What scientists discovered in Kā piti
Evidence of ‘massive’ tsunami found in research
Locally, this would have caused widespread coastal abandonment by Mā ori, causing a movement both inland and uphill, something that we know happened because of other research done in the region — but this confirms it for the Kā piti Coast. Professor James Goff
Scientists have uncovered evidence of a massive tsunami that slammed into the North Island’s west coast some 600 years ago, raising intriguing questions about what might have caused it.
The event, thought to have occurred in the mid-15th century, is among dozens of prehistoric “paleotsunamis” around the country that researchers have documented through ancient geological deposits.
In the latest case, a team of researchers were working on dune systems on the Kāpiti Coast when they found evidence to suggest sand had suddenly and violently been shifted inland at some point in the past.
After measuring the extent and height of the dunes, the team dug trenches to explore material beneath it, before taking samples of old tree stumps submerged in a nearby wetland formed when the sand blocked drainage.
Further analysis and radiocarbon dating allowed them to reconstruct the environment — including a large podocarp forest and sand dunes that offered Māori inhabitants a refuge from the wind, as indicated by old shell middens — as it would have appeared in the 1400s.
More interestingly, they were able to show how this area was instantly transformed by an event that drowned the forest, wiped out the dunes and seemingly caused Māori to abandon the coastal area, with no
further signs of occupation after that date.
Given how far inland the deposits had been traced, the researchers concluded they weren’t dealing with a small, local event, but a possible region-wide tsunami that could have hit coastlines stretching up to Taranaki.
“This was no small tsunami,” said study leader Professor James Goff, of the University of New South Wales’ Earth and Sustainability Science Research Centre.
“Locally, this would have caused widespread coastal abandonment by
Māori, causing a movement both inland and uphill, something that we know happened because of other research done in the region — but this confirms it for the Kāpiti Coast.”
For scientists investigating huge tsunamis that hit our shores many centuries ago, Goff said the study came with some further interesting takeaways.
One was that changes in local geomorphology and human activity — as well as sediment buried in the geological record — could offer a useful window to the past.
Another was the need to understand
just what might have triggered the tsunami.
New Zealand’s largest paleotsunamis — including metreshigh surges caused by giant “megathrust” earthquakes — have often been linked to the Hikurangi margin, stretching along the opposite side of the country.
Partly by drawing on some of those events, scientists recently estimated a 26 per cent chance of a subduction zone quake 8.0 or larger striking beneath the lower North Island within the next 50 years.
But Goff said there was still much to learn about the tsunami threat to the west coast.
“We do not know how big and how often these events happen.”
The study, published in scientific journal the Holocene, comes after scientists last year reconstructed the impacts of the largest underwater landslides ever documented in New Zealand.
While these west coast events could have produced tsunami measuring up to 70m high — and sending waves as far as eastern Australia — they were extremely rare, occurring about once every million years.