Abstract

The Edgecumbe earthquake of March 1987 had a Richter magnitude of 6.3 and a maximum felt intensity of IX on the Modified Mercalli scale. The earthquake took place at the northeast end of the Taupo volcanic zone, a graben that is widening at a rate of about 7 mm/yr, according to retriangulation. During the earthquake there was tectonic widening of about 1.5 m across a newly formed 7-km-long rent, and subsidence was up to 1.5 m. Widening took place in the Taupo volcanic zone during an earthquake swarm in 1922 and during the Tarawera eruption of 1886. Widening of an unknown amount probably also took place during two earlier historical earthquakes. The rate of widening for the past 150 yr, estimated by adding individual events, agrees well with that inferred from retriangulation.

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