Two New Zealand Tsunamis

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Abstract
The only well-observed tsunamis oflocal origin to reach the New Zealand coast occurred on 1947 March 25 and 1947 Mav 17. Coastal structures between Anaura Bay and Mahia Peninsula were damaged by waves up to 10 m high. They were generated by earthquakes of less than magnitude six. These earthquakes possessed the "slow" character found in certain shallow tsunamicausing shocks defining a belt within the inner margin of the Japan Trench. Low frequencies predominate in the records of these earthquakes. Their character has been attributed to the weakness of the sediments in which they occur. The New Zealand shocks originated beneath a coastal platform area over which the water is no deeper than 200 metres. This platform, of Miocene and more recent sediments, is penetrated and fractured by diapiric intrusions that have from time to time explosively expelled mud and breccia originating at a depth of several kilometres. The rocks at the earthquake source are as weak as or weaker than those in Japan. Because of this structure, the risk of tsunami damage on this stretch of coast is greater than elsewhere in New Zealand.

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