Systematic effects of the introduction in 1977 of a new magnitude scale for New Zealand earthquakes

Abstract Before 1977, local magnitudes for New Zealand earthquakes were calculated using a method based on that for crustal earthquakes in Southern California. Since then they have been calculated using separate procedures for crustal earthquakes, subcrustal earthquakes in or north of the Main Seismic Region, and subcrustal earthquakes in the Fiordland Seismic Region. These procedures take into account the varying characteristics of seismic wave propagation in New Zealand. For each earthquake the magnitudes determined from individual stations with the new procedures are usually much more consistent than those calculated by the old method. The introduction of the new scale has had a systematic effect on the magnitudes, but the effect varies with region, magnitude range, and epoch, the last because of changes to the seismograph network. A procedure is given for converting from the magnitude assigned by the old method to the new magnitude and vice‐versa.