A new local magnitude scale for South Australian earthquakes has been developed from first principles. The scale is defined by


where AWA is the equivalent Wood-Anderson trace amplitude (in millimeters), Δ is the epicentral distance in kilometers, and Ci is a station correction.

The formula accommodates the local attenuation function for South Australia. It is significantly less than Richter's -log A0 function but agrees closely with the recent results of Boore and Hutton (1984) for southern California. The attenuation constants and the station corrections were determined by detailed statistical analysis of data from hundreds of earthquakes. Station corrections vary from −0.6 to 0.2 magnitude units and show a broad correlation with the regional geology.

South Australian earthquake magnitudes are obtained from short-period, vertical-component seismographs of dissimilar frequency response to that of the horizontal Wood-Anderson instrument. Preliminary analysis of three-component data from the longer period characteristic Adelaide (ADE) seismic station suggests that the departure from standard instrument type makes little difference to actual magnitude determinations.

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