lead to significantly erroneous estimates of magnitudes. The South Australian scale has resulted from an application of the “time-term” method to the study of amplitudes. This has produced satisfactory agreement with a theoretical amplitude-distance relation. Tentative relations are given between the South Australian scale and other scales and an energy-magnitude relation is derived. The maximum S ground velocity between 1° and 3° is due to the reflection from the Mohorovičić discontinuity.