KEY POINTS:
New Zealand has been hit by a tsunami travelling at the speed of a jetplane - but few people noticed despite the wave bouncing around the coast for three days.
The tsunami, triggered by a big earthquake in the Kuril Islands in Russia's Far East, hit on Thursday last week and was still affecting parts of New Zealand on Saturday.
The magnitude 8.3 earthquake, at 11.14pm on November 15 (NZST) was on a subduction zone where one tectonic plate overrides another between Japan and Russia's Kamchatka Peninsula.
The initial wave took just over 14 hours to travel the 9600km to the Bay of Plenty and Chatham Islands - at an average 685km/h - but a bit longer to reach the South Island, said Rob Bell, principal scientist at the National Institute for Water and Atmospheric Research (Niwa).
Waves more than half a metre high were recorded by at least seven sea-level gauges on the east coast of the South Island and the Chatham Islands.
Dr Bell said the tsunami then bounced off undersea ridges and moved to and fro along indented sections of coastline.
"This explains why the highest waves ... occurred just over 40 hours after the first wave arrived in New Zealand waters," he said.
The tsunami waves were recorded by several sea-level gauges including those at Timaru, Lyttelton, Sumner Head (Christchurch), Kaikoura, and on the Chatham Islands. The gauges are operated by Niwa, Port of Timaru and the Port of Lyttelton.
The largest wave heights were 56cm at the Chatham Islands on November 16 and 58cm at Timaru on November 18.
Dr Bell said New Zealand was never in any danger. "The subduction zone in the Kuril Islands is oriented to beam more directly at Hawaii, US Pacific coast and Central America."
The tsunami impact was greatest at Crescent City in northern California, where a peak wave height of 1.76m caused strong surging currents, resulting in more than US$1 million ($1.5 million) in damage to docks, slipways, floats and electrical equipment.
- NZPA
