West coast is tsunami 'blind spot' - new study

New Zealand's west coast is its tsunami "blind spot", researchers say, after finding evidence of at least three large tsunamis in the last 1000 years.

While most New Zealand tsunami research focuses on the Pacific coast, a newly published paper in the journal Marine Geology describes three catastrophic events on the west coast including a 12-metre wall of water that hit Westport in 1870.

Two other ancient tsunamis were identified - one associated with the South Taranaki Bight - possibly extending down to Abel Tasman National Park - between 1470 and 1510, and one associated with about 150km of coastline centred on the western Waikato between 1320 and 1450.

Study author Professor James Goff, of the University of New South Wales, said the Tasman Sea was commonly thought to be "benign", but the study, drawing on 20 years of research in the area, showed tsunamis, sometimes reasonably localised events, affected the coast.

The 1870 Westport tsunami was described in a 1912 account in the Evening Post newspaper, though its exact date was a matter of debate.

A Mr Nees said the "old town of Westport ... is now resting under the sea - many fathoms deep - the result of a catastrophe that brought ruin financially to many."

Nees said one afternoon he and others were standing on Dryer's Wharf, "like the town, about a mile distant from the Buller bar" up the Buller River.

"The wind had dropped, and everything was very still, when, looking out westward, we saw the sea and the horizon apparently meeting, and moving towards the land. Then came a great rushing sound of water, and a huge bank of water, about 40 feet high, struck the shore, and rushed up the river, raising its level many feet."

Nees said the water receded "leaving the sea beach quite dry for a long distance seaward, and the Buller River ran out to sea so swiftly that it nearly ran dry". The next surge flooded "the brewery, the Maori pa, all buildings along the river side, and the South Spit"."But the worst was to come."

Nees said the river "suddenly changed its outlet" and carried away "Dryer's wharf, Beauchamp's wharf, and its wharf stores".

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"The slaughter-yards, a mile below the town, went next, then the large public cemetery began to go, until the river was strewn with coffins."

A tide gauge in Sydney registered a small tsunami on August 12, 1870 though no record of the wave could be found in nearby Greymouth or Hokitika, Goff said.

The pre-historic tsunamis were identified through sediment layers deposited by tsunamis coupled with archaeological evidence of Maori villages being abandoned and Maori oral history of the events.

The proposed South Taranaki Bight tsunami between 1470 and 1510 had strong evidence drawn from D'Urville and Kapiti Islands and Waitori in South Taranaki.

The older West Waikato tsunami drew on evidence, mainly marine gravel deposits, from 32 sites extending northward along the coast from Awakino, 90km north of New Plymouth, where State Highway 3 turns inland.

A purakau (legend) from the New Plymouth area called "Coming of the Sand" also described what could be interpreted as a tsunami inundating the area far inland and covering everything in a  thick layer of sand.

Goff said the Westport tsunami probably came from a submarine landslide and the South Tarankai Bight event from activity from submarine faults in the area.

The west Waikato event probably came from a "submarine slope failure possibly from the flanks of the Aotea Seamount", about 240km west of Raglan.

"We don't need to have a fault line off the western side of New Zealand to give us nasty tsunamis," Goff said.

"I'm not saying divert all our funding to the west coast - but let's not forget coastlines that we think are devoid of problems." 

 - The Dominion Post

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