Scientists have uncovered evidence that a massive tsunami slammed into New Zealand’s northwest coast more than 600 years ago, forever changing the forest ecology of Kāpiti – the country’s North Island.
The latest findings came from Australian scientists from the University of New South Wales, who now use samples from surviving tree stumps found within sand dunes to understand the future threat of tsunamis.
The 15th-century weather event is amongst dozens of prehistoric “paleo tsunamis” that have now been documented by archaeologists combing New Zealand’s coastline for geological deposits.
Using radiocarbon dating, they successfully reconstructed the environment – as it would appear in the 1400s – revealing a large “podocarp forest” offering Māori inhabitants refuge from wind and rain.
Podocarp forests are a mix of tall podocarps and smaller trees (or hardwoods) native to New Zealand with an understorey of shrubs, plants and ferns – today, more than 2 million hectares of the forests hog the country’s northern coastlines.
However, thanks to the massive tsunami, the forests, which once thrived on the coastline, drowned, and the supporting sand dunes were wiped out – resulting in the Māori population, which occupied the coastline for centuries, abandoning the area for several centuries after the event.
“This was no small tsunami,” according to Professor James Goff from the University of New South Wales’ Earth and Sustainability Science Research Centre, whose findings were published in the Holocene scientific journal.
“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.”
According to Professor Goff, 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.
After measuring the extent and height of the dunes, the team dug trenches to explore the material beneath it before taking samples of old tree stumps submerged in a nearby wetland formed when the sand blocked drainage.
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.
For scientists investigating giant tsunamis that hit NZ shores many centuries ago, Professor Goff revealed some interesting takeaways from the research.
One was that changes in local geomorphology, human activity, and sediment buried in the geological record could offer a valuable window into the past – and also the risk of reoccurrence.
Another was the need to understand just what might have triggered the tsunami.
New Zealand’s most significant paleo tsunamis – including metres-high surges caused by giant “megathrust” earthquakes – have often been linked to the Hikurangi margin, stretching along the opposite side of the country.
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 Professor Goff said there was still much to learn about the tsunami threat to the West Coast, adding that “we do not know how big and how often these events happen.”
While these west coast events could have produced tsunamis measuring up to 70 metres high – and sending waves as far as eastern Australia – they were rare, with Professor Goff saying that they occur about once every million years.
