To minimize dispersion in local magnitude estimates due to different instrumental bandwidths, a scale has been established to allow for the average source spectrum, geometrical attenuation, and frequency-dependent absorption. The data used to derive the scale parameters were from S waves recorded in South Australia from 1967 through 1970, in the frequency range 1 to 10 Hz, and for epicentral distances up to 5°. The magnitudes were mainly in the range 1.5 to 3.5. The local scale MN is given by


where Ag mm is the ground amplitude at f Hz at Δ km epicentral distance, and c is a station correction.

The dispersion in observations of magnitude has probably been reduced by use of the scale to near the theoretical limits, allowing for possible source radiation patterns. The relationship of the scale to other measures of magnitude is uncertain, but MN may be approximately equivalent to the local Richter magnitude ML for the magnitude range (1.5 < ML < 3.5) commonly observed in South Australia.

The scle is limited in use to data in the ranges given above, for local earthquakes in South Australia. Modification is necessary before such a magnitude scale can be applied elsewhere or to different data ranges.

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