abstract

New procedures have been used since 1977 for assigning local earthquake magnitudes in New Zealand, based on a slightly modified form of Richter's definition and derived from amplitude-distance relations determined from New Zealand earthquakes. The magnitudes for all crustal earthquakes, and for subcrustal earthquakes in or north of the Main Seismic Region, are determined using a formula for geometric spreading which is appropriate to head-wave propagation; for subcrustal earthquakes south of the Main Seismic Region, they are determined assuming body-wave propagation. Inelastic attenuation needs to be taken into account for subcrustal earthquakes south of the Main Seismic Region, and when paths from crustal earthquakes pass through a region which includes the zone of recent volcanism in the center of the North Island.

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